Because a satellite directly over you is easier to contact than one over the horizon with way more atmospheric distortion to deal with. So best to send up a lot of them so that you can get a good signal from anywhere.
And the old ones stay up because they weren't designed to come back down. You'd need to sent up another space thingy with a big lasso or grabby arms and enough fuel to make the old satellite fall down. Not to mention you'd probably need to bring up extra heat shielding to protect the not-for-reentry satellite from the heat of reentry. All together, too expensive to do it constantly.
Very, very minor quibble, most satellites do come down eventually. There is atmospheric drag out to a rather surprising distance (well past the ISS apoapsis), even if it is very small. Very small things add up eventually, so they do and will come down eventually if not constantly boosted higher.
Eventually can be decades or centuries for the really high sub-GEO stuff, admittedly.
Some things are boosted into graveyard orbits where they won't 'come down', but when practical it's considered better to de-orbit/burn up a satellite at the end of it's life then to leave it unpowdered and dead in a useful orbit.
Those are almost all GEO orbiting satts, it takes a lot of propellant to get up into those graveyard orbits.
And absolutely, it is best practice to intentionally de-orbit, there's even a patch of the Indian Ocean designated as the "doom zone" for owners who are conscientious to drop their birds into when they reach end of life.
Not everybody put the spare fuel into their birds for that, or the fuel just evaporated away before they could de-orbit. And those ones make for some interesting space encounters on occasion.
Do you have the numbers to back that up? I’m not saying that I do, but just the initial thought of boosting to a higher orbit seems like it should require a lot more fuel.
Entering a higher, stable orbit would take at least two boosts, one to get to that higher energy state and another to get a circular orbit. A de-orbiting would only require one properly timed boost and could be in just about any direction.
That's a factor of 2. We're talking about much bigger factors here. US government minimum for "graveyard orbit" is 300km above geostationary orbit. Let's be generous and aim for 1000km Geostationary orbit is at 35,786km. The boost burn to lift your apogee from 35,786km to 36,786km is 17.96m/s, and the circularisation burn at the other end is 17.86m/s (these are close because the two orbits are close in the grand scheme of things) for a total of 35.82m/s. Dropping your apogee to, say, 100km (might be a bit high for a reliable reentry from there, not sure, but whatever) from 35,786km is 1,487.83m/s. It's not even on the same order of magnitude.
"A de-orbiting would only require one properly timed boost and could be in just about any direction."
The de-orbit burn has to be in the opposite direction from flight. The goal is to slow down the satellite. You don't need to push it towards Earth, gravity will do that for you.
Also, we'd rather satellites hit the middle of the uninhabited bit of the Pacific ocean. We don't want them to come down on land and leave litter, someone would have to pay to pick up that litter. We don't want them to come down on someone and hurt or kill them. We don't want them to come down in a populated area and hit multiple people. We don't want them to come down on Washington or Moscow, be mistaken for an ICBM, and trigger a nuclear war.
That's probably the biggest reason the Space Force puts so much effort into publicizing reentry predictions aside from the occasional rocket body that's large enough to reenter and do major damage on the ground.
I’m going to disagree on the “opposite direction from flight”. Slowing it down is one way to de-orbit, but increasing the speed to generate an elliptical orbit can also result in de-orbiting.
Increasing speed will only move the opposite side of your orbit higher. More elliptical, but still higher. It is not possible to lower your orbit at any point by increasing speed. You need to either slow down, or burn directly towards earth, but that requires much much more fuel.
Trying to imagine a situation in Kerbal where I'd burn radially to shift my orbit in order to get some interaction with the atmosphere (argument of perigee?), I feel like that'd be the only circumstance that I can think of besides negative in-track thrust to achieve a reentry.
bluesam has an excellent response, but I wanted to give some additional intuition:
in space, ascending from a circular 500km to 1000km orbit takes (roughly) the same amount of fuel as going from 1000 to 500. The difference between your ultimate velocities is the same, and the burn to get from altitude A to B is similar as B to A.
It is true that once your orbit touches the thick parts of the atmosphere, you get a bunch of "free" deceleration. However, the atmosphere is tightly held around Earth's surface: geosynchronous orbit is almost 36000 km, whereas the atmosphere is mostly done with at 500. The graveyard orbit is just a few hundred km further out than geosynchronous - going the other way wouldn't get you anywhere near Earth's atmosphere, even if you spent the circularization burn too.
IIRC there is an international agreement now that what goes up, must come down at the end of its lifespan. Of course, there is no enforcement of this, more of a nice to have but it costs money to de-orbit a satellite. Easier to say sorry and leave it up there. Still, some are now being made to come back down when they are no longer being used.
Oh I'm bummed to find out that I can't attach an image of the chart I made here. I just pulled the orbit data for LAGEOS-1 and you seem to be right, since it was launched in 1976 its perigee hasn't even remotely decayed from 5840km.
When you consider the design life expectency and the insane cadence that they're being launched, you have to imagine that they'll eventually be coming down at the same rate that they were sent up. Pretty cool, and a pain in the ass to do reentry predictions for. But SpaceX is very cooperative.
For the Geo synchronous and other satellites that are higher than low Earth orbit they are supposed to be pushed higher into a graveyard orbit that's even higher. This doesn't always happen though because it uses fuel to move it up to the graveyard orbit.
When I played lots of Kerbal, I liked to test launch platforms by orbiting these one-shot de-orbiters. They'd have grabbers, fins and heat shields and a bunch of parachutes (in case whatever I was grabbing needed to survive re-entry). So I'd end up with a dozen of these things in various orbits.
Every time I messed up a fuel calculation on a crewed mission, or forgot to put parachutes or an antennae on a science probe, I'd just find the nearest one, RV and bob's your uncle
To get something out of all the effort. If you're not going to at least use it as scrap or something, why bother bringing it back at all? I'd also rather have a floating pile of junk in space than evaporated particles of metal, rubber, and who knows what else hanging around in our atmosphere.
That makes no sense to do, the scrap value of a satellite is nothing compared to what it would cost to bring it back intact. When dead satellites are deorbited they are just allowed to disintegrate in the atmosphere. The point of deorbiting expired satellites is so they don't become space junk and a hazard to active satellites, not to recover the satellite for scrap.
The ELI5 answer: You don’t have to “go get them”. They fall back down eventually. This is caused from them running into gasses up there slowing them down. Think about running into the wind, but on a really small scale. Eventually they lose enough speed to catch the atmosphere and reenter quickly.
Extra: the satellites that will take a really REALLY long time to fall back down are actually more likely to get sling shotted out into space due to the moon.
Orbital mechanics doesn't care about intention. Leave a spacecraft in a high Earth orbit for long enough and there is a good chance it happens eventually.
