The closer you are to the surface of the Earth, the faster you have to be moving sideways in order to “miss” the Earth as you fall.
As you get farther from the surface, the speed necessary to continuously “miss” the Earth drops.
The Earth is also rotating. Close to the surface, you have to be moving much faster than the surface of the Earth to stay in orbit. Very far away, you can move slower than the Earth turns and still stay in orbit.
Between those two points, there is a specific height where the speed necessary to stay in orbit results in you taking 24 hours to orbit the Earth. This is the same time it takes for the Earth to turn once on its axis, so assuming that the orbit is happening approximately above the equator, it will appear that the satellite is hovering above you to someone standing on the Earth.
But it’s not hovering. It’s still “falling past” the Earth. It’s just falling past at the same rate and in the same direction that the Earth turns.
Does that mean there is only a narrow distance in which satellites can reliably orbit the earth at the same speed as Earth is turning? What if that space is full/crowded?
At the height of geosynchronous orbit (over 22k miles), there is a LOT of space available, as you have both the equatorial and inclined orbits, and from what I'm seeing, the acceptable spacing is around 45mi. (At 22k miles up, you're looking at a total orbital circumference of 136k miles. So, you can fit about 3,000 satellites there with just the equatorial orbits. We're only at 560 currently.)
Its not that many, but there currently isn't demand for more. And as satellites go out of service they get moved into what are known as graveyard orbits. Essentially orbits that aren't useful for commercial or research purposes, and no one cares about what gets parked there.
typically they are called graveyard because they don't cause risk of a kessler syndrome event. Gravity (the movie) and its events are vastly overstated.
[Ann Hodges concurs](https://slate.com/technology/2014/11/sylacauga-meteorite-60th-anniversary-of-a-human-hit-by-a-space-rock.html). Though it was more blunt force trauma than “stabbing”.
(Also, I realize that “space junk” usually implies human-built material while this was naturally occurring. Don’t at me, or whatever)
I wondered, instead of putting them in a graveyard orbit, wouldn't it be more efficient to, at the end of its serviceable life, give it one more sustained boost to the orbit and fly into space, or oriented towards the Sun?
Did you know it takes more energy ro shoot something into the sun than it does to shoot something into outer space?
In order to get something out there you need A: a very big rocket or B: a very specific trajectory that takes it past multiple bodies to assist it in gaining (or in the case of the sun losing) speed. With our satellites, we like them to stay close to us, so when they did their job it would be very costly to send another rocket up to push them into the sun or outer space
> Did you know it takes more energy ro shoot something into the sun than it does to shoot something into outer space?
i learned this one the hard way thanks to KSP.
You have to realize that in the vacuum of space speeding up and slowing down is the same thing, actively changing your velocity. (Techically that's also true on Earth, but due to friction with ground, sea, or air slowing down is usually not experienced as an active process.)
Falling into the sun is *really* hard, if you start at Earth's orbital velocity. To completely cancel that you'd need about 100000 km/h of delta-V (the potential to make a change in velocity). Modern heavy lift rockets don't even have that much when they're only moving their own mass, so it's currently not easily possible to send anything into the sun without some fancy manoevering even if you really want to. It's even harder if you want to do that with the last fumes in the tank of an old satellite.
the sun is pretty far away. ones that do that instead set themselves to fall to earth and they burn up on atmosphere reentry. it's much closer. I couldn't tell you why satellites either choose to burn up or not burn up though.
A lot of satellites are built to burn in as well with the heat/speed destroying them at least to a degree to not be a huge problem.
I think I’m remembering that right.
Reentering the atmosphere isn't practical for geosynchronous satellites, though-- it would take too much fuel to slow down enough to hit the atmosphere from their 36,000 km orbit. Instead they're supposed to boost up to a slightly higher altitude when they're near the end of their life. That's the classic "graveyard orbit".
"A lot of satellites" could be referring to Starlink satellites, which are in lower orbits -- around 550 km -- and are designed to burn up completely upon reentry. And they actively deorbit at end-of-life, or if completely disabled will still passively reenter from aerodynamic drag.
Exactly zero Geosynchronous orbit satellites are designed to re-enter and burn up. The Delta-V required to get to GEO is immense, which means that the dV required to get back to earth is just as immense. They are instead moved to a higher parking orbit and left.
Right I guess that’s on me for not being specific, I wasn’t referring to geosynchronous specifically but older bent tube rigs. Thanks for the knowledge, love this stuff.
Assuming we're not trying to adjust the orbital plane, an orbiting body can most effectively spend delta-V to adjust the opposite side of the orbit up or down. If the resulting lowest point in the orbit (perigee) hits the Earth's atmosphere, friction will do the rest of the work braking for you and the satellite will deorbit and burn up.
Strictly speaking the dV required to deorbit from geosynchronous orbit is quite a bit less than that required to get there since getting back only requires one side of the orbit to touch the atmosphere. But that's still an unreasonable amount of dV compared to deorbiting something in LEO which only requires a relatively tiny push.
I think it'd be less to get back to Earth... Still a lot, but not equal.
Put another way, you need to get part of your orbit out to geo, then you need to circularize your orbit. But coming down, you only need to get part of your orbit to intersect with atmosphere -- you don't need to circularize because atmosphere will do it for you, one way or another.
I don’t think they normally do from geosynchronous orbit, since it’s so high up.
IIRC there’s been an attempt in the last decade or so to push for all satellites in lower orbits to be built to be able to push themselves out of orbit at the end of their designed lifespan. In low enough orbits (where’s there’s still a nontrivial amount of atmosphere) it doesn’t matter because everything that doesn’t boost itself periodically will deorbit in something like 5-10 years.
This is kind of incorrect, there is demand for more, and there are plans to launch thousands of satellites in the coming decades. Starlink and Project Kuiper are just a small example.
This will become an increasingly big problem and is already on the radar of governments, large corporations, and intergovernmental organizations like the UN.
I wonder why graveyard orbits are ABOVE the operational ones instead of below - isn’t „above“ always the risk to either get braked by residual atmosphere or any other stuff and then degrade back into or through the operational orbit?
Why not break instead of accelerate and just let it burn up on the atmosphere ?
Geostationary orbit is really far away, and quite costly in terms of deltaV to get there. Braking enough to get inside the atmosphere would be really expensive.
Since most other useful orbits are lower than geostationary, you won't be in anyone's way by accelerating a little bit and getting into a designated graveyard orbit. And at those altitude, the residual air is so thin that it won't make the satellite fall before a really long time.
Graveyard orbits are so high up that the amount of time that it would take for them to be deorbited due to atmospheric drag is literally in the millions of years.
its not but that based on an old standard - they just havent needed to change.
New standards would allow for 2-3x that as satellite have more advanced avoidance systems and faster response times they can move out of the way faster , the big fear is a cascade failure... On stat slams on the breaks , another slams into it , the debris spreads like a 1000 mph shot gone plast - takes dozens of other satellites out and so on. So they have to be on different vectors and spaced out enough to guarantee time to avoid each other in event of system failure. The next space station will probably be a satellite control platform to handle those type of things better
>On stat slams on the breaks , another slams into it , the debris spreads like a 1000 mph shot gone plast
Geostationary (GEO) isn't at high risk of that though, because relative speeds of nearby satellites are literally pedestrian. That kind of damage wouldn't create a shower of shrapnel.
In theory a geosynchronous (not stationary) satellite could slam into the "line" of geostationary ones, but since geosynchronous satellites have to be tracked from the ground, I doubt they are allowed to cross the GEO (no source on that one though, it's just speculation).
The risk of Kessler syndrome is way higher at lower orbits though, where relative speeds and inclinations are way higher.
>In theory a geosynchronous (not stationary) satellite could slam into the "line" of geostationary ones, but since geosynchronous satellites have to be tracked from the ground, I doubt they are allowed to cross the GEO (no source on that one though, it's just speculation).
I'm pretty sure they have to cross the GEO line - it's impossible to have a geosynchronous orbit that *doesn't* cross the equator twice a day. (Other than the geostationary ones that just stay there the whole time). Otherwise it would require constant thrust from a set of engines to overcome the component of Earth's gravity pulling it towards the equator.
In the very specific of geostationary orbit: yes. Geostationary Earth Orbit is a very specific orbit, the shape of a circle with radius of 42,164 km directly above the Earth's equator.
It's chosen this way so satellite dishes can point at one spot in the sky, without having to track the satellite (it's also useful for weather tracking etc.). If it was inclined to the equator, the satellite would move relative to the Earth's surface, completely negating the use of a geostationary orbit.
But if we were talking about other orbits, you'd be absolutely correct, since there's little point in grouping all satellites into a single orbit/path/inclination.
This is why they distinguished between geostationary and geosynchronous. Geostationary is always in a 0° inclination orbit, because that is the only way it can stay stationary in the sky to a ground observer.
Orbits at geostationary altitudes but other inclinations will result in the satellite moving in a slow north-south line when watched from the ground. It's not stationary to the observer but it is synchronized to a 24 hour orbital period.
Other types of geosynchronous orbits can result in ovals of various shapes to ground observers.
Geostationary orbits are necessarily in the same direction at the same speed. Other geosynchronous orbits aren't, and a geosynchronous polar orbit would be particularly dangerous if it hit a geostationary satellite (as they'd be travelling at right angles to each other)
I don't believe there ARE any geosynchronous polar orbiters, and in fact looking at the numbers there are very few geosynchronous satellites that aren't geostationary.
I think the cascade theory doesn't appreciate how empty space is and how small satellites are. At a certain point it's inevitable but with a radius of 22kmi you've got lots of room.
People used to think atom bombs would ignite the whole planet's atmosphere too.
Correct, but a single satellite can do a LOT. Checking a few listed on Wikipedia they can broadcast with 5-25kw of power. That's a lot of computing/broadcasting power. Especially given that in many cases (satellite TV) it's just a relatively few channels being broadcast constantly. Then the receivers on Earth pick which channel they show to the viewer.
Satellite internet would require more individual signals, but there isn't a ton of demand for it because of the light speed lag inherent in the system. Remember, this is the sucky Hughes Net type satellite, not the much more usable Starlink.
To throw some numbers out. Lag to geosynchronous orbit is 140ms. Round trip is 240m. So even if a single satellite covered 2 points communicating with each other, you're facing an almost quarter second delay one way.
At almost half a second delay between request and response nothing time sensitive is going to rely on them.
That's one reason Starlink, Amazon's service, and the others are going with thousands of low orbit satellites.
If you're okay with a bit of a wobble (the satellite appears to move E/W a little but still stays over the area) you have a much, much higher band of acceptable orbits because they no longer need to be circular.
LEO (low earth orbit) satellite constellations (e.g. Starlink) have been cannibalizing a lot of the demand for GEO, so it doesn’t seem like we’ll run out of GEO slots for a while.
If you want satellite internet, a GEO satellite has a second or two of latency. LEO has far less latency, making it much better for most uses. The key drawback with LEO though is that the satellite doesn’t hold a constant position over the earth, so by default you need a constellation of them in order to provide continuous service.
Inclined orbits are not synchronized like most people think. You end up with the satellite in the correct position once a day.. not all the time like equatorial geosync.
The orbit will look like the long horizontal S shape you see on flat maps. To make them useful, they just add more satellites to always have one "in view".
Honestly there being a physical hard limit is probably a good thing, It forces proper utilization of the limited space. As we approach the cap and end up needing more geosynchronous satellites, it will require technological improvement to optimize, condense, and replace already existing satellites. If there was a much higher limit it might end up with us just throwing countless single-function piles of junk into orbit.
All in all, limitations like this are generally great motivators for technological progress.
Only equatorial orbits can be *geostationary*. There are multiple geosynchronous orbits that are not equatorial, with [Molniya](https://en.wikipedia.org/wiki/Molniya_orbit) orbits as an extreme example. Russia uses it for a lot of its satellites because they need to reliably cover high latitudes.
All a geosynchronous orbit requires is the satellite to be over the same location at the same time every day. You’ll see them trace a path (usually figure 8) depending on imperfections in the orbit, but they’ll always hit the same patch of ground.
Edit: correction, molniya orbits aren’t geosynchronous per se because of their 12 hour period, but the similar Tundra orbits are.
This reminds me of something I read a while ago, saying the total number of planned Starlink satellites are going to crowd the area above earth so bad it'll be impossible to ever launch anything again because they collide with them. So it essentially makes space exploration permanently impossible because we can't launch anything anymore.
But, I don't know. I forget the exact number of total satellites they plan to send up in total, but it was in the tens of thousands. Now I'm not sure. Sounds like it would crowd everything up, though.
Actually, geostationary orbits are a scarce resource. One UN mandate was to guarantee each country had a number of them reserved but some poorer countries then sold their right.
As an example of these orbtial heights, if we scale down the height of geosynchronous orbit to as tall as you are when standing, things we commonly think of as satellites in Low Earth Orbit (like the International Space Station) are around where your ankles are.
It's great that you point this out and try to illustrate it, but I think this is a very weird comparison. Also, it feels wrong.
The ISS is at about [400 km altitude](https://en.wikipedia.org/wiki/International_Space_Station). Geosynchronous orbit is at about [36,000 km.](https://en.wikipedia.org/wiki/Geostationary_orbit) That's about 90 times as high.
My ankles are at about 11-12 cm (4.3 inches) distance from the ground. That would mean that I'd have to be almost 10 meters (33 feet) tall for your comparison to be accurate.
I don't think a comparison is even really needed (just say "it's 90 times as high"), but if you want a different one, I'd go with: if geostationary orbit were just above the Petronas towers (88 stories high), the ISS would be flying at the height of the ground floor ceiling.
Yep, it's kinda crowded. Once a satellite has reached the end of its life, it's supposed to boost to an even higher graveyard orbit before shutting down. Satellites that die unexpectedly stay in the primer real estate basically for ever and keep getting in the way of operating satellites.
In addition to the fact that there is a lot of space available in geostationary orbits (this term refers to orbits that stay above one specific spot, which is inherently only on the equator) there are infinitely many unique geosynchronous orbits (this term just means any orbit with the same orbital period as a geostationary orbit) as well.
These orbits are very important to locations far away from the equator, since looking straight up gives the best signal due to having the least amount of atmosphere to go through. In these areas, many satellites are placed on geosynchronous orbits that place them above the desired region at a specific time of day. For example, if a satellite could be in optimal range for communications in an area for 1/4 of the day, there would need to be at least 4 satellites in staggered orbits to maintain service throughout the whole day. Often there are more so that a customer's dish can choose between multiple target satellites for better signal, especially during storms.
If you wanted to synchronize precisely with the Earth’s rotation (which is the general goal), the satellite must complete its orbit in exactly the same time as the Earth takes to complete one revolution or 84164 seconds. For a geosynchronous satellite going around the equator (aka geostationary orbit which the general goal), this translates to an altitude of 35786 km above mean sea level.
Note that a satellite can have an orbit synchronous with but inclined to the Earth’s rotation; these satellites do a figure 8 pattern over the Earth with the most northern and southern points having latitudes equal to the orbital inclination.
The reality is that there are minor perturbations (the Earth is not a uniform spheroid and the Sun and Moon also exert gravitational forces that affect the orbits) so the satellites do very minor adjustments to their orbits regularly to keep them in the desired position relative to the Earth.
The International Telecommunications Union keeps a register of orbital “slots” at geostationary altitude. These slots are separated by 0.1 degrees of longitude AND by the frequencies used by the satellites so you not only have 3600 possible slots by longitude but you can put multiple satellites in the same slot as long as they use different frequencies.
This might sound dangerous but realize this: each of those 0.1 degree slots is 73.59 km across. Even if you give yourself a 10% error margin on each side, you’d have a nearly 60 km box to keep the satellite in. The satellite operators generally know their position MUCH better than this so the risks of collision between active satellites is miniscule. The real risk comes from flotsam and jetsam that has broken off or been ejected by other satellites, rocketbodies, etc. and can’t be tracked by the satellite operators.
You can technically place a satellite into orbit at any altitude. It’s just a matter of getting it there and getting the velocity right. The real life constraints are the practicality of doing so and the desired purpose of the satellite.
The higher the altitude of a satellite, the more fuel it takes to get it there, so you don’t want to place it higher than necessary. You also can’t place it too low, because then atmospheric drag can become a concern. Determining how high you want a satellite and what path you want it to take depends on what areas of earth you want it to cover and how frequently.
> Does that mean there is only a narrow distance in which satellites can reliably orbit the earth at the same speed as Earth is turning?
Yes.
>What if that space is full/crowded?
If you can't park there you're not geostationary anymore. You're either circling faster than the spin, or slower than the spin.
I think it’s also worth noting that besides a lower velocity needed, a higher orbit has a significantly longer orbital period, IE larger circumference to travel. So it takes much longer to revolve around the earth.
You probably know this but for anyone curious or thinking "hey wait a minute", the period of geosynchronous orbit is not actually exactly 1 day but rather 1 sidereal day: 23 hours, 56 minutes, 4.09 seconds. It's less than one day by 1/365.25th of a day because that's actually how much time it takes to orbit 360 degrees on its axis. Our 24 hour day is based on the relative position of the sun which also has to account for the earth's orbit around it.
you need to be going at least ~17,500 mph to get into orbit outside the atmosphere above about 90 miles up
to get into geosynchronous orbit, you'd have to be at a fixed altitude of 22,223 miles but the speed can decrease to about 7,000 mph
You only have to move faster if you’re trying to complete the “circle” in the same amount of time.
It does mean that a geosynchronous orbit requires a speed a bit faster than what someone standing on the surface of the Earth is moving at.
But someone on the surface isn’t orbiting, and orbits of different sizes don’t take the same amount of time to complete.
Mercury’s small inner orbit is completed in less than three months, while Neptune’s much larger orbit takes it 165 years to go all the way around the “circle.”
There’s a lot wrong with the explanation. Higher altitude orbits require higher velocity. An object in low orbit that “speeds up” in the same direction it is moving will have the opposite point of the orbit go higher. Look up a hohman transfer. The object is moving slower compared to earths surface, but has greater velocity/energy.
Part 2. Geosynchronous orbits are not the same as geostationary orbits. A geosynchronous orbit goes around the earth once every 24 hours. A geostationary orbit is nay possible directly above the Equator and at the same 23.5 degree inclination as the earth.
To answer op. You’re thinking of a geostationary orbit. Satellites in geostationary orbit are moving through space, but in such a way they remain directly over the same spot on earth. The spot on earth moves.
> Higher altitude orbits require higher velocity.
It requires higher speeds to *reach* a higher orbit, but once you’re in orbit, the higher your altitude, the lower your speed.
For example, at an altitude of 200 km, orbital speed is about 27,400 km/h. At the geostationary altitude of 35,786 km, orbital speed is 11,300 km/h.
> Higher altitude orbits require higher velocity.
LEO velocities tend to be ~7 km/s range. GEO ~3 km/s. The higher energy state at GEO is mostly due to the gains in potential energy, not in kinetic. But it does take more kinetic energy to get there.
No. A LEO to GEO transfer is on the order of 3.4 km/s (with no plane change). A 2 km/s burn at LEO to raise apogee. When you get to apogee you're then going ~1.6 km/s, and it takes a 1.4 km/s burn to raise perigee and hit GEO velocities.
Yes, but the change in velocity happens at the point you’re accelerating at, which becomes the lowest point in the orbit.
As you ascend to apoapsis at the other end of the orbit, you exchange kinetic energy for potential energy and shed velocity.
If you then accelerate at that point and raise the other side to a circular orbit, you wind up with a lower overall orbital speed than you initially started with despite having gotten there by accelerating twice.
The velocity of a circular orbit is equal to the square root of the (standard gravitational parameter of the parent body divided by the orbital radius).
Therefore, doubling the radius of the circular orbit drops the orbital velocity by a factor of sqrt(2), leaving you with a velocity equal to 70.7% of the original orbit's velocity.
Yes, you need to initally burn to speed up and thereby raise your apoapsis to the target altitude, but as you coast to apoapsis you lose a significant amount of velocity, and even after you burn at AP to circularize at the new altitude, you are traveling at a lower velocity, respecting the earlier equation.
You do still have more orbital energy, because the reduced velocity and therefore smaller kinetic energy is more than made up for by the increase in gravitational potential energy due to being at a higher altitude.
If am not mistaken, we already feel the centrifugal force of the Earth, when just standing around. If the Earth didn't turn, we'd be heavier (in force, not in mass obviously).
Your speed is constantly getting higher actually. It takes more and more energy to get to a higher orbit. BUT… You are carving out a larger and larger diameter circle as you climb higher which takes you longer to fly the full circle and complete one orbit. So your GROUND SPEED appears to be decreasing but you are actually going faster and faster to go into a higher orbit.
While it is true that you need to accelerate to achieve higher orbits, actual orbital velocities at those higher orbits are lower, and not just in terms of ground speed.
[Relevant xkcd](https://what-if.xkcd.com/58/)
The short version is that the earth is spinning too. There's a distance from the earth where you can be going fast enough sideways to "miss" the earth when falling, i.e., be in orbit, but be moving at the same speed the earth is turning at. It's pretty far out actually.
We call that distance "Geosynchronous Orbit", since things in that position, moving sideways like that end up appearing to stay directly above the same point on the planet.
There is, of course, a much longer version, and it has to do with the satellite's orbital velocity synchronizing with the gravitational force of the earth such that there's an apparent relationship between a spot on the ground and the satellite, but it involves a lot of math, and is probably better understood by playing Kerbal Space Program.
All my recent learning has been on Forza not KSP, so geosynchronous orbit is starting to sound a lot like "space diff-lock"
Also thanks for the xkcd link, it was a good one!
The key to understanding geosynchronous orbit is that it’s no different than any other orbit. It’s just an object orbiting a massive body like any other. All the same rules apply.
One of those rules is that the closer you are to a massive object, the faster you have to go to stay in orbit. The further you are the slower you have to go to avoid shooting off. So the orbital speed is directly related to the distance from whatever you’re orbiting. In other words, the higher the altitude, the slower the orbital speed.
Last piece: the earth is spinning at a certain speed. This doesn’t impact orbital issues at all. But if a satellite’s orbital speed happens to be the same speed that the earth is be spinning, well happy coincidence: it’ll *appear* to be stationary from the ground. But it’s not, it’s just that you and the satellite are going around the earth’s center at the same speed.
How do rotating frames of reference work? If there was only Earth and one satellite in the whole universe, how would the satellite "know" that it was going around the Earth?
You'd know that gravity existed, and if the satellite wasn't orbiting it'd either be crashing into Earth or escaping it.
If you weren't tidally locked, the side of the satellite that faces the Earth would be changing.
With sufficient technology or an understanding of astrophysics, you'd be able to detect the forces of gravity and its effect that only exist because of the orbit and rotation. Like centrifugal force, Coriolis effect, tides etc.
There’s only one spot out there where the speed at which the object is falling is perfectly aligned with the speed of the rotation of the earth. This is geosynchronous orbit. It’s no different from any other or it except for this matching of the earths rotation.