There are only a handful of satellites that far out, however.
It happens frequently, that’s how comets and other things get ejected from our solar system all together. There’s an entire theory on how we had a 5th gas giant that got ejected from our solar system due to a close encounter with Jupiter billions of years ago.
If you leave anything in space for millions or billions of years it either comes back or gets thrown out.
I think that’s where people are getting confused. Because I wasn’t speaking of those, I’m speaking of satellites that are pushed up and out past geostationary orbit before they’re decommissioned.
You say that it never happens, but we haven’t been around long enough to see it. Much less had satellites up there long enough.
When I say a really REALLY long time to come down I’m talking about super synchronous orbits where our grave yard orbits are. It will take longer for them to come down than humans have been on this planet currently. Millions of years. With this in mind, i speculate that they won’t fall back to earth normally but will instead get shot out into space by either the earth, moon, or both due to their orbits becoming destabilized from tidal forces, radiation, or even light pressure.
I could be wrong on that speculation, but I don’t think there’s been a proper study on the orbital life of any satellite that reaches out to a million years either. And if there has I’d love to read it.
There don't need to be million year studies to know whether this is possible/likely. This isn't a new medicine or food additive. Astrophysics, at least regarding orbital mechanics with things like satellites, is well understood.
I don't understand it well enough to make a call here and the fact that you think million year studies would be needed strongly suggests you aren't qualified to make the judgment either.
Lmao yeah this is pure physics/orbital mechanics. I'm not educated enough to make the judgment on its possibility but I am informed enough to know that this could be accurately simulated by a computer and wouldn't require a long-term study.
Let me be more clear, there isn’t a study that simulates the orbit of any satellite or debris in that orbit for a million or more years that I have seen. And part of that is because you’re not very right in your assumption that it could be accurately modeled on that time scale, and also because it’s not really useful information to us at this moment.
You cannot accurately model or simulate their precise orbit for millions of years into the future because of the n-body problem, which has been discussed in this very same sub
And also further discussed in association with this very topic in this paper which basically states that the lunar effects were the cause of uncertainty in their findings.
Oof I assumed a satellite was too low mass to be a factor in the three body problem so I didn't consider it at all but according to AI at least a satellite does constitute a significant third body.
My assumption is still that the moon would just cause it to fly off into space but not actually sling it around itself, considering there's so much lateral speed and way more influence from the earth.
My curiosity is going now so I'm probably gonna make an askscience/askphysics post about it. Are you interested in me linking any answers? I'm not trying to be vindictive or anything, even if it turns out your assumption was the correct one I'll still link it. I just want to hear from someone who really knows orbital mechanics in depth, figured you might be interested too.
We do “de-orbit” some satellites by letting atmospheric drag slow them down until they (hopefully) disintegrate in the atmosphere, but that’s only feasible for satellites that orbit at relatively low altitudes where atmospheric drag is significant.
Satellites that are higher up are usually boosted into a “graveyard” orbit that’s only occupied by other similarly cooked satellites.
It’s no small feat to go fetch a satellite and they don’t usually carry enough fuel to de-orbit themselves from higher orbits, so this is the most efficient solution.
De-orbiting satellites don’t always fully disintegrate either, there’s always the risk that a large component survives and crash lands somewhere.
The number of them is because they either die from solar radiation/mechanical breakdown over time and need to be replaced, or a combination of so many entities wanting/needing their own units. Then you get into conversations about coverage… a satellite can only cover so much area efficiently, so you need several to cover more of the planet.
The reason why they get left is because it’s a MASSIVE amount of space… so even with the junk still there, and it being a looming problem for the future, it’s pretty rare to have a collision at present.
The other part of it is, those satellites are moving at 10’s of thousands of miles per hour or more. It’s not as simple as just letting them fall. You’d have to put the brakes on for it to lose enough speed to fall. And that takes energy/effort/cost.
Over time, they will naturally slow enough to fall on their own, it can just take a long time because there’s very very little resistance there. Near zero. Most of them are small enough that they’ll burn up on reentry and won’t pose a threat to the ground.
Stuff that is big enough to reach the ground usually IS brought down in a controlled manner on purpose to ensure where and when it’s going to come down
Some modern satellites incorporate mechanisms to take them out for orbit when they are no longer in use. Things like sails or boosters which put them on a declining orbit.
Many are also designed to fall out of orbit towards the end of their useful life just through natural orbit changes, for instance many low orbit satellites.
I was wondering if this was a thing, makes sense to install like a deadmans switch on them tied to a single use booster or compressed gas tank or whatever to kick their orbit into decay if they shut down. I guess the problem with that is that you can't control if that deadmans switch device puts it into the path of something else like a rocket launch or other satellites orbit path.
Yeah it's cool. I think they are remotely activated. I.e. activated from earth at the end of the satellite's life, so it goes onto a known path rather than a random one.
There are also junk orbits where you can move your satellites to so it doesn't stay in the way.
The number of interesting orbits is limited which means that there are still concerns about the amount of "junk" in those specific orbits or orbits that you need to go through to get to those interesting orbits.
The dangerous junk pieces are actually tracked, some satellites can be maneuvered to avoid junk if you know there's something coming.
okay, another question. how do we know how much exact force it takes to send a satellite to orbit but not so far it floats away or so low it just comes back down? or do they fly them up therev
Orbital mechanics and laws of motion are well-known and thoroughly understood by those people whose jobs depend on it. The math to get something from the ground into orbit, and stay there, is a solved problem.
I'm not saying the math is necessarily easy - I sure as hell couldn't do it - but it's not a mystery. We have figured it all out, and so when we want to launch something into orbit, we just...do the math.
It doesn't need to be exact. If you accelerate them a bit more than planned, they will be in a slightly higher orbit. If you accelerate them a bit less than planned, they will be in a slightly lower orbit. Almost all spacecraft have their own propulsion to make finer adjustments, too. They only fall down if you have a serious issue with the rocket, and making them fly away needs so much extra propulsion that you don't do that on accident.
Satellites are almost all in LEO, low earth orbit, meaning if left alone long enough they will eventually come back down. Floating away is never a threat because escape velocity is much higher than the orbital speed they achieve. It would be like trying to throw a baseball to your friend and accidentally throwing it several miles away.
Some satellites have little engines on them so they can give themselves a boost to maintain their orbit.
How do we know how much force? well, math... to put it bluntly. Complex math called orbital mechanics.
Longer answer: All of this can be calculated precisely if you know Earth’s gravity (we do) and how far away from earth your object is. We can calculate how fast that object will “fall” towards the center earth, and we make it go fast enough to the side that, instead of coming back down, it makes a circle instead. (And if we know how fast we need it and how much it weights, we can calculate the energy (force x time) needed to get it to that speed.