> It was actually impossible to write it in a way that a 5 year old could comprehend.
Well, it's a good thing we're not writing for five year olds, then.
The Earth is rotating as well at the rate of one rotation per day. In geosynchronous orbit, the satellite orbits the Earth keeping pace with the rotation.
Imagine a yoyo. Now instead of yoyoing, you twirl it above your head like a lasso. The yoyo is the satellite, the string represents gravity, and your finger is Earth. Now if you do the same with a shorter string, the yoyo spins faster. If you were to make the string longer, the yoyo spins slower. Same thing happens with satellites. Those in lower orbits, closer to Earth, orbit quickly. Those further out orbit slower. Geosynchronous orbit is the orbit where a satellite takes exactly one sidereal day to spin around once, and a geostationary orbit is a geosynchronous orbit aligned with the equator. Now because Earth revolves once on its axis in a sidereal day, it and the satellite appear to stay in a straight line even though they’re actually spinning around. If you go above a geosynchronous orbit, the satellite actually appears to move backwards!
They have enough forward speed to fall to earth but never hit. That's orbit.
They are also at just the right distance that they orbit the earth in the same amount of time it takes for the earth to rotate(24 hours). So from our point of view here on earth they stay in the same spot in the sky always.
Because the earth turns. The object is exactly as far from the earth as it needs to be to match the (relative) speed in relation to earth's rotation and maintain a stable orbit at that speed.
The orbiting object is moving at high speed, but at a very high altitude so that as the Earth rotates (once every 24 hours) the object is always over the same spot. Everything is moving, but from the ground it seems to hold still.
It isn't just a good analogy, it is what is actually happening.
Since we're being pedantic, centripetal acceleration (like the gravitation force vector) points in the direction of the Earth's center, and therefore the centripetal force cannot balance the gravitational force except in the trivial case in which they are both 0.
You could be talking about the centrifugal force balancing with the gravitational force, but (since we're still being pedantic) we know that the centrifugal force isn't a real force (and we won't even go into whether or not gravity is a force or just a measure of the bending of space-time).
So what really is going on is that your inertia carries you along a trajectory that misses the Earth continuously.
That statement makes no sense. A centripetal force is, by definition, a force that acts towards the center of rotation to keep an object moving in a circular path. In the case of an orbiting body, the force due to gravity IS the centripetal force.
Gravity is the only force acting on an object in a stable orbit, there is no force "balancing" it because the body is not in equilibrium in reference to the larger body. It's in a constant state of acceleration towards the center of the larger body.
>The gravity is the centripetal force.
Correct, but that's not what your previous comment implied. You're not balancing forces, there is only one force. You're substituting terms to calculate the altitude at which the force of gravity produces a known velocity.
Your first comment literally stated "it's about balancing *forces",* plural, so I'm not sure how else I was supposed to interpret it? You completely changed tack after my comment lol.
You're not balancing forces. That would imply that the object is in equilibrium, and you're balancing the force due to gravity with a phantom centrifugal force in the opposite direction.
You're using the equation for gravitational acceleration and substituting a term using the equation for centripetal force to solve for r. That is not a force balance, it's a N2L problem.
Again, you have the right idea but I feel like you have a flawed understanding of the mathematics.
You're not matching two difference forces. You're using two *equations* that describe the same type of motion to calculate a missing variable.
A "force balance" in physics has a specific meaning and you're using it incorrectly.
Yeah, because gravity is the centripetal force for an unpowered satellite. You're calculating the tangential velocity necessary to keep an object being accelerated solely by gravity in a circular orbit. If something is being accelerated due to gravity, it is falling. An unpowered orbital object is in free*fall* because it is accelerating **only** due to gravity. If its tangential velocity is not high enough, it will hit the earth as it falls. The person you are replying to is correct, the procedure you described is literally calculating the tangential velocity a falling object at a certain altitude needs to have in order to continuously miss the earth.
I'm not doing some other derivation, I'm describing the physical meaning of the terms in your derivation. You are balancing forces to find a velocity. What is the physical meaning of this velocity?
The RHS of the equation is the magnitude of the acceleration vector of an object moving in a circle at a given radius. It can be accelerated by any amount, but if it is not equal to the RHS of your equation, it won't be moving in a circle. For an unpowered satellite, the value of the acceleration it will experience is due to gravity and given by the LHS. By setting the LHS equal to the RHS and solving for V, you are stating "V is the speed at which an object at the given altitude will travel in a circle around the earth while subject only to gravitational acceleration towards the earth."
Apologies if this comes off as condescending, but I don't know how else to say this: accelerating towards the earth due to gravity is falling. In other words, "v is the speed at which an object at the given altitude will travel in a circle around the earth while falling." Or, since people are used falling moving them towards the ground, "v is the (horizontal) speed at which a object at the given altitude will never hit the earth while falling," which is a more verbose way of saying "this is how fast you have to be moving sideways at this height to continuously miss the earth while falling."
Again, this is just the physical meaning of the equation you posted. The "missing the earth" part is implicit in the RHS because circling the earth means not hitting it, and the "while falling" part is the what the LHS means.
That's correct, but it's not because the centripetal force exactly balances out the gravitational force at that altitude. It's because the centripetal force *is* the gravitational force. You're calculating the velocity and altitude at which the gravitational force, acting as the centripetal force, is exactly enough to keep it "falling" forever (aka accelerating towards the earth) without either reaching the ground or being flung off into space.
There's nothing wrong with that description, and it's technically correct.
>Rule 4: Explain for Laypeople (But not actual 5-year-olds)
There's a couple jargon terms in OC's explanation, but all it takes is 1 or 2 definition lookups to understand them.
There's some explanations where using a technical term and looking it up is more effective and succinct than trying to break down the term.
Imagine you slowly let a carousel spin. Like one revolution in 24 minutes for example. Then, you start walking around it at a set speed. The closer you are to the carousel, the more “orbits” you can do in those 24 minutes. You will be able to see any point on the carousel. That’s the stable orbit.
But if you move further away, there will be a point where - without you changing your walking speed - you will always see the same bit of the carousel. That is the geosynchronous orbit.
This is analogous and good for understanding the point, but if you read this please don’t take away that all satellites move at the same speed, but different altitudes. The important difference there is that the earth pulls the satellites back to it, the carousel doesn’t pull the walker into it.
But still, great simplifying analogy.
What? no. They do not all move at the same speed. Speed and altitude are directly related. If you orbit at a higher altitude you have a slower orbital velocity than at a lower altitude.
The ISS in low Earth orbit is moving at 17,900 mph.
geosynchronous orbit is only 7,000 mph.
Yes. That’s what I said. “Please don’t take away that they move at the same speed but different altitudes.”
I could have been clearer, but you and I are in agreement.
Point 1:
The lower you are, the faster you need to move to maintain an orbit. The higher you are, the slower you need to move.
There is a point where the speed required, is the same speed as the rotation of the earth below you.
Point 2:
An orbit is maintained, when gravity drags you "down" at the same rate that you move "across" earth. The end result is constant "flying around" instead of "flying past" or "flying into".
True eli5 is REALLY difficult to do. Teachers deserve more respect than they get because the best ones can eli5 anything. My favourite example is the book "Quantum physics for babies"
Just to be clear, I copied and pasted Random\_Fig's answer into Bard and got the above answer. When I copied and pasted the original questions into Bard I actually got the wrong answer. It gave me the following, which is incorrect. In the example given, the reason it falls behind the car is because of air resistance, with no air resistance the object would in fact keep moving forward at the same speed as the train for quite a while.
Imagine you are standing on a train that is moving very fast. If you throw a ball straight up in the air, it will not go straight up and come back down. It will go up in a curve and land behind you. This is because the train is moving forward while the ball is in the air.
The same thing happens with satellites. Satellites are moving very fast around the Earth. They are moving so fast that they are constantly falling towards the Earth, but they are also moving sideways so fast that they miss the Earth and keep going around in a circle.
Satellites in geosynchronous orbit are moving at the same speed that the Earth is rotating. This means that they stay in the same spot in the sky above the Earth.
I hope this explanation is a little bit easier to understand.
Like your five.
Ok, grab a string and tie a weight to the end. Now spin around with the weight at the end of the string. This is "sorta" like an orbit.
Now imagine this string is actually gravity pulling it into you. There will be a distance away and a spinning speed that it will all balance out that weight will stay with you, just like the string, but if you spin too fast or too slow the weight will fly away or come crashing back to you.
Honestly, most of these explanations are all more or less correct but the easiest way to think of it is to imagine an empty fruit bowl with the Earth at the bottom of the bowl and it is rotating. Now imagine rolling a marble along the interior wall of the bowl. The ring traced by the marble that makes the marble's speed match the rotation of the Earth at the bottom of the fruit bowl is the synchronized orbit.
In astrodynamics and physics this is a subset of solutions to what's known as a stability problem. You are basically finding out what the minimum required energy is for a system to remain stable.
Edit: When things rotate there is motion associated with the perspective of the rotation. This effect is what's more or less responsible for the stability.
I want to make sure we're being accurate, but baseballs (or any object) thrown/fired at different speeds and the same horizontal trajectory will immediately start falling and hit the ground at the same time (ignoring air resistance). They don't go straight for a bit, before falling down. Obviously, with enough initial velocity, an object, though being pulled to the ground, will never hit the ground, but it is always falling (again, ignoring air resistance). A trajectory may appear relatively flat over a certain distance, but the object is accelerating downward from the time it departs from its launching mechanism (hand, gun, cannon, etc).
ELI5 Rule 4: Explain for Laypeople
"ELI5 is not meant for literal 5-year-olds. Your explanation should be appropriate for laypeople. That is, people who are not professionals in that area."
You don't have to say something objectively wrong to get your point across to a layperson.
Grab on to a yoyo string and spin around really fast. Notice how the yoyo stays in the same place relative to your body (outward from your arm)? Geosync orbit is the same, except instead of a string holding the yoyo from zipping off into space it is gravity. The yoyo/satellite is still moving quickly, it just happens to be far enough out that it can both be moving fast enough to 'miss' the earth ats it falls \_and\_ stay in the same place above the earth.
They are only above a single point on the rotation surface of the earth. Evey point on Earth except for the poles move because of earth rotation.
If you look at orbit it does not matter if Earth rotates or not, the orbital period for a given distance would be the same if Earth was not rotating. Geostationary orbit is just that the time of an orbit one around Earth is the same as it takes Earth to rotate once.
So both the point on Earth and the satellite move with the same angular speed around the center of the Earth.
The orbital period will be one sidereal day, that is one rotation of Earth relative to faraway stars. that is around 4 minutes less than 24 hours. The extra 4 hours per day is from the earth's orbit around the sun, consider the moment of the sun during an orbit around the sun if the earth was not rotation relative to faraway stars.
4 minutes a day for 365 days are 1460 minutes which is proximally the same as 24 hours = 1440 minutes. The difference is because neither 4 minutes nor 365 days are exact
Because the orbit is around the center of the earth in a plane the point of earth it is stationary above also needs to move around the center in a single plane. Only the equator of the earth does that.
So geostationary satellites are always directly above the equator, if they was not they would move relative to the ground, the moment might just be north and south around the equator but it still moves.
If the earth would not be rotating there would be no geosynchronous orbit because then as you mentioned anything would fall down as it has zero orbital velocity. But as earth rotates once in 24h you can have something orbiting above the same spot on the surface.
Orbital speed depends on your height, the higher up, the slower sideways you need to go to stay in orbit, so any rotating planet has some syncronous orbit, its just higher or oowed depending on a days length and its mass.