Orbit is cool and weird and I highly recommend playing the original Kerbal Space Program to really understand it.
It's not like you fire them out of a cannon, where you only apply force once, and hope you got it exactly right. The rockets have a guidance system that continuously updates thrust levels and steering to keep them on course. Calculating where they need to be, and how to get there is fairly simple.
How do you know how hard to throw a ball so it lands right there where you want it to land?
You've got lots of experience with how things fall through the air, so you can estimate how much force to use when throwing to get a ball to land where you want it. You use your arm to apply a sideways force to the ball. Meanwhile, the earth's gravity is applying a downwards force to the ball. Together, these cause the ball to go forwards while also falling down at the same time, until it hits the ground.
Throwing a satellite into orbit works pretty much the same way. We use a rocket to apply a sideways force to the satellite, while the earth's gravity applys a downwards force. Together, these call the satellite to go forwards while also falling down at the same time.
The trick is to throw the satellite forwards so fast that by the time it's fallen down, it's already past the ground. Then it just keeps falling around the earth.
The rocket deploys the satellite at a specific altitude and speed. Satellites then use their onboard engines to move into the exact position they want to be in.
Both the rocket and satellite are keeping track of where they are, so they adjust thrust and direction accordingly.
There are actually factors like atmospheric density (varies due to weather) that affect the launch, so mathematics alone can’t calculate an exact destination. They need to adjust during flight.
Each satellite has a specific purpose, and can only do what it was designed to do. It also can only do so much. If you need more stuff done, you need more satellites to do it. As far as keeping old ones in orbit, it takes energy to take them out of orbit (in a controlled manner), and they may not have the fuel or may not be able to receive instructions to do so safely. Newer satellites are more commonly sent up with plans for being able to be de-orbited if they're in a low enough orbit where that is a good idea.
Because we use satellites for a lot of things. Communicate and imaging being the biggest things.
Lots of people communicate so we have a lot of satellites.
Why do we keep unused ones up there?
Because its hard to bring one back. It requires a bunch of extra fuel and thus extra cost. And most (all?) will naturally slow down eventually and fall back to earth anyway. Might take a few 100 years but it will happen.
Satellites may be divided into three general categories:
Low-earth-orbit satellites need to expend fuel to stay in orbit because of atmospheric drag, and will naturally fall out of orbit due to atmospheric drag when they stop doing so.
Geosynchronous satellites are much further from Earth and have an orbital period of about 23 hours and 56 minutes (the time between consecutive noons is slightly longer than the earth's rotation period because of the planet's motion around the Sun). Realistically speaking, a satellite in a geosynchronous orbit will stay there for more than a century if it doesn't use fuel to go elsewhere and nothing hits it.
Retired geosynchronous satellites are a little higher still. It doesn't take much fuel to move from a geosynchronous orbit to a higher one with a different inclination (moving higher is slightly cheaper than moving lower). Although space is very big, Sir Isaac has decreed that only an infinitesimal fraction of the space around Earth (a circle, with only one degree of freedom) can be usefully inhabited by geosynchronous satellites, so things that aren't geosynchronous satellites are kept outside the circle.
Some low-earth-orbit satellites are higher than others, and will thus last longer, but generally the only reason to expend fuel putting a satellite into an orbit that would be stable for decades would be to either put it into or take it out of a geosynchronous orbit.
To the first question: we've been launching them a long time, but more importantly: recently we've learned ways to launch lots of them at once more cheaply than in the past.
To the second question: So the cheap constellation-style ones are too low, catch enough air to get dragged down, and burn up in the atmosphere after weeks or months.
For higher ones that we leave up there: it actually costs a lot of fuel to make some satellites crash to the ground, especially if it's waaaaay up in geosynchronous orbit (22,300-ish miles). You can fit two entire Earths between those satellites and the ground! So it's much cheaper to instead just push them even higher, so they're drifting around up beyond where they're unlikely to ever touch anything again.
Developing technology, differing needs, and control are all the main reasons why there are so many satellites up in space, and why more are being added.
As for why they tend to stay up there, well for LEO (Low Earth Orbit) satellites they will eventually deorbit by themselves thanks to the extremely tenuous atmosphere up there.
Otherwise, to deorbit, you need enough fuel to slow down, which adds cost and shortens the operational lifespan of the satellite as that fuel could instead have been used for orbital maneuvers to keep the satellite where you want it. Also there are only specific places on Earth that satellites are supposed to be deorbited to. For geostationary satellites it's often more cost effective to push the satellite into a higher graveyard orbit than to get back to Earth.
Most of them are internet satellites at low altitudes, where you need a lot of them to be sure there's always a satellite in range. The lower ones will reenter relatively quickly if they stop getting boosts. You want them low to reduce ping times, and fewer people all trying to use the same satellite at once..
and why do we just keep the ones not in use still up there?
For geostationary satellites, because you'd need a large burn to bring them back down, so instead they're put in a graveyard orbit where they won't interfere with anything when they reach end of useful life(which is usually when they're running out of fuel for stationkeeping).
First off, because they are useful. The more satellites up there, the better transmission quality in terms of latency, strength of signal, etc you have for communication satellites. And the more science satellites you have, the most things you can keep track of. And the more military you have again, the more things you can keep track of. The bulk are communication satellites, and simply having more means better bandwidth of communication.
As for why they stay up there? They don't. They constantly fall back to earth and burn up in the atmosphere. The ISS is in a major band of satellite traffic and it needs to do a couple maneuvers a year to prevent it from falling back to earth, and we plan on stopping those maneuvers in a couple years. The satellites out in geosynchronous orbit don't fall back, but these are 1) uncommon and 2) in such an open area that it's not worth bothering with keeping track of how many are there. They just exist there and it's fine, there is so much empty space that it doesn't matter.
old satellites are basically space junk at this point but it costs wayyy more to send something up there to collect them than to just track where they are and avoid them.
THey will come down eventually by themselves but forcing them to deorbit requires fuel that the satellites just don't have on them. And adding the fuel (and rockets) to do it would just make them more expensive to make and launch,
Earlier satellites were often put up without the ability to deorbit safely. They either didn't have the fuel to deorbit or were massive enough to survive reentry without the precise control to aim for a safe impact spot.
But newer satellites are generally designed to be brought down at end of life. Of course sometimes those systems fail.
You can’t do maintenance or upgrades on a satellite. Imagine space like a circular highway, do you really want it full of 30 year old vehicles which have never been maintained?
Hubble was not only designed to be serviced—it was several times more expensive than the cost of a servicing mission. For most satellites that are not NASA science platforms, the cost of replacing them with a new one is barely more
than the cost of a servicing mission would be. It’s like asking why you would take a $50 microwave oven to the repair shop—the repair bill would be nearly as high as getting a new one.