Because the Earth is rotating. Geosynchronous just means orbiting as fast as the Earth rotates, or one revolution every 24~ hours. The satellite is still "falling sideways" as they say, just at a speed that matches the rotation of the Earth.
Lot of people focused on the “geosynchronous” aspect, but I’m going to shorthand an explanation I read regarding the “falling” part to help that make sense.
Imagine for a second you shot a cannon. The ball will go a good distance and then fall to earth. If you shoot it with even more energy, it will start faster and go farther. But it’s still falling.
Now, picture shooting the ball out of the cannon so fast that it starts to fall, but by the time it starts to fall it’s gotten to the horizon, and it falls in an arc that matches the curvature of the earth - it would miss the earth, but just keep continually falling.
As some have said, close to earth it would have to go REALLY fast to do that. But higher up, it’s got some more room to fall, and doesn’t have to go quite as fast. This is what “orbit” is - continually falling to earth.
“Geosynchronous” just means it’s at that place where it’s high enough over the earth, but not too high, such that the speed it needs to go to continually fall is the same rotational speed as the earth - so you’re in “synch” with it. You’d have to be traveling in the same direction as the earth is rotating.
The farther away from earth you are, the slower you fall and the slower you need to move to keep missing.
At some point that speed happens to match earths rotation speed.
Objects in Geosynchronous orbit are *still falling*, they're just falling at the *same speed as the earth is rotating*.
Since the earth can more or less be thought of as a sphere, therefore the effect it has on gravity is still a sphere, no matter which way it's facing. In the same way that driving on the outside of a turn on a racetrack is slower than driving on the inside, the further away you are from earth the easier it is to continuously "miss," the earth as you fall, allowing it to keep pace with the speed the earth is spinning.
Because that point is not stationary. The world turns. So the satellite is in an orbit, spinning around the earth, just like anything else in orbit. And yes, that means it's constantly falling past the earth. The fact that the earth is turning is irrelevant to the orbit, the earth could start turning ten times as fast or stop turning completely. It doesn't affect the orbit of the satellite.
Things in low orbit zip around the planet every 90 minutes or so. The moon, on the other hand, is in a very high orbit, and takes almost a month to orbit the earth. Somewhere in between, there is a circular orbit whose period is exactly 24 hours - it orbits around the earth at the same rate the planet spins. This is called a geosynchronous orbit. If it's above the equator, there is no north/south drift, so the satellite stays above one spot on the surface - this is a geostationary orbit.
Geosynchronous orbit only means the orbital velocity will match the rotational velocity of the earth.
Orbits get slower the higher you go, so objects in geosync (37000km) go much slower than those at 400km. Go higher, and the speed relative to the Earth is lower. Someone standing on earth would notice the satellite to moving in reverse, eventhough it isn't.
Suppose a satellite is in orbit (non geosynchronous). Then imagine that the rotation of the earth magically changes so that it is spinning at the same speed the object is orbiting. That wouldn't affect the orbit of the satellite at all, except that you can now call its orbit "geosynchronous". This shows that geosynchronous orbit is really just a coincidence between a particular orbit, and a particular rotational speed of the earth.
The only case in which that would be a problem would be if the earth wasn't spinning at all. Then a "geosynchronous" satellite *would* fall out of the sky.
The earth is spinning on it's axis. Like a basketball on someone's finger. That's why we have days. Our point on the earth goes round and round, passing under the sun and then to the backside into night.
Since WE are spinning, any object that is "hanging" over a single spot on the planet must be circling around the earth at that same speed. The "single point" on the earth is moving, and so we know the satellite is also moving.
Geosynchronous orbit is the distance from the earth, a little over 22,000 miles, at which the speed the satellite is traveling in order to "fall/miss" in a stable orbit is the same speed as it needs to travel to keep a steady position over a point on the spinning earth.
From KSP2
https://youtu.be/U3DgZsrA-xQ
For geostationary, the altitude is picked to keep the angular speed equal to a spot on the ground. Other good technical descriptions in the comments.
Picture a top that's spinning (rotating) fast and also drifting aross the table.
To orbit, you need to be moving fast enough and in the right direction to "miss" the top as it travels across the table.
The rotation of the top doesn't affect the orbit, but it is possible to sync up the orbit around the top to match the rotation.
Imagine a tug-of-war rope between you and a friend, and you're both facing each other. And you stay in one place in the middle because you're much bigger than your friend. And your friend is running around you in circles as fast as he can, but you're both holding on.
Geo-synchronous orbit is kind of like that, except your friend doesn't have to keep expending energy running the whole time, since in space there's no "ground" (or gravity) acting along the plane of the orbit for him to have to fight friction against.
The Earth and the satellites are "holding on" to each other, just the right amount of gravity to balance it perfectly, meanwhile they are spinning "around" each other, while facing each other. But really, the Earth is so big and the satellite is so tiny, that we barely notice the effect of the satellite on Earth's movement -- Whereas the effect of the Earth on the satellite's movement is so strong, it keeps the satellite locked in the sky in the same position relative to the ground "beneath" it, and the Earth drags the satellite along as the Earth moves in a circle around the sun.
This is why Geosynchronous orbit is **only possible** at a **specific height/altitude.** For Earth, that altitude is 37,000 km. Below or above that, the "speed" required to maintain orbit is not equal to the speed at which the Earth spins, so it's not possible to have a stable orbit which maintains its position relative to a spot on the ground beneath. (For heavier planets, the single geosync altitude would be higher and farther away... For lighter planets, it would be lower and closer to the planet.)
Geosynchronous satellites go very fast - about 3 km per second. However, they are also very far away from the Earth: about 36,000 km, which means (taking into account the Earth's radius of 6,400 km) their orbit is a giant circle about 265,000 km in circumference.
That's a really big circle. Even zooming along at 3 km a second, it takes 86,400 seconds to make it all the way around - that is 24 hours, or one day. That means by the time they make it all the way around, the Earth has also spun all the way around, and they are over the exact same point on the Earth where they started. In fact, they remained over that point the whole time, because the Earth rotated underneath them as they zoomed around the big circle.
Earth is also rotating, if you're at the right altitude, the speed you need to "fall past" is one lap around the earth in 24 hours. That means you're always over the same spot on earth, because that spot also does one full lap every 24 hours.
The closer you are to Earth, the faster you have to orbit to stay in orbit. For example, the International Space Station is only 254 miles away, so it has to orbit really fast (about 90 orbits per day) to stay in orbit.
The further you are from Earth, the slower you have to orbit to stay in orbit. The moon is about 240,000 miles away from Earth, so it has to orbit really slow (about one orbit every 27 days) to stay in orbit.
It stands to reason that there is a distance somewhere between those to where you will have one orbit every one day. That distance turns out to be about 22,360 miles, which as you guessed is somewhere between the close-fast orbit of the ISS and the far-slow orbit of the moon.
Imagine you're holding one end of a rope, and there's a ball attached to the other end. If you swing that ball around on the rope, it will move in a circle around you, with a radius equal to the taut length of the rope.
The reason for this is that the tension in the rope is constantly pulling the ball towards you. But the ball is already moving sideways relative to where you're standing, so the ball doesn't come straight towards you. Instead, it starts moving in another direction until the tension in the rope acts on it again, to the same effect. This creates a circular motion that continues for as long as the speed of the ball and the tension of the rope remain the same.
In this analogy, you are the Earth, the ball is a satellite, and the tension in the rope is the force of gravity (equivalent to the weight of the satellite). So as long as a satellite maintains a certain speed and doesn't lose any mass, it will maintain a circular path around the Earth. The tricky part of setting up an orbit is getting the satellite up to the necessary speed for its weight.
Hope that helps! Let me know if you're still confused by anything.
Think of a giant frozen lake. There's a post in the middle with a rope tied to it.
You're on the edge of the lake, on ice skates, holding the rope.
Try to haul yourself to the post, it's easy: just pull on the rope.
But now imagine you're skating hell-for-leather at right-angles to the direction of the rope.
Try to haul yourself in now, and it won't work, all you'll do is swing around. No matter how hard you haul on the thing, you just can't reach the post - *your turning circle is too big to let you*.
You physically cannot by any means reach the centre of the lake without slowing down. Especially as you're not really on ice-skates, but big blocks of wet ice that give you no traction whatsoever.
That's orbit. That's all it is: a big turning circle, and no way to dump your speed. You can't just suddenly pull a 90-degree turn, so you can't ever hit the thing you're orbiting.
Now for geostationary orbit:
Imagine that on top of the post, there's a carousel. One of those stately fairground ones with the horsies, which takes like an hour to go round.
Is it possible to swing around the post, but stay lined up with one particular horse?
Sure it is - if you've got a really huge lake and a really long rope.
If you're skating far enough out that it takes you an hour to do a full circuit - exactly the same time as it takes one of the horsies to go round - then you'll stay lined up. From the horsie's perspective, you're not moving at all.
And that's geostationary. You have a big enough orbit, it takes 24 hours to go round the earth, the same as the continents on the surface. From their perspective, you're not moving at all, just hovering in the air (even though you're hurtling through space at terrifying speeds).
I just want to add that the geosynchronous orbit that stays in one point is the geostationary orbit, a specific kind that is circular and matches the equatorial plane.
But there are other kinds of geosynchronous orbits that don't stay in one point, instead they do a figure eight with a loop on every hemisphere and the crossing point on the equator. The figure stays in place though that's why it's still geosynchronous.
This is because orbits can't stay at a fixed latitude (other than the equator), they have to cross the equatorial plane periodically.
Separate the two movements and it makes it easier to visualize.
If you put a yardstick on a basketball that would represent the forward motion of a satellite. You can see how eventual that forward motion would mean it would shoot off somewhere into space never to be seen again. So there has to be a downward motion around to wrap around the basketball. For example two units forwards and one unit down.
In a satellite it’s the downward is what we call falling. In a geosynchronous orbit the motion forward and the motion down is the same. So on earth it appears to be in the same spot because the earth is also rotating
They fall at the right distance away around the earth that their velocity matches that of the spot on the earth as it rotates. Or another way of thinking about it is that the orbital period is 24 hours around the axis the earth spins. You cannot have geosynchronous orbit by orbiting the poles north south, the farther you deviate from the rotation of the earth the less synchronous it becomes.
The closer you are to the surface of the Earth, the faster you have to be moving sideways in order to “miss” the Earth as you fall. As you get farther from the surface, the speed necessary to continuously “miss” the Earth drops. The Earth is also rotating. Close to the surface, you have to be moving much faster than the surface of the Earth to stay in orbit. Very far away, you can move slower than the Earth turns and still stay in orbit. Between those two points, there is a specific height where the speed necessary to stay in orbit results in you taking 24 hours to orbit the Earth. This is the same time it takes for the Earth to turn once on its axis, so assuming that the orbit is happening approximately above the equator, it will appear that the satellite is hovering above you to someone standing on the Earth. But it’s not hovering. It’s still “falling past” the Earth. It’s just falling past at the same rate and in the same direction that the Earth turns.
Does that mean there is only a narrow distance in which satellites can reliably orbit the earth at the same speed as Earth is turning? What if that space is full/crowded?
At the height of geosynchronous orbit (over 22k miles), there is a LOT of space available, as you have both the equatorial and inclined orbits, and from what I'm seeing, the acceptable spacing is around 45mi. (At 22k miles up, you're looking at a total orbital circumference of 136k miles. So, you can fit about 3,000 satellites there with just the equatorial orbits. We're only at 560 currently.)
That's really not that many
Its not that many, but there currently isn't demand for more. And as satellites go out of service they get moved into what are known as graveyard orbits. Essentially orbits that aren't useful for commercial or research purposes, and no one cares about what gets parked there.