Most satellites do come down. At low Earth orbit there's just enough atmosphere to eventually drag satellites back down if they don't have fuel to maintain their orbit. The ones that don't are the ones much further up, and at that distance it would take basically as much effort to get them down as it did to get them up there in the first place. Since that's expensive and not really necessary we don't do it.
The answer to your first question is that they're fulfilling lots of functions for lots of different people; weather, maps, traffic, etc etc. Much of our technology is increasingly reliant upon satellite data, so we need more nad more satellites.
The answer to your second question is two-fold. First, the reason we put things into orbit is because that's the most stable, energy-neutral way that it can stay above the earth. What that means is that it takes energy to move something out of orbit - you have to either speed up or slow down. For old satellites they generally don't have the spare energy to deorbit themselves. But second, we kind of don't leave there; the thing about orbit being stable is that it's not perfectly stable. The atmosphere, even being very thin at that altitude, is still present and still slows things down, so things actually require a bit of energy to be added periodically (usually via stationkeeping thrusters) to stay in a particular orbit. What that means is that pretty much everything in orbit, if left unattended, will eventually slow down enough to be recaptured by earth's gravity and burn up in the thicker parts of the atmosphere. It's just a question of how long that takes (which is determined by how high the orbit is.) For large, high-orbit, or otherwise potentially problematic satellites, they sometimes have the thrust and fuel budget built into them intentionally so that they can deorbit themselves at the end of their lifespan.
So, that gives another answer to your first question: because it takes time for the atmosphere to slow them down enough to re-enter and burn up.
Those in lower orbits naturally decay due to air drag within a few years, of not getting maintenance boosts, so are less problematic.
Those that served in orbits that are too high for air drag, would need to keep around fuel for use at their end of life. If they are out of fuel or there is a malfunction, then they become problematic space debris. If they did have functioning propulsion, then they could reverse thrust for de-orbit or boost to higher graveyard orbit. (usually boosting to graveyard orbit requires less fuel, so...)
Satellites in low orbits are supposed to fall into the atmosphere and burn up when their useful life is expired. Sometimes they push themselves down orbit at end of life to burn up sooner. Sometimes that doesn’t happen and they stay up for a long time. Starlink satellites are made to burn up within a few years even if they go dead.
Satellites in geostationary orbit are supposed to burn more fuel to push themselves out into what’s called a graveyard orbit, clear of other satellites. It would take far too much fuel to push themselves down into the atmosphere, and otherwise they’d stay in orbit for thousands of years.
Most of our satellites are communications satellites. Think about how many millions of cell phone towers you have on earth. Well, all of these communications satellites are doing the same job as a cell phone towers, but because they’re higher up they can cover a wider area, so you need thousands rather than millions to get planet wide coverage.
There’s also a lot of GPS satellites. A gps network needs a minimum of 24 satellites. Once a decade or so, you’ll want to send up a new set with all the upgraded modern tech for more precision. Then there’s more because china and Russia don’t want to have to piggyback off of anyone else’s satellites, so they send up their own set of GPS satellites. There’s a few hundred of these things in orbit now.
All the big countries have a couple of spy satellites with really big cameras to see what the neighbors are getting up to. Official numbers on how many of these everyone has are of course going to be kept secret, but there ought to be a few dozen.
Weather satellites are very important to have, but you don’t need a lot of them. These give us advance warning of hurricanes and other large storms.
There are lots of satellites because we use them for many things like internet, GPS, weather, and TV, and when they stop working, it’s usually too hard and expensive to bring them back, so we either leave them where they are or move them to a parking orbit instead of pulling them down.
There are so many satellites in space because we keep putting more up there.
We aren't really keeping the dead, unused satellites up there so much as nobody made any plans to deorbit them before they stopped working. Once something is in orbit it stays in orbit until something else causes the orbit to decay or change. For satellites in low Earth orbit atmospheric drag causes the orbit to decay and the satellite to eventually reenter the atmosphere, but this process can be slow and unpredictable. For satellites in higher orbits (such as geosynchronous orbit) gravitational interactions from the Moon and planets may eventually perturb the orbit but that can have timescales of years or longer. Deliberately deorbiting a satellite in a planned way would require keeping some reserve propellant in the satellite to slow it down enough to cause its orbit to enter the atmosphere, and also not having the satellite malfunction badly enough that we could no longer control it.
Because there's so much stuff up there now, it's now more common for satellites to have planned lifetimes and to be intentionally deorbited rather than just leaving them up until they fail and letting whatever happens happen after that.
There really aren't that many in space. Right now there are about 15,000 total orbiting the earth. Even if they were exactly at the same altitude (they aren't), that isn't that many compared to cars on the roads.
Right now, satellites have to file a deorbit plan with the government of their launch state, as well as any state that issues them a spectrum licence. It the operators responsibility throughout the lifecycle of the rocket.
If it's not in use, it's probably is GEO orbit and it's probably a country. But even in LEO satellites do NOT touch each other except in some rare circumstances; like the ISS. The technologies to refurbish satellites in situ is just now being developed. (Yes, yes, I'm sure theres some random Intel agency that does it. But it isn't available in the commercial market). So that means they must deorbit at the end of their usable life.
What happens if they don't comply and just abandon the satellite? It's never come up (in Canada/United States at least), but technically they could take control of the spectrum that controls the downlink and lawfully force it to deorbit. But someone has to get it out before it gets smashed, and causes runaway chain reactions, Kessler Syndrome
The point is that sustainability of orbital slots is a focus of NASA, ESA, CSA. It's incredibly important that everyone act responsibly to prevent the tragedy of the Commons. That means noone gets to pollute space with debris under theOuter Space Treaty
Because ... we are human! At one point our pop bottles were recyclable. Then we started making plastic bottles, and recycling became optional. Now, although these plastic bottles made there own island in the Pacific, it's too costly to retrieve and recycle them. So what do we do ... screw it just make more cheaper plastic bottles so we can fulfill the needs of a capitalistic society.
So if we make cheaper, better satellites, who cares what happens to the EOLN satellites.
Think of the "twilight bark" or the "beacons" from Lord of the Rings. We can only talk directly to satellites that we can "see" right now. Since they move around the sky and go around (relative to) the Earth, just like the moon and the sun do, we need a long line of similar ones travelling in a row (like cars on the highway) so that we can always see and talk to at least one at a time. And if we want to send a message to the other side of the world, we can do that because one satellite can pass a message on to the next one, and the next one, and so on, until the satellite on top of Australia (or wherever) sends it back down to the ground.