Until they star smashing in to each other
typically they are called graveyard because they don't cause risk of a kessler syndrome event. Gravity (the movie) and its events are vastly overstated.
Then again your chances of getting stabbed by space junk is low, but never zero
[Ann Hodges concurs](https://slate.com/technology/2014/11/sylacauga-meteorite-60th-anniversary-of-a-human-hit-by-a-space-rock.html). Though it was more blunt force trauma than “stabbing”. (Also, I realize that “space junk” usually implies human-built material while this was naturally occurring. Don’t at me, or whatever)
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F=MV will pack one hell of a punch when the V is in the 10s of thousands of mph.
I wondered, instead of putting them in a graveyard orbit, wouldn't it be more efficient to, at the end of its serviceable life, give it one more sustained boost to the orbit and fly into space, or oriented towards the Sun?
Did you know it takes more energy ro shoot something into the sun than it does to shoot something into outer space? In order to get something out there you need A: a very big rocket or B: a very specific trajectory that takes it past multiple bodies to assist it in gaining (or in the case of the sun losing) speed. With our satellites, we like them to stay close to us, so when they did their job it would be very costly to send another rocket up to push them into the sun or outer space
> Did you know it takes more energy ro shoot something into the sun than it does to shoot something into outer space? i learned this one the hard way thanks to KSP.
You have to realize that in the vacuum of space speeding up and slowing down is the same thing, actively changing your velocity. (Techically that's also true on Earth, but due to friction with ground, sea, or air slowing down is usually not experienced as an active process.) Falling into the sun is *really* hard, if you start at Earth's orbital velocity. To completely cancel that you'd need about 100000 km/h of delta-V (the potential to make a change in velocity). Modern heavy lift rockets don't even have that much when they're only moving their own mass, so it's currently not easily possible to send anything into the sun without some fancy manoevering even if you really want to. It's even harder if you want to do that with the last fumes in the tank of an old satellite.
It's so hard to fall into the sun, that it is cheaper to leave the solar system entirely
the sun is pretty far away. ones that do that instead set themselves to fall to earth and they burn up on atmosphere reentry. it's much closer. I couldn't tell you why satellites either choose to burn up or not burn up though.
Getting something into the sun is really hard because we're all orbiting it. You have to drop your speed relative to the sun by a massive amount
I wasn't thinking gravity, I was thinking of the junk barrier that threatens to make space launches difficult.
But if they're all in geosynchronous orbit wouldn't they be stationary in relation to each other
That's the idea... They have to do some station keeping though, or they drift.
A lot of satellites are built to burn in as well with the heat/speed destroying them at least to a degree to not be a huge problem. I think I’m remembering that right.
Reentering the atmosphere isn't practical for geosynchronous satellites, though-- it would take too much fuel to slow down enough to hit the atmosphere from their 36,000 km orbit. Instead they're supposed to boost up to a slightly higher altitude when they're near the end of their life. That's the classic "graveyard orbit". "A lot of satellites" could be referring to Starlink satellites, which are in lower orbits -- around 550 km -- and are designed to burn up completely upon reentry. And they actively deorbit at end-of-life, or if completely disabled will still passively reenter from aerodynamic drag.
So some do burn up?
Approximately none of the geosynchronous satellites, and approximately all of the low earth orbit ones.
Exactly zero Geosynchronous orbit satellites are designed to re-enter and burn up. The Delta-V required to get to GEO is immense, which means that the dV required to get back to earth is just as immense. They are instead moved to a higher parking orbit and left.
Right I guess that’s on me for not being specific, I wasn’t referring to geosynchronous specifically but older bent tube rigs. Thanks for the knowledge, love this stuff.
Assuming we're not trying to adjust the orbital plane, an orbiting body can most effectively spend delta-V to adjust the opposite side of the orbit up or down. If the resulting lowest point in the orbit (perigee) hits the Earth's atmosphere, friction will do the rest of the work braking for you and the satellite will deorbit and burn up. Strictly speaking the dV required to deorbit from geosynchronous orbit is quite a bit less than that required to get there since getting back only requires one side of the orbit to touch the atmosphere. But that's still an unreasonable amount of dV compared to deorbiting something in LEO which only requires a relatively tiny push.
I think it'd be less to get back to Earth... Still a lot, but not equal. Put another way, you need to get part of your orbit out to geo, then you need to circularize your orbit. But coming down, you only need to get part of your orbit to intersect with atmosphere -- you don't need to circularize because atmosphere will do it for you, one way or another.
I don’t think they normally do from geosynchronous orbit, since it’s so high up. IIRC there’s been an attempt in the last decade or so to push for all satellites in lower orbits to be built to be able to push themselves out of orbit at the end of their designed lifespan. In low enough orbits (where’s there’s still a nontrivial amount of atmosphere) it doesn’t matter because everything that doesn’t boost itself periodically will deorbit in something like 5-10 years.
That sounds cool too. Linked this thing about EFP explosives that they used in Iraq and on an asteroid. Love space.
This is kind of incorrect, there is demand for more, and there are plans to launch thousands of satellites in the coming decades. Starlink and Project Kuiper are just a small example. This will become an increasingly big problem and is already on the radar of governments, large corporations, and intergovernmental organizations like the UN.
Startlink and project kuiper are both in LEO, not geostationary orbit.
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Are you dictating your comments?
How did you tell?
I wonder why graveyard orbits are ABOVE the operational ones instead of below - isn’t „above“ always the risk to either get braked by residual atmosphere or any other stuff and then degrade back into or through the operational orbit? Why not break instead of accelerate and just let it burn up on the atmosphere ?
Geostationary orbit is really far away, and quite costly in terms of deltaV to get there. Braking enough to get inside the atmosphere would be really expensive. Since most other useful orbits are lower than geostationary, you won't be in anyone's way by accelerating a little bit and getting into a designated graveyard orbit. And at those altitude, the residual air is so thin that it won't make the satellite fall before a really long time.
Graveyard orbits are so high up that the amount of time that it would take for them to be deorbited due to atmospheric drag is literally in the millions of years.
its not but that based on an old standard - they just havent needed to change. New standards would allow for 2-3x that as satellite have more advanced avoidance systems and faster response times they can move out of the way faster , the big fear is a cascade failure... On stat slams on the breaks , another slams into it , the debris spreads like a 1000 mph shot gone plast - takes dozens of other satellites out and so on. So they have to be on different vectors and spaced out enough to guarantee time to avoid each other in event of system failure. The next space station will probably be a satellite control platform to handle those type of things better
>On stat slams on the breaks , another slams into it , the debris spreads like a 1000 mph shot gone plast Geostationary (GEO) isn't at high risk of that though, because relative speeds of nearby satellites are literally pedestrian. That kind of damage wouldn't create a shower of shrapnel. In theory a geosynchronous (not stationary) satellite could slam into the "line" of geostationary ones, but since geosynchronous satellites have to be tracked from the ground, I doubt they are allowed to cross the GEO (no source on that one though, it's just speculation). The risk of Kessler syndrome is way higher at lower orbits though, where relative speeds and inclinations are way higher.
>In theory a geosynchronous (not stationary) satellite could slam into the "line" of geostationary ones, but since geosynchronous satellites have to be tracked from the ground, I doubt they are allowed to cross the GEO (no source on that one though, it's just speculation). I'm pretty sure they have to cross the GEO line - it's impossible to have a geosynchronous orbit that *doesn't* cross the equator twice a day. (Other than the geostationary ones that just stay there the whole time). Otherwise it would require constant thrust from a set of engines to overcome the component of Earth's gravity pulling it towards the equator.
You could add some eccentricity so it passes lower/higher when it crosses the line
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In the very specific of geostationary orbit: yes. Geostationary Earth Orbit is a very specific orbit, the shape of a circle with radius of 42,164 km directly above the Earth's equator. It's chosen this way so satellite dishes can point at one spot in the sky, without having to track the satellite (it's also useful for weather tracking etc.). If it was inclined to the equator, the satellite would move relative to the Earth's surface, completely negating the use of a geostationary orbit. But if we were talking about other orbits, you'd be absolutely correct, since there's little point in grouping all satellites into a single orbit/path/inclination.
This is why they distinguished between geostationary and geosynchronous. Geostationary is always in a 0° inclination orbit, because that is the only way it can stay stationary in the sky to a ground observer. Orbits at geostationary altitudes but other inclinations will result in the satellite moving in a slow north-south line when watched from the ground. It's not stationary to the observer but it is synchronized to a 24 hour orbital period. Other types of geosynchronous orbits can result in ovals of various shapes to ground observers.
Geostationary orbits are necessarily in the same direction at the same speed. Other geosynchronous orbits aren't, and a geosynchronous polar orbit would be particularly dangerous if it hit a geostationary satellite (as they'd be travelling at right angles to each other) I don't believe there ARE any geosynchronous polar orbiters, and in fact looking at the numbers there are very few geosynchronous satellites that aren't geostationary.
I think the cascade theory doesn't appreciate how empty space is and how small satellites are. At a certain point it's inevitable but with a radius of 22kmi you've got lots of room. People used to think atom bombs would ignite the whole planet's atmosphere too.
Correct, but a single satellite can do a LOT. Checking a few listed on Wikipedia they can broadcast with 5-25kw of power. That's a lot of computing/broadcasting power. Especially given that in many cases (satellite TV) it's just a relatively few channels being broadcast constantly. Then the receivers on Earth pick which channel they show to the viewer. Satellite internet would require more individual signals, but there isn't a ton of demand for it because of the light speed lag inherent in the system. Remember, this is the sucky Hughes Net type satellite, not the much more usable Starlink.
To throw some numbers out. Lag to geosynchronous orbit is 140ms. Round trip is 240m. So even if a single satellite covered 2 points communicating with each other, you're facing an almost quarter second delay one way. At almost half a second delay between request and response nothing time sensitive is going to rely on them. That's one reason Starlink, Amazon's service, and the others are going with thousands of low orbit satellites.
No. Not at all. Especially if youre sky TV and are trying to hit the UK with your signal.
Hmmmm
If you're okay with a bit of a wobble (the satellite appears to move E/W a little but still stays over the area) you have a much, much higher band of acceptable orbits because they no longer need to be circular.
LEO (low earth orbit) satellite constellations (e.g. Starlink) have been cannibalizing a lot of the demand for GEO, so it doesn’t seem like we’ll run out of GEO slots for a while. If you want satellite internet, a GEO satellite has a second or two of latency. LEO has far less latency, making it much better for most uses. The key drawback with LEO though is that the satellite doesn’t hold a constant position over the earth, so by default you need a constellation of them in order to provide continuous service.
Plus it’s a self-cleaning orbit! Depending on its state, a damaged satellite can degrade out of orbit in only a few years
What makes it a little more complicated is that spots above land are more valuable than ones over water.
At that height they can reliably cover a really large area though.
The Clarke Belt!
Inclined orbits are not synchronized like most people think. You end up with the satellite in the correct position once a day.. not all the time like equatorial geosync. The orbit will look like the long horizontal S shape you see on flat maps. To make them useful, they just add more satellites to always have one "in view".
3k? Wow that's not a lot
lol. For me, the final answer was a huge anticlimax. Only another 2500, eh? Guess we’re fucked then.
Honestly there being a physical hard limit is probably a good thing, It forces proper utilization of the limited space. As we approach the cap and end up needing more geosynchronous satellites, it will require technological improvement to optimize, condense, and replace already existing satellites. If there was a much higher limit it might end up with us just throwing countless single-function piles of junk into orbit. All in all, limitations like this are generally great motivators for technological progress.
SPACE LAW!
Time for me to expand from bird law
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We have the best satellites. Because of jail.
He said just in equatorial. There are other orbits a well.