Why we need to replace them:
Think of things that wear out and need to be replaced, like shoes. Sometimes your shoes wear down the soles, or get holes in the top, and you need new shoes. Sometimes your needs change, feet grow, and you need new shoes. Also, think about your XBOX or your PlayStation. For a while you can play new games on the same machine, but every few years when new games start to require better graphics, sometimes you need to get an entirely new game console in order to handle those games.
Why we leave them up there:
When you lose a ball or a frisbee on your neighbour's roof, you have to ask the neighbour to go up and get it back down for you. That is a lot to ask and the neighbour can't always do this (public/private space program resource limits). Worse, if no neighbour lives in that house (no active agency responsible for junk sat), you have no choice but to just wait until it falls down on its own, which it might never do (or at least take a really, really long time). The only alternative is to build a three-story ladder and climb up yourself, which is too complicated and dangerous for a child to do on their own (satellite retrieval and de-orbiting tech is in infancy and prohibitively expensive right now).
Because a satellite directly over you is easier to contact than one over the horizon with way more atmospheric distortion to deal with. So best to send up a lot of them so that you can get a good signal from anywhere.
And the old ones stay up because they weren't designed to come back down. You'd need to sent up another space thingy with a big lasso or grabby arms and enough fuel to make the old satellite fall down. Not to mention you'd probably need to bring up extra heat shielding to protect the not-for-reentry satellite from the heat of reentry. All together, too expensive to do it constantly.
Very, very minor quibble, most satellites do come down eventually. There is atmospheric drag out to a rather surprising distance (well past the ISS apoapsis), even if it is very small. Very small things add up eventually, so they do and will come down eventually if not constantly boosted higher.
Eventually can be decades or centuries for the really high sub-GEO stuff, admittedly.
Some things are boosted into graveyard orbits where they won't 'come down', but when practical it's considered better to de-orbit/burn up a satellite at the end of it's life then to leave it unpowdered and dead in a useful orbit.
Those are almost all GEO orbiting satts, it takes a lot of propellant to get up into those graveyard orbits.
And absolutely, it is best practice to intentionally de-orbit, there's even a patch of the Indian Ocean designated as the "doom zone" for owners who are conscientious to drop their birds into when they reach end of life.
Not everybody put the spare fuel into their birds for that, or the fuel just evaporated away before they could de-orbit. And those ones make for some interesting space encounters on occasion.
it takes a lot of propellant to get to GEO. It takes much less to get from GEO to graveyard than from GEO to reentry, or they would get reentered.
Do you have the numbers to back that up? I’m not saying that I do, but just the initial thought of boosting to a higher orbit seems like it should require a lot more fuel.
Entering a higher, stable orbit would take at least two boosts, one to get to that higher energy state and another to get a circular orbit. A de-orbiting would only require one properly timed boost and could be in just about any direction.
That's a factor of 2. We're talking about much bigger factors here. US government minimum for "graveyard orbit" is 300km above geostationary orbit. Let's be generous and aim for 1000km Geostationary orbit is at 35,786km. The boost burn to lift your apogee from 35,786km to 36,786km is 17.96m/s, and the circularisation burn at the other end is 17.86m/s (these are close because the two orbits are close in the grand scheme of things) for a total of 35.82m/s. Dropping your apogee to, say, 100km (might be a bit high for a reliable reentry from there, not sure, but whatever) from 35,786km is 1,487.83m/s. It's not even on the same order of magnitude.
Thanks for the info. Rocket science isn’t always intuitive.
It's not exactly brain surgery, is it?
*Rocket engineering
"A de-orbiting would only require one properly timed boost and could be in just about any direction."
The de-orbit burn has to be in the opposite direction from flight. The goal is to slow down the satellite. You don't need to push it towards Earth, gravity will do that for you.
Also, we'd rather satellites hit the middle of the uninhabited bit of the Pacific ocean. We don't want them to come down on land and leave litter, someone would have to pay to pick up that litter. We don't want them to come down on someone and hurt or kill them. We don't want them to come down in a populated area and hit multiple people. We don't want them to come down on Washington or Moscow, be mistaken for an ICBM, and trigger a nuclear war.
That's probably the biggest reason the Space Force puts so much effort into publicizing reentry predictions aside from the occasional rocket body that's large enough to reenter and do major damage on the ground.
I’m going to disagree on the “opposite direction from flight”. Slowing it down is one way to de-orbit, but increasing the speed to generate an elliptical orbit can also result in de-orbiting.
Increasing speed will only move the opposite side of your orbit higher. More elliptical, but still higher. It is not possible to lower your orbit at any point by increasing speed. You need to either slow down, or burn directly towards earth, but that requires much much more fuel.
Trying to imagine a situation in Kerbal where I'd burn radially to shift my orbit in order to get some interaction with the atmosphere (argument of perigee?), I feel like that'd be the only circumstance that I can think of besides negative in-track thrust to achieve a reentry.
You should look into orbital mechanics. It’s clear you don’t have any experience with it. KSP is a great way to figure it out too.
bluesam has an excellent response, but I wanted to give some additional intuition:
in space, ascending from a circular 500km to 1000km orbit takes (roughly) the same amount of fuel as going from 1000 to 500. The difference between your ultimate velocities is the same, and the burn to get from altitude A to B is similar as B to A.
It is true that once your orbit touches the thick parts of the atmosphere, you get a bunch of "free" deceleration. However, the atmosphere is tightly held around Earth's surface: geosynchronous orbit is almost 36000 km, whereas the atmosphere is mostly done with at 500. The graveyard orbit is just a few hundred km further out than geosynchronous - going the other way wouldn't get you anywhere near Earth's atmosphere, even if you spent the circularization burn too.
IIRC there is an international agreement now that what goes up, must come down at the end of its lifespan. Of course, there is no enforcement of this, more of a nice to have but it costs money to de-orbit a satellite. Easier to say sorry and leave it up there. Still, some are now being made to come back down when they are no longer being used.
Even something in a graveyard orbit will come down eventually, it's just in a really long timescale (millions to billions of years).
Thank you KSP for teaching me what a apoapsis means (and the whole space stuff, orbital mechanic, and other thing maybe)
Upvote, but now i am sad.
Don't think about what could have been. Think about the good times we had.
Lots of centuries. Once you get up to medium orbit, you're looking at an orbital lifespan in the millions of years.
The LAGEOS satellites orbiting at ~5800km are expected to stay up there for about 8 million years.
Oh I'm bummed to find out that I can't attach an image of the chart I made here. I just pulled the orbit data for LAGEOS-1 and you seem to be right, since it was launched in 1976 its perigee hasn't even remotely decayed from 5840km.
Literally why imgur.com was created
Sorry I'm not a terminally online redditor but thanks
wtf? you dont have to apologize, it's the easy, free tool to use for what you wanted to do.
sounded cool. now you sound like a dickhead
Fun fact, an average of one to two Starlink satellites de-orbit every day
That is a fun fact!
When you consider the design life expectency and the insane cadence that they're being launched, you have to imagine that they'll eventually be coming down at the same rate that they were sent up. Pretty cool, and a pain in the ass to do reentry predictions for. But SpaceX is very cooperative.
For the Geo synchronous and other satellites that are higher than low Earth orbit they are supposed to be pushed higher into a graveyard orbit that's even higher. This doesn't always happen though because it uses fuel to move it up to the graveyard orbit.
A couple years ago Dish was fined $150K for not reserving enough fuel to push one of their satellites all the way into the agreed graveyard orbit.
Wow
A whole half hour of their CEO's salary
I'm sure they learned their lesson
Ah, so you too have played Kerbal Space Program
When I played lots of Kerbal, I liked to test launch platforms by orbiting these one-shot de-orbiters. They'd have grabbers, fins and heat shields and a bunch of parachutes (in case whatever I was grabbing needed to survive re-entry). So I'd end up with a dozen of these things in various orbits.
Every time I messed up a fuel calculation on a crewed mission, or forgot to put parachutes or an antennae on a science probe, I'd just find the nearest one, RV and bob's your uncle
Cool, that's a neat idea!
You don't need heat shielding on a satellite you don't plan on using any more!
You also don't need to bother retrieving a satellite if you don't plan on using it anymore. There's a lot more room up in space.
why protect a defunct satellite?
To get something out of all the effort. If you're not going to at least use it as scrap or something, why bother bringing it back at all? I'd also rather have a floating pile of junk in space than evaporated particles of metal, rubber, and who knows what else hanging around in our atmosphere.
That makes no sense to do, the scrap value of a satellite is nothing compared to what it would cost to bring it back intact. When dead satellites are deorbited they are just allowed to disintegrate in the atmosphere. The point of deorbiting expired satellites is so they don't become space junk and a hazard to active satellites, not to recover the satellite for scrap.
This is a great answer! Thank you
Please tell me that "grabby arms" is the technical term. 😉
The ELI5 answer: You don’t have to “go get them”. They fall back down eventually. This is caused from them running into gasses up there slowing them down. Think about running into the wind, but on a really small scale. Eventually they lose enough speed to catch the atmosphere and reenter quickly.
Extra: the satellites that will take a really REALLY long time to fall back down are actually more likely to get sling shotted out into space due to the moon.
That literally never happens.
A few spacecraft have used the Moon's and Sun's gravity to go from a high Earth orbit to some other orbit. Here is a paper discussing this.
Yes, but only intentionally. Nothing would ever “get sling shotted”.
Orbital mechanics doesn't care about intention. Leave a spacecraft in a high Earth orbit for long enough and there is a good chance it happens eventually.
There are only a handful of satellites that far out, however.
It happens frequently, that’s how comets and other things get ejected from our solar system all together. There’s an entire theory on how we had a 5th gas giant that got ejected from our solar system due to a close encounter with Jupiter billions of years ago.
If you leave anything in space for millions or billions of years it either comes back or gets thrown out.
Previous commenter was referring to human-launched satellites in low earth orbit, not all bodies in space.
I think that’s where people are getting confused. Because I wasn’t speaking of those, I’m speaking of satellites that are pushed up and out past geostationary orbit before they’re decommissioned.
You say that it never happens, but we haven’t been around long enough to see it. Much less had satellites up there long enough.
When I say a really REALLY long time to come down I’m talking about super synchronous orbits where our grave yard orbits are. It will take longer for them to come down than humans have been on this planet currently. Millions of years. With this in mind, i speculate that they won’t fall back to earth normally but will instead get shot out into space by either the earth, moon, or both due to their orbits becoming destabilized from tidal forces, radiation, or even light pressure.
I could be wrong on that speculation, but I don’t think there’s been a proper study on the orbital life of any satellite that reaches out to a million years either. And if there has I’d love to read it.
There don't need to be million year studies to know whether this is possible/likely. This isn't a new medicine or food additive. Astrophysics, at least regarding orbital mechanics with things like satellites, is well understood.
I don't understand it well enough to make a call here and the fact that you think million year studies would be needed strongly suggests you aren't qualified to make the judgment either.
Well, you get points for imagination I guess, but this isn't "explain like I'm high and making stuff up"!
Lmao yeah this is pure physics/orbital mechanics. I'm not educated enough to make the judgment on its possibility but I am informed enough to know that this could be accurately simulated by a computer and wouldn't require a long-term study.
Let me be more clear, there isn’t a study that simulates the orbit of any satellite or debris in that orbit for a million or more years that I have seen. And part of that is because you’re not very right in your assumption that it could be accurately modeled on that time scale, and also because it’s not really useful information to us at this moment.
If you think this can't be accurately simulated you are seriously ignorant of the power of modern computers.
Your whole argument essentially boils down to "you can't prove it can't happen," and that's how religious people go about debates.
You cannot accurately model or simulate their precise orbit for millions of years into the future because of the n-body problem, which has been discussed in this very same sub
And also further discussed in association with this very topic in this paper which basically states that the lunar effects were the cause of uncertainty in their findings.
Oof I assumed a satellite was too low mass to be a factor in the three body problem so I didn't consider it at all but according to AI at least a satellite does constitute a significant third body.
My assumption is still that the moon would just cause it to fly off into space but not actually sling it around itself, considering there's so much lateral speed and way more influence from the earth.
My curiosity is going now so I'm probably gonna make an askscience/askphysics post about it. Are you interested in me linking any answers? I'm not trying to be vindictive or anything, even if it turns out your assumption was the correct one I'll still link it. I just want to hear from someone who really knows orbital mechanics in depth, figured you might be interested too.
We do “de-orbit” some satellites by letting atmospheric drag slow them down until they (hopefully) disintegrate in the atmosphere, but that’s only feasible for satellites that orbit at relatively low altitudes where atmospheric drag is significant.
Satellites that are higher up are usually boosted into a “graveyard” orbit that’s only occupied by other similarly cooked satellites.
It’s no small feat to go fetch a satellite and they don’t usually carry enough fuel to de-orbit themselves from higher orbits, so this is the most efficient solution.
De-orbiting satellites don’t always fully disintegrate either, there’s always the risk that a large component survives and crash lands somewhere.
Unfortunately, even when something big crash lands, it never lands in the target in the middle of the ocean that would give everyone a free taco.
Tacos? 👀
Free Tacos for U.S. If Mir Hits Floating Taco Bell Ocean Target – Taco Bell sets 40 by 40 foot target in South Pacific for Mir’s Re-Entry
🤮 fuck taco bell!!
🤣
If they can be controlled, reentries target the southern Pacific as there is a huge area without any islands.