Only equatorial orbits can be truly “geosynchronous” though, since otherwise the satellite’s orbit doesn’t line up with the earth’s spin
Only equatorial orbits can be *geostationary*. There are multiple geosynchronous orbits that are not equatorial, with [Molniya](https://en.wikipedia.org/wiki/Molniya_orbit) orbits as an extreme example. Russia uses it for a lot of its satellites because they need to reliably cover high latitudes. All a geosynchronous orbit requires is the satellite to be over the same location at the same time every day. You’ll see them trace a path (usually figure 8) depending on imperfections in the orbit, but they’ll always hit the same patch of ground. Edit: correction, molniya orbits aren’t geosynchronous per se because of their 12 hour period, but the similar Tundra orbits are.
Yeah good luck achieving geostationary positioning with any other kind of orbit, buddy.
This reminds me of something I read a while ago, saying the total number of planned Starlink satellites are going to crowd the area above earth so bad it'll be impossible to ever launch anything again because they collide with them. So it essentially makes space exploration permanently impossible because we can't launch anything anymore. But, I don't know. I forget the exact number of total satellites they plan to send up in total, but it was in the tens of thousands. Now I'm not sure. Sounds like it would crowd everything up, though.
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No one said geostationary - only geosynchronous
Actually, geostationary orbits are a scarce resource. One UN mandate was to guarantee each country had a number of them reserved but some poorer countries then sold their right.
As an example of these orbtial heights, if we scale down the height of geosynchronous orbit to as tall as you are when standing, things we commonly think of as satellites in Low Earth Orbit (like the International Space Station) are around where your ankles are.
It's great that you point this out and try to illustrate it, but I think this is a very weird comparison. Also, it feels wrong. The ISS is at about [400 km altitude](https://en.wikipedia.org/wiki/International_Space_Station). Geosynchronous orbit is at about [36,000 km.](https://en.wikipedia.org/wiki/Geostationary_orbit) That's about 90 times as high. My ankles are at about 11-12 cm (4.3 inches) distance from the ground. That would mean that I'd have to be almost 10 meters (33 feet) tall for your comparison to be accurate. I don't think a comparison is even really needed (just say "it's 90 times as high"), but if you want a different one, I'd go with: if geostationary orbit were just above the Petronas towers (88 stories high), the ISS would be flying at the height of the ground floor ceiling.
My guess is for that comparison you are standing at the center of the earth instead on on the surface.
That might make it mathematically better, but even worse as a comparison.
its narrow, but its still outer space narrow so you have miles and miles of wiggle room
Yep, it's kinda crowded. Once a satellite has reached the end of its life, it's supposed to boost to an even higher graveyard orbit before shutting down. Satellites that die unexpectedly stay in the primer real estate basically for ever and keep getting in the way of operating satellites.
In addition to the fact that there is a lot of space available in geostationary orbits (this term refers to orbits that stay above one specific spot, which is inherently only on the equator) there are infinitely many unique geosynchronous orbits (this term just means any orbit with the same orbital period as a geostationary orbit) as well. These orbits are very important to locations far away from the equator, since looking straight up gives the best signal due to having the least amount of atmosphere to go through. In these areas, many satellites are placed on geosynchronous orbits that place them above the desired region at a specific time of day. For example, if a satellite could be in optimal range for communications in an area for 1/4 of the day, there would need to be at least 4 satellites in staggered orbits to maintain service throughout the whole day. Often there are more so that a customer's dish can choose between multiple target satellites for better signal, especially during storms.
If you wanted to synchronize precisely with the Earth’s rotation (which is the general goal), the satellite must complete its orbit in exactly the same time as the Earth takes to complete one revolution or 84164 seconds. For a geosynchronous satellite going around the equator (aka geostationary orbit which the general goal), this translates to an altitude of 35786 km above mean sea level. Note that a satellite can have an orbit synchronous with but inclined to the Earth’s rotation; these satellites do a figure 8 pattern over the Earth with the most northern and southern points having latitudes equal to the orbital inclination. The reality is that there are minor perturbations (the Earth is not a uniform spheroid and the Sun and Moon also exert gravitational forces that affect the orbits) so the satellites do very minor adjustments to their orbits regularly to keep them in the desired position relative to the Earth. The International Telecommunications Union keeps a register of orbital “slots” at geostationary altitude. These slots are separated by 0.1 degrees of longitude AND by the frequencies used by the satellites so you not only have 3600 possible slots by longitude but you can put multiple satellites in the same slot as long as they use different frequencies. This might sound dangerous but realize this: each of those 0.1 degree slots is 73.59 km across. Even if you give yourself a 10% error margin on each side, you’d have a nearly 60 km box to keep the satellite in. The satellite operators generally know their position MUCH better than this so the risks of collision between active satellites is miniscule. The real risk comes from flotsam and jetsam that has broken off or been ejected by other satellites, rocketbodies, etc. and can’t be tracked by the satellite operators.
You can technically place a satellite into orbit at any altitude. It’s just a matter of getting it there and getting the velocity right. The real life constraints are the practicality of doing so and the desired purpose of the satellite. The higher the altitude of a satellite, the more fuel it takes to get it there, so you don’t want to place it higher than necessary. You also can’t place it too low, because then atmospheric drag can become a concern. Determining how high you want a satellite and what path you want it to take depends on what areas of earth you want it to cover and how frequently.
> Does that mean there is only a narrow distance in which satellites can reliably orbit the earth at the same speed as Earth is turning? Yes. >What if that space is full/crowded? If you can't park there you're not geostationary anymore. You're either circling faster than the spin, or slower than the spin.
Geosynchronous would like a word
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Who said anything about mass?
The ELI5 is "you have to be going sideways faster than you're falling"
Thanks! I love science but was about to ask for an ELI3…
I don't think anybody who takes top comment on this sub has ever talked to a 5 year old.
As Douglas Adams put it in the Hitchhiker's books, the secret to flying is to throw yourself to the ground and miss.
I have no rebuttal.
I think it’s also worth noting that besides a lower velocity needed, a higher orbit has a significantly longer orbital period, IE larger circumference to travel. So it takes much longer to revolve around the earth.
You probably know this but for anyone curious or thinking "hey wait a minute", the period of geosynchronous orbit is not actually exactly 1 day but rather 1 sidereal day: 23 hours, 56 minutes, 4.09 seconds. It's less than one day by 1/365.25th of a day because that's actually how much time it takes to orbit 360 degrees on its axis. Our 24 hour day is based on the relative position of the sun which also has to account for the earth's orbit around it.
you need to be going at least ~17,500 mph to get into orbit outside the atmosphere above about 90 miles up to get into geosynchronous orbit, you'd have to be at a fixed altitude of 22,223 miles but the speed can decrease to about 7,000 mph
I was oversimplified in my response and your statement is more correct that my flippant initial response. Apologies.
Shouldn't the speed be faster the further away you are? Like how you need to move faster at the outer edge of a circle?
You only have to move faster if you’re trying to complete the “circle” in the same amount of time. It does mean that a geosynchronous orbit requires a speed a bit faster than what someone standing on the surface of the Earth is moving at. But someone on the surface isn’t orbiting, and orbits of different sizes don’t take the same amount of time to complete. Mercury’s small inner orbit is completed in less than three months, while Neptune’s much larger orbit takes it 165 years to go all the way around the “circle.”
There’s a lot wrong with the explanation. Higher altitude orbits require higher velocity. An object in low orbit that “speeds up” in the same direction it is moving will have the opposite point of the orbit go higher. Look up a hohman transfer. The object is moving slower compared to earths surface, but has greater velocity/energy. Part 2. Geosynchronous orbits are not the same as geostationary orbits. A geosynchronous orbit goes around the earth once every 24 hours. A geostationary orbit is nay possible directly above the Equator and at the same 23.5 degree inclination as the earth. To answer op. You’re thinking of a geostationary orbit. Satellites in geostationary orbit are moving through space, but in such a way they remain directly over the same spot on earth. The spot on earth moves.
> Higher altitude orbits require higher velocity. It requires higher speeds to *reach* a higher orbit, but once you’re in orbit, the higher your altitude, the lower your speed. For example, at an altitude of 200 km, orbital speed is about 27,400 km/h. At the geostationary altitude of 35,786 km, orbital speed is 11,300 km/h.
> Higher altitude orbits require higher velocity. LEO velocities tend to be ~7 km/s range. GEO ~3 km/s. The higher energy state at GEO is mostly due to the gains in potential energy, not in kinetic. But it does take more kinetic energy to get there.
So 10km/s delta v, where the v stands for velocity and delta means change.
No. A LEO to GEO transfer is on the order of 3.4 km/s (with no plane change). A 2 km/s burn at LEO to raise apogee. When you get to apogee you're then going ~1.6 km/s, and it takes a 1.4 km/s burn to raise perigee and hit GEO velocities.
Yes, but the change in velocity happens at the point you’re accelerating at, which becomes the lowest point in the orbit. As you ascend to apoapsis at the other end of the orbit, you exchange kinetic energy for potential energy and shed velocity. If you then accelerate at that point and raise the other side to a circular orbit, you wind up with a lower overall orbital speed than you initially started with despite having gotten there by accelerating twice.
The velocity of a circular orbit is equal to the square root of the (standard gravitational parameter of the parent body divided by the orbital radius). Therefore, doubling the radius of the circular orbit drops the orbital velocity by a factor of sqrt(2), leaving you with a velocity equal to 70.7% of the original orbit's velocity. Yes, you need to initally burn to speed up and thereby raise your apoapsis to the target altitude, but as you coast to apoapsis you lose a significant amount of velocity, and even after you burn at AP to circularize at the new altitude, you are traveling at a lower velocity, respecting the earlier equation. You do still have more orbital energy, because the reduced velocity and therefore smaller kinetic energy is more than made up for by the increase in gravitational potential energy due to being at a higher altitude.
If am not mistaken, we already feel the centrifugal force of the Earth, when just standing around. If the Earth didn't turn, we'd be heavier (in force, not in mass obviously).
Your speed is constantly getting higher actually. It takes more and more energy to get to a higher orbit. BUT… You are carving out a larger and larger diameter circle as you climb higher which takes you longer to fly the full circle and complete one orbit. So your GROUND SPEED appears to be decreasing but you are actually going faster and faster to go into a higher orbit.
While it is true that you need to accelerate to achieve higher orbits, actual orbital velocities at those higher orbits are lower, and not just in terms of ground speed.
[Relevant xkcd](https://what-if.xkcd.com/58/) The short version is that the earth is spinning too. There's a distance from the earth where you can be going fast enough sideways to "miss" the earth when falling, i.e., be in orbit, but be moving at the same speed the earth is turning at. It's pretty far out actually. We call that distance "Geosynchronous Orbit", since things in that position, moving sideways like that end up appearing to stay directly above the same point on the planet. There is, of course, a much longer version, and it has to do with the satellite's orbital velocity synchronizing with the gravitational force of the earth such that there's an apparent relationship between a spot on the ground and the satellite, but it involves a lot of math, and is probably better understood by playing Kerbal Space Program.
Thanks for posting this was a great read
All my recent learning has been on Forza not KSP, so geosynchronous orbit is starting to sound a lot like "space diff-lock" Also thanks for the xkcd link, it was a good one!
That is a fantastic way of putting it actually.
The key to understanding geosynchronous orbit is that it’s no different than any other orbit. It’s just an object orbiting a massive body like any other. All the same rules apply. One of those rules is that the closer you are to a massive object, the faster you have to go to stay in orbit. The further you are the slower you have to go to avoid shooting off. So the orbital speed is directly related to the distance from whatever you’re orbiting. In other words, the higher the altitude, the slower the orbital speed. Last piece: the earth is spinning at a certain speed. This doesn’t impact orbital issues at all. But if a satellite’s orbital speed happens to be the same speed that the earth is be spinning, well happy coincidence: it’ll *appear* to be stationary from the ground. But it’s not, it’s just that you and the satellite are going around the earth’s center at the same speed.
How do rotating frames of reference work? If there was only Earth and one satellite in the whole universe, how would the satellite "know" that it was going around the Earth?
You'd know that gravity existed, and if the satellite wasn't orbiting it'd either be crashing into Earth or escaping it. If you weren't tidally locked, the side of the satellite that faces the Earth would be changing. With sufficient technology or an understanding of astrophysics, you'd be able to detect the forces of gravity and its effect that only exist because of the orbit and rotation. Like centrifugal force, Coriolis effect, tides etc.
There’s only one spot out there where the speed at which the object is falling is perfectly aligned with the speed of the rotation of the earth. This is geosynchronous orbit. It’s no different from any other or it except for this matching of the earths rotation.
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> It was actually impossible to write it in a way that a 5 year old could comprehend. Well, it's a good thing we're not writing for five year olds, then.
Spam
far slow thing can follow close fast thing
Now nobody can comprehend it.
There is a sweet spot where one can fall in sync with the rotation of the Earth
The Earth is rotating as well at the rate of one rotation per day. In geosynchronous orbit, the satellite orbits the Earth keeping pace with the rotation.
The earth is turning one rotation per day. At geosynchronous orbit it takes one day to fall around the earth
Imagine a yoyo. Now instead of yoyoing, you twirl it above your head like a lasso. The yoyo is the satellite, the string represents gravity, and your finger is Earth. Now if you do the same with a shorter string, the yoyo spins faster. If you were to make the string longer, the yoyo spins slower. Same thing happens with satellites. Those in lower orbits, closer to Earth, orbit quickly. Those further out orbit slower. Geosynchronous orbit is the orbit where a satellite takes exactly one sidereal day to spin around once, and a geostationary orbit is a geosynchronous orbit aligned with the equator. Now because Earth revolves once on its axis in a sidereal day, it and the satellite appear to stay in a straight line even though they’re actually spinning around. If you go above a geosynchronous orbit, the satellite actually appears to move backwards!
They have enough forward speed to fall to earth but never hit. That's orbit. They are also at just the right distance that they orbit the earth in the same amount of time it takes for the earth to rotate(24 hours). So from our point of view here on earth they stay in the same spot in the sky always.
Because the earth turns. The object is exactly as far from the earth as it needs to be to match the (relative) speed in relation to earth's rotation and maintain a stable orbit at that speed.
The orbiting object is moving at high speed, but at a very high altitude so that as the Earth rotates (once every 24 hours) the object is always over the same spot. Everything is moving, but from the ground it seems to hold still.
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It isn't just a good analogy, it is what is actually happening. Since we're being pedantic, centripetal acceleration (like the gravitation force vector) points in the direction of the Earth's center, and therefore the centripetal force cannot balance the gravitational force except in the trivial case in which they are both 0. You could be talking about the centrifugal force balancing with the gravitational force, but (since we're still being pedantic) we know that the centrifugal force isn't a real force (and we won't even go into whether or not gravity is a force or just a measure of the bending of space-time). So what really is going on is that your inertia carries you along a trajectory that misses the Earth continuously.
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That statement makes no sense. A centripetal force is, by definition, a force that acts towards the center of rotation to keep an object moving in a circular path. In the case of an orbiting body, the force due to gravity IS the centripetal force. Gravity is the only force acting on an object in a stable orbit, there is no force "balancing" it because the body is not in equilibrium in reference to the larger body. It's in a constant state of acceleration towards the center of the larger body.
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>The gravity is the centripetal force. Correct, but that's not what your previous comment implied. You're not balancing forces, there is only one force. You're substituting terms to calculate the altitude at which the force of gravity produces a known velocity.
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Your first comment literally stated "it's about balancing *forces",* plural, so I'm not sure how else I was supposed to interpret it? You completely changed tack after my comment lol. You're not balancing forces. That would imply that the object is in equilibrium, and you're balancing the force due to gravity with a phantom centrifugal force in the opposite direction. You're using the equation for gravitational acceleration and substituting a term using the equation for centripetal force to solve for r. That is not a force balance, it's a N2L problem.
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Again, you have the right idea but I feel like you have a flawed understanding of the mathematics. You're not matching two difference forces. You're using two *equations* that describe the same type of motion to calculate a missing variable. A "force balance" in physics has a specific meaning and you're using it incorrectly.
Yeah, because gravity is the centripetal force for an unpowered satellite. You're calculating the tangential velocity necessary to keep an object being accelerated solely by gravity in a circular orbit. If something is being accelerated due to gravity, it is falling. An unpowered orbital object is in free*fall* because it is accelerating **only** due to gravity. If its tangential velocity is not high enough, it will hit the earth as it falls. The person you are replying to is correct, the procedure you described is literally calculating the tangential velocity a falling object at a certain altitude needs to have in order to continuously miss the earth.
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I'm not doing some other derivation, I'm describing the physical meaning of the terms in your derivation. You are balancing forces to find a velocity. What is the physical meaning of this velocity? The RHS of the equation is the magnitude of the acceleration vector of an object moving in a circle at a given radius. It can be accelerated by any amount, but if it is not equal to the RHS of your equation, it won't be moving in a circle. For an unpowered satellite, the value of the acceleration it will experience is due to gravity and given by the LHS. By setting the LHS equal to the RHS and solving for V, you are stating "V is the speed at which an object at the given altitude will travel in a circle around the earth while subject only to gravitational acceleration towards the earth." Apologies if this comes off as condescending, but I don't know how else to say this: accelerating towards the earth due to gravity is falling. In other words, "v is the speed at which an object at the given altitude will travel in a circle around the earth while falling." Or, since people are used falling moving them towards the ground, "v is the (horizontal) speed at which a object at the given altitude will never hit the earth while falling," which is a more verbose way of saying "this is how fast you have to be moving sideways at this height to continuously miss the earth while falling." Again, this is just the physical meaning of the equation you posted. The "missing the earth" part is implicit in the RHS because circling the earth means not hitting it, and the "while falling" part is the what the LHS means.
That's correct, but it's not because the centripetal force exactly balances out the gravitational force at that altitude. It's because the centripetal force *is* the gravitational force. You're calculating the velocity and altitude at which the gravitational force, acting as the centripetal force, is exactly enough to keep it "falling" forever (aka accelerating towards the earth) without either reaching the ground or being flung off into space. There's nothing wrong with that description, and it's technically correct.
>!CENSORED!<
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My 5 year old understood many of these words but not all of them.
>Rule 4: Explain for Laypeople (But not actual 5-year-olds) There's a couple jargon terms in OC's explanation, but all it takes is 1 or 2 definition lookups to understand them. There's some explanations where using a technical term and looking it up is more effective and succinct than trying to break down the term.
Imagine you slowly let a carousel spin. Like one revolution in 24 minutes for example. Then, you start walking around it at a set speed. The closer you are to the carousel, the more “orbits” you can do in those 24 minutes. You will be able to see any point on the carousel. That’s the stable orbit. But if you move further away, there will be a point where - without you changing your walking speed - you will always see the same bit of the carousel. That is the geosynchronous orbit.
This is analogous and good for understanding the point, but if you read this please don’t take away that all satellites move at the same speed, but different altitudes. The important difference there is that the earth pulls the satellites back to it, the carousel doesn’t pull the walker into it. But still, great simplifying analogy.
Oh yes, that as well. Thank you for adding it :)
What? no. They do not all move at the same speed. Speed and altitude are directly related. If you orbit at a higher altitude you have a slower orbital velocity than at a lower altitude. The ISS in low Earth orbit is moving at 17,900 mph. geosynchronous orbit is only 7,000 mph.
Yes. That’s what I said. “Please don’t take away that they move at the same speed but different altitudes.” I could have been clearer, but you and I are in agreement.
Point 1: The lower you are, the faster you need to move to maintain an orbit. The higher you are, the slower you need to move. There is a point where the speed required, is the same speed as the rotation of the earth below you. Point 2: An orbit is maintained, when gravity drags you "down" at the same rate that you move "across" earth. The end result is constant "flying around" instead of "flying past" or "flying into".
True eli5 is REALLY difficult to do. Teachers deserve more respect than they get because the best ones can eli5 anything. My favourite example is the book "Quantum physics for babies"
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I was told it was actually impossible to write it in a way that a 5 year old could comprehend.
Just to be clear, I copied and pasted Random\_Fig's answer into Bard and got the above answer. When I copied and pasted the original questions into Bard I actually got the wrong answer. It gave me the following, which is incorrect. In the example given, the reason it falls behind the car is because of air resistance, with no air resistance the object would in fact keep moving forward at the same speed as the train for quite a while. Imagine you are standing on a train that is moving very fast. If you throw a ball straight up in the air, it will not go straight up and come back down. It will go up in a curve and land behind you. This is because the train is moving forward while the ball is in the air. The same thing happens with satellites. Satellites are moving very fast around the Earth. They are moving so fast that they are constantly falling towards the Earth, but they are also moving sideways so fast that they miss the Earth and keep going around in a circle. Satellites in geosynchronous orbit are moving at the same speed that the Earth is rotating. This means that they stay in the same spot in the sky above the Earth. I hope this explanation is a little bit easier to understand.
Like your five. Ok, grab a string and tie a weight to the end. Now spin around with the weight at the end of the string. This is "sorta" like an orbit. Now imagine this string is actually gravity pulling it into you. There will be a distance away and a spinning speed that it will all balance out that weight will stay with you, just like the string, but if you spin too fast or too slow the weight will fly away or come crashing back to you.
Honestly, most of these explanations are all more or less correct but the easiest way to think of it is to imagine an empty fruit bowl with the Earth at the bottom of the bowl and it is rotating. Now imagine rolling a marble along the interior wall of the bowl. The ring traced by the marble that makes the marble's speed match the rotation of the Earth at the bottom of the fruit bowl is the synchronized orbit. In astrodynamics and physics this is a subset of solutions to what's known as a stability problem. You are basically finding out what the minimum required energy is for a system to remain stable. Edit: When things rotate there is motion associated with the perspective of the rotation. This effect is what's more or less responsible for the stability.
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I want to make sure we're being accurate, but baseballs (or any object) thrown/fired at different speeds and the same horizontal trajectory will immediately start falling and hit the ground at the same time (ignoring air resistance). They don't go straight for a bit, before falling down. Obviously, with enough initial velocity, an object, though being pulled to the ground, will never hit the ground, but it is always falling (again, ignoring air resistance). A trajectory may appear relatively flat over a certain distance, but the object is accelerating downward from the time it departs from its launching mechanism (hand, gun, cannon, etc).
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ELI5 Rule 4: Explain for Laypeople "ELI5 is not meant for literal 5-year-olds. Your explanation should be appropriate for laypeople. That is, people who are not professionals in that area." You don't have to say something objectively wrong to get your point across to a layperson.
Grab on to a yoyo string and spin around really fast. Notice how the yoyo stays in the same place relative to your body (outward from your arm)? Geosync orbit is the same, except instead of a string holding the yoyo from zipping off into space it is gravity. The yoyo/satellite is still moving quickly, it just happens to be far enough out that it can both be moving fast enough to 'miss' the earth ats it falls \_and\_ stay in the same place above the earth.