The number of them is because they either die from solar radiation/mechanical breakdown over time and need to be replaced, or a combination of so many entities wanting/needing their own units. Then you get into conversations about coverage… a satellite can only cover so much area efficiently, so you need several to cover more of the planet.
The reason why they get left is because it’s a MASSIVE amount of space… so even with the junk still there, and it being a looming problem for the future, it’s pretty rare to have a collision at present.
The other part of it is, those satellites are moving at 10’s of thousands of miles per hour or more. It’s not as simple as just letting them fall. You’d have to put the brakes on for it to lose enough speed to fall. And that takes energy/effort/cost.
Over time, they will naturally slow enough to fall on their own, it can just take a long time because there’s very very little resistance there. Near zero. Most of them are small enough that they’ll burn up on reentry and won’t pose a threat to the ground.
Stuff that is big enough to reach the ground usually IS brought down in a controlled manner on purpose to ensure where and when it’s going to come down
Some modern satellites incorporate mechanisms to take them out for orbit when they are no longer in use. Things like sails or boosters which put them on a declining orbit. Many are also designed to fall out of orbit towards the end of their useful life just through natural orbit changes, for instance many low orbit satellites.
I was wondering if this was a thing, makes sense to install like a deadmans switch on them tied to a single use booster or compressed gas tank or whatever to kick their orbit into decay if they shut down. I guess the problem with that is that you can't control if that deadmans switch device puts it into the path of something else like a rocket launch or other satellites orbit path.
Yeah it's cool. I think they are remotely activated. I.e. activated from earth at the end of the satellite's life, so it goes onto a known path rather than a random one.
There are also junk orbits where you can move your satellites to so it doesn't stay in the way.
The number of interesting orbits is limited which means that there are still concerns about the amount of "junk" in those specific orbits or orbits that you need to go through to get to those interesting orbits.
The dangerous junk pieces are actually tracked, some satellites can be maneuvered to avoid junk if you know there's something coming.
Because going up there to get them is more trouble than it's worth.
Same answer.
okay, another question. how do we know how much exact force it takes to send a satellite to orbit but not so far it floats away or so low it just comes back down? or do they fly them up therev
We just do the math.
Orbital mechanics and laws of motion are well-known and thoroughly understood by those people whose jobs depend on it. The math to get something from the ground into orbit, and stay there, is a solved problem.
I'm not saying the math is necessarily easy - I sure as hell couldn't do it - but it's not a mystery. We have figured it all out, and so when we want to launch something into orbit, we just...do the math.
It doesn't need to be exact. If you accelerate them a bit more than planned, they will be in a slightly higher orbit. If you accelerate them a bit less than planned, they will be in a slightly lower orbit. Almost all spacecraft have their own propulsion to make finer adjustments, too. They only fall down if you have a serious issue with the rocket, and making them fly away needs so much extra propulsion that you don't do that on accident.
Satellites are almost all in LEO, low earth orbit, meaning if left alone long enough they will eventually come back down. Floating away is never a threat because escape velocity is much higher than the orbital speed they achieve. It would be like trying to throw a baseball to your friend and accidentally throwing it several miles away.
Some satellites have little engines on them so they can give themselves a boost to maintain their orbit.
How do we know how much force? well, math... to put it bluntly. Complex math called orbital mechanics.
Short answer: Math and Isaac Newton.
Longer answer: All of this can be calculated precisely if you know Earth’s gravity (we do) and how far away from earth your object is. We can calculate how fast that object will “fall” towards the center earth, and we make it go fast enough to the side that, instead of coming back down, it makes a circle instead. (And if we know how fast we need it and how much it weights, we can calculate the energy (force x time) needed to get it to that speed.
Orbit is cool and weird and I highly recommend playing the original Kerbal Space Program to really understand it.
It's not like you fire them out of a cannon, where you only apply force once, and hope you got it exactly right. The rockets have a guidance system that continuously updates thrust levels and steering to keep them on course. Calculating where they need to be, and how to get there is fairly simple.
but even if you did, we can also do that pretty spot on too
Eh, you'd still need guidance to do the apogee kick
You could probably do that with just a timer.
Simple orbital mechanics. High school maths and physics.
How do you know how hard to throw a ball so it lands right there where you want it to land?
You've got lots of experience with how things fall through the air, so you can estimate how much force to use when throwing to get a ball to land where you want it. You use your arm to apply a sideways force to the ball. Meanwhile, the earth's gravity is applying a downwards force to the ball. Together, these cause the ball to go forwards while also falling down at the same time, until it hits the ground.
Throwing a satellite into orbit works pretty much the same way. We use a rocket to apply a sideways force to the satellite, while the earth's gravity applys a downwards force. Together, these call the satellite to go forwards while also falling down at the same time.
The trick is to throw the satellite forwards so fast that by the time it's fallen down, it's already past the ground. Then it just keeps falling around the earth.
The rocket deploys the satellite at a specific altitude and speed. Satellites then use their onboard engines to move into the exact position they want to be in.
Both the rocket and satellite are keeping track of where they are, so they adjust thrust and direction accordingly. There are actually factors like atmospheric density (varies due to weather) that affect the launch, so mathematics alone can’t calculate an exact destination. They need to adjust during flight.
Because it's cheaper to let their orbit slowly decay until they fall on their own than spending extra effort planning to bring them down.
Each satellite has a specific purpose, and can only do what it was designed to do. It also can only do so much. If you need more stuff done, you need more satellites to do it. As far as keeping old ones in orbit, it takes energy to take them out of orbit (in a controlled manner), and they may not have the fuel or may not be able to receive instructions to do so safely. Newer satellites are more commonly sent up with plans for being able to be de-orbited if they're in a low enough orbit where that is a good idea.
Why are there so many?
Because we use satellites for a lot of things. Communicate and imaging being the biggest things. Lots of people communicate so we have a lot of satellites.
Why do we keep unused ones up there?
Because its hard to bring one back. It requires a bunch of extra fuel and thus extra cost. And most (all?) will naturally slow down eventually and fall back to earth anyway. Might take a few 100 years but it will happen.
Satellites may be divided into three general categories:
Low-earth-orbit satellites need to expend fuel to stay in orbit because of atmospheric drag, and will naturally fall out of orbit due to atmospheric drag when they stop doing so.
Geosynchronous satellites are much further from Earth and have an orbital period of about 23 hours and 56 minutes (the time between consecutive noons is slightly longer than the earth's rotation period because of the planet's motion around the Sun). Realistically speaking, a satellite in a geosynchronous orbit will stay there for more than a century if it doesn't use fuel to go elsewhere and nothing hits it.