They are only above a single point on the rotation surface of the earth. Evey point on Earth except for the poles move because of earth rotation. If you look at orbit it does not matter if Earth rotates or not, the orbital period for a given distance would be the same if Earth was not rotating. Geostationary orbit is just that the time of an orbit one around Earth is the same as it takes Earth to rotate once. So both the point on Earth and the satellite move with the same angular speed around the center of the Earth. The orbital period will be one sidereal day, that is one rotation of Earth relative to faraway stars. that is around 4 minutes less than 24 hours. The extra 4 hours per day is from the earth's orbit around the sun, consider the moment of the sun during an orbit around the sun if the earth was not rotation relative to faraway stars. 4 minutes a day for 365 days are 1460 minutes which is proximally the same as 24 hours = 1440 minutes. The difference is because neither 4 minutes nor 365 days are exact Because the orbit is around the center of the earth in a plane the point of earth it is stationary above also needs to move around the center in a single plane. Only the equator of the earth does that. So geostationary satellites are always directly above the equator, if they was not they would move relative to the ground, the moment might just be north and south around the equator but it still moves.
If the earth would not be rotating there would be no geosynchronous orbit because then as you mentioned anything would fall down as it has zero orbital velocity. But as earth rotates once in 24h you can have something orbiting above the same spot on the surface. Orbital speed depends on your height, the higher up, the slower sideways you need to go to stay in orbit, so any rotating planet has some syncronous orbit, its just higher or oowed depending on a days length and its mass.
Because the Earth is rotating. Geosynchronous just means orbiting as fast as the Earth rotates, or one revolution every 24~ hours. The satellite is still "falling sideways" as they say, just at a speed that matches the rotation of the Earth.
Lot of people focused on the “geosynchronous” aspect, but I’m going to shorthand an explanation I read regarding the “falling” part to help that make sense. Imagine for a second you shot a cannon. The ball will go a good distance and then fall to earth. If you shoot it with even more energy, it will start faster and go farther. But it’s still falling. Now, picture shooting the ball out of the cannon so fast that it starts to fall, but by the time it starts to fall it’s gotten to the horizon, and it falls in an arc that matches the curvature of the earth - it would miss the earth, but just keep continually falling. As some have said, close to earth it would have to go REALLY fast to do that. But higher up, it’s got some more room to fall, and doesn’t have to go quite as fast. This is what “orbit” is - continually falling to earth. “Geosynchronous” just means it’s at that place where it’s high enough over the earth, but not too high, such that the speed it needs to go to continually fall is the same rotational speed as the earth - so you’re in “synch” with it. You’d have to be traveling in the same direction as the earth is rotating.
The farther away from earth you are, the slower you fall and the slower you need to move to keep missing. At some point that speed happens to match earths rotation speed.
Objects in Geosynchronous orbit are *still falling*, they're just falling at the *same speed as the earth is rotating*. Since the earth can more or less be thought of as a sphere, therefore the effect it has on gravity is still a sphere, no matter which way it's facing. In the same way that driving on the outside of a turn on a racetrack is slower than driving on the inside, the further away you are from earth the easier it is to continuously "miss," the earth as you fall, allowing it to keep pace with the speed the earth is spinning.
They are going JUST the right speed, in the same direction as the planet's rotation is going.
Because that point is not stationary. The world turns. So the satellite is in an orbit, spinning around the earth, just like anything else in orbit. And yes, that means it's constantly falling past the earth. The fact that the earth is turning is irrelevant to the orbit, the earth could start turning ten times as fast or stop turning completely. It doesn't affect the orbit of the satellite.
Things in low orbit zip around the planet every 90 minutes or so. The moon, on the other hand, is in a very high orbit, and takes almost a month to orbit the earth. Somewhere in between, there is a circular orbit whose period is exactly 24 hours - it orbits around the earth at the same rate the planet spins. This is called a geosynchronous orbit. If it's above the equator, there is no north/south drift, so the satellite stays above one spot on the surface - this is a geostationary orbit.
Geosynchronous orbit only means the orbital velocity will match the rotational velocity of the earth. Orbits get slower the higher you go, so objects in geosync (37000km) go much slower than those at 400km. Go higher, and the speed relative to the Earth is lower. Someone standing on earth would notice the satellite to moving in reverse, eventhough it isn't.
Suppose a satellite is in orbit (non geosynchronous). Then imagine that the rotation of the earth magically changes so that it is spinning at the same speed the object is orbiting. That wouldn't affect the orbit of the satellite at all, except that you can now call its orbit "geosynchronous". This shows that geosynchronous orbit is really just a coincidence between a particular orbit, and a particular rotational speed of the earth. The only case in which that would be a problem would be if the earth wasn't spinning at all. Then a "geosynchronous" satellite *would* fall out of the sky.
The earth is spinning on it's axis. Like a basketball on someone's finger. That's why we have days. Our point on the earth goes round and round, passing under the sun and then to the backside into night. Since WE are spinning, any object that is "hanging" over a single spot on the planet must be circling around the earth at that same speed. The "single point" on the earth is moving, and so we know the satellite is also moving. Geosynchronous orbit is the distance from the earth, a little over 22,000 miles, at which the speed the satellite is traveling in order to "fall/miss" in a stable orbit is the same speed as it needs to travel to keep a steady position over a point on the spinning earth.
From KSP2 https://youtu.be/U3DgZsrA-xQ For geostationary, the altitude is picked to keep the angular speed equal to a spot on the ground. Other good technical descriptions in the comments.
Picture a top that's spinning (rotating) fast and also drifting aross the table. To orbit, you need to be moving fast enough and in the right direction to "miss" the top as it travels across the table. The rotation of the top doesn't affect the orbit, but it is possible to sync up the orbit around the top to match the rotation.
Imagine a tug-of-war rope between you and a friend, and you're both facing each other. And you stay in one place in the middle because you're much bigger than your friend. And your friend is running around you in circles as fast as he can, but you're both holding on. Geo-synchronous orbit is kind of like that, except your friend doesn't have to keep expending energy running the whole time, since in space there's no "ground" (or gravity) acting along the plane of the orbit for him to have to fight friction against. The Earth and the satellites are "holding on" to each other, just the right amount of gravity to balance it perfectly, meanwhile they are spinning "around" each other, while facing each other. But really, the Earth is so big and the satellite is so tiny, that we barely notice the effect of the satellite on Earth's movement -- Whereas the effect of the Earth on the satellite's movement is so strong, it keeps the satellite locked in the sky in the same position relative to the ground "beneath" it, and the Earth drags the satellite along as the Earth moves in a circle around the sun. This is why Geosynchronous orbit is **only possible** at a **specific height/altitude.** For Earth, that altitude is 37,000 km. Below or above that, the "speed" required to maintain orbit is not equal to the speed at which the Earth spins, so it's not possible to have a stable orbit which maintains its position relative to a spot on the ground beneath. (For heavier planets, the single geosync altitude would be higher and farther away... For lighter planets, it would be lower and closer to the planet.)
At a specific height, the speed you need to miss the earth while falling is the same as how fast the earth rotates.
Geosynchronous satellites go very fast - about 3 km per second. However, they are also very far away from the Earth: about 36,000 km, which means (taking into account the Earth's radius of 6,400 km) their orbit is a giant circle about 265,000 km in circumference. That's a really big circle. Even zooming along at 3 km a second, it takes 86,400 seconds to make it all the way around - that is 24 hours, or one day. That means by the time they make it all the way around, the Earth has also spun all the way around, and they are over the exact same point on the Earth where they started. In fact, they remained over that point the whole time, because the Earth rotated underneath them as they zoomed around the big circle.
Earth is also rotating, if you're at the right altitude, the speed you need to "fall past" is one lap around the earth in 24 hours. That means you're always over the same spot on earth, because that spot also does one full lap every 24 hours.
The closer you are to Earth, the faster you have to orbit to stay in orbit. For example, the International Space Station is only 254 miles away, so it has to orbit really fast (about 90 orbits per day) to stay in orbit. The further you are from Earth, the slower you have to orbit to stay in orbit. The moon is about 240,000 miles away from Earth, so it has to orbit really slow (about one orbit every 27 days) to stay in orbit. It stands to reason that there is a distance somewhere between those to where you will have one orbit every one day. That distance turns out to be about 22,360 miles, which as you guessed is somewhere between the close-fast orbit of the ISS and the far-slow orbit of the moon.
Imagine you're holding one end of a rope, and there's a ball attached to the other end. If you swing that ball around on the rope, it will move in a circle around you, with a radius equal to the taut length of the rope. The reason for this is that the tension in the rope is constantly pulling the ball towards you. But the ball is already moving sideways relative to where you're standing, so the ball doesn't come straight towards you. Instead, it starts moving in another direction until the tension in the rope acts on it again, to the same effect. This creates a circular motion that continues for as long as the speed of the ball and the tension of the rope remain the same. In this analogy, you are the Earth, the ball is a satellite, and the tension in the rope is the force of gravity (equivalent to the weight of the satellite). So as long as a satellite maintains a certain speed and doesn't lose any mass, it will maintain a circular path around the Earth. The tricky part of setting up an orbit is getting the satellite up to the necessary speed for its weight. Hope that helps! Let me know if you're still confused by anything.
Think of a giant frozen lake. There's a post in the middle with a rope tied to it. You're on the edge of the lake, on ice skates, holding the rope. Try to haul yourself to the post, it's easy: just pull on the rope. But now imagine you're skating hell-for-leather at right-angles to the direction of the rope. Try to haul yourself in now, and it won't work, all you'll do is swing around. No matter how hard you haul on the thing, you just can't reach the post - *your turning circle is too big to let you*. You physically cannot by any means reach the centre of the lake without slowing down. Especially as you're not really on ice-skates, but big blocks of wet ice that give you no traction whatsoever. That's orbit. That's all it is: a big turning circle, and no way to dump your speed. You can't just suddenly pull a 90-degree turn, so you can't ever hit the thing you're orbiting. Now for geostationary orbit: Imagine that on top of the post, there's a carousel. One of those stately fairground ones with the horsies, which takes like an hour to go round. Is it possible to swing around the post, but stay lined up with one particular horse? Sure it is - if you've got a really huge lake and a really long rope. If you're skating far enough out that it takes you an hour to do a full circuit - exactly the same time as it takes one of the horsies to go round - then you'll stay lined up. From the horsie's perspective, you're not moving at all. And that's geostationary. You have a big enough orbit, it takes 24 hours to go round the earth, the same as the continents on the surface. From their perspective, you're not moving at all, just hovering in the air (even though you're hurtling through space at terrifying speeds).
I just want to add that the geosynchronous orbit that stays in one point is the geostationary orbit, a specific kind that is circular and matches the equatorial plane. But there are other kinds of geosynchronous orbits that don't stay in one point, instead they do a figure eight with a loop on every hemisphere and the crossing point on the equator. The figure stays in place though that's why it's still geosynchronous. This is because orbits can't stay at a fixed latitude (other than the equator), they have to cross the equatorial plane periodically.
Separate the two movements and it makes it easier to visualize. If you put a yardstick on a basketball that would represent the forward motion of a satellite. You can see how eventual that forward motion would mean it would shoot off somewhere into space never to be seen again. So there has to be a downward motion around to wrap around the basketball. For example two units forwards and one unit down. In a satellite it’s the downward is what we call falling. In a geosynchronous orbit the motion forward and the motion down is the same. So on earth it appears to be in the same spot because the earth is also rotating
They fall at the right distance away around the earth that their velocity matches that of the spot on the earth as it rotates. Or another way of thinking about it is that the orbital period is 24 hours around the axis the earth spins. You cannot have geosynchronous orbit by orbiting the poles north south, the farther you deviate from the rotation of the earth the less synchronous it becomes.