Retired geosynchronous satellites are a little higher still. It doesn't take much fuel to move from a geosynchronous orbit to a higher one with a different inclination (moving higher is slightly cheaper than moving lower). Although space is very big, Sir Isaac has decreed that only an infinitesimal fraction of the space around Earth (a circle, with only one degree of freedom) can be usefully inhabited by geosynchronous satellites, so things that aren't geosynchronous satellites are kept outside the circle.
Some low-earth-orbit satellites are higher than others, and will thus last longer, but generally the only reason to expend fuel putting a satellite into an orbit that would be stable for decades would be to either put it into or take it out of a geosynchronous orbit.
To the first question: we've been launching them a long time, but more importantly: recently we've learned ways to launch lots of them at once more cheaply than in the past.
To the second question: So the cheap constellation-style ones are too low, catch enough air to get dragged down, and burn up in the atmosphere after weeks or months.
For higher ones that we leave up there: it actually costs a lot of fuel to make some satellites crash to the ground, especially if it's waaaaay up in geosynchronous orbit (22,300-ish miles). You can fit two entire Earths between those satellites and the ground! So it's much cheaper to instead just push them even higher, so they're drifting around up beyond where they're unlikely to ever touch anything again.
Developing technology, differing needs, and control are all the main reasons why there are so many satellites up in space, and why more are being added.
As for why they tend to stay up there, well for LEO (Low Earth Orbit) satellites they will eventually deorbit by themselves thanks to the extremely tenuous atmosphere up there.
Otherwise, to deorbit, you need enough fuel to slow down, which adds cost and shortens the operational lifespan of the satellite as that fuel could instead have been used for orbital maneuvers to keep the satellite where you want it. Also there are only specific places on Earth that satellites are supposed to be deorbited to. For geostationary satellites it's often more cost effective to push the satellite into a higher graveyard orbit than to get back to Earth.
Most of them are internet satellites at low altitudes, where you need a lot of them to be sure there's always a satellite in range. The lower ones will reenter relatively quickly if they stop getting boosts. You want them low to reduce ping times, and fewer people all trying to use the same satellite at once..
For geostationary satellites, because you'd need a large burn to bring them back down, so instead they're put in a graveyard orbit where they won't interfere with anything when they reach end of useful life(which is usually when they're running out of fuel for stationkeeping).
First off, because they are useful. The more satellites up there, the better transmission quality in terms of latency, strength of signal, etc you have for communication satellites. And the more science satellites you have, the most things you can keep track of. And the more military you have again, the more things you can keep track of. The bulk are communication satellites, and simply having more means better bandwidth of communication.
As for why they stay up there? They don't. They constantly fall back to earth and burn up in the atmosphere. The ISS is in a major band of satellite traffic and it needs to do a couple maneuvers a year to prevent it from falling back to earth, and we plan on stopping those maneuvers in a couple years. The satellites out in geosynchronous orbit don't fall back, but these are 1) uncommon and 2) in such an open area that it's not worth bothering with keeping track of how many are there. They just exist there and it's fine, there is so much empty space that it doesn't matter.
Space is a very hostile environment the satellites do not have a long lifespan
old satellites are basically space junk at this point but it costs wayyy more to send something up there to collect them than to just track where they are and avoid them.
The ones not in use will fall down to earth in a matter of years, so the ones up there are all in use. Not sure why you think otherwise.
We don't keep the useless ones up there.
Point nemo Pacific ocean is the dumping ground for old space tech
THey will come down eventually by themselves but forcing them to deorbit requires fuel that the satellites just don't have on them. And adding the fuel (and rockets) to do it would just make them more expensive to make and launch,
Earlier satellites were often put up without the ability to deorbit safely. They either didn't have the fuel to deorbit or were massive enough to survive reentry without the precise control to aim for a safe impact spot.
But newer satellites are generally designed to be brought down at end of life. Of course sometimes those systems fail.
You can’t do maintenance or upgrades on a satellite. Imagine space like a circular highway, do you really want it full of 30 year old vehicles which have never been maintained?
Except for the Hubble Space Telescope.
Hubble was not only designed to be serviced—it was several times more expensive than the cost of a servicing mission. For most satellites that are not NASA science platforms, the cost of replacing them with a new one is barely more than the cost of a servicing mission would be. It’s like asking why you would take a $50 microwave oven to the repair shop—the repair bill would be nearly as high as getting a new one.
It was not designed to need to be serviced.
Most satellites do come down. At low Earth orbit there's just enough atmosphere to eventually drag satellites back down if they don't have fuel to maintain their orbit. The ones that don't are the ones much further up, and at that distance it would take basically as much effort to get them down as it did to get them up there in the first place. Since that's expensive and not really necessary we don't do it.
The answer to your first question is that they're fulfilling lots of functions for lots of different people; weather, maps, traffic, etc etc. Much of our technology is increasingly reliant upon satellite data, so we need more nad more satellites.
The answer to your second question is two-fold. First, the reason we put things into orbit is because that's the most stable, energy-neutral way that it can stay above the earth. What that means is that it takes energy to move something out of orbit - you have to either speed up or slow down. For old satellites they generally don't have the spare energy to deorbit themselves. But second, we kind of don't leave there; the thing about orbit being stable is that it's not perfectly stable. The atmosphere, even being very thin at that altitude, is still present and still slows things down, so things actually require a bit of energy to be added periodically (usually via stationkeeping thrusters) to stay in a particular orbit. What that means is that pretty much everything in orbit, if left unattended, will eventually slow down enough to be recaptured by earth's gravity and burn up in the thicker parts of the atmosphere. It's just a question of how long that takes (which is determined by how high the orbit is.) For large, high-orbit, or otherwise potentially problematic satellites, they sometimes have the thrust and fuel budget built into them intentionally so that they can deorbit themselves at the end of their lifespan.
So, that gives another answer to your first question: because it takes time for the atmosphere to slow them down enough to re-enter and burn up.
Those in lower orbits naturally decay due to air drag within a few years, of not getting maintenance boosts, so are less problematic.
Those that served in orbits that are too high for air drag, would need to keep around fuel for use at their end of life. If they are out of fuel or there is a malfunction, then they become problematic space debris. If they did have functioning propulsion, then they could reverse thrust for de-orbit or boost to higher graveyard orbit. (usually boosting to graveyard orbit requires less fuel, so...)
There are so many so we can do so many things.
Satellites in low orbits are supposed to fall into the atmosphere and burn up when their useful life is expired. Sometimes they push themselves down orbit at end of life to burn up sooner. Sometimes that doesn’t happen and they stay up for a long time. Starlink satellites are made to burn up within a few years even if they go dead.
Satellites in geostationary orbit are supposed to burn more fuel to push themselves out into what’s called a graveyard orbit, clear of other satellites. It would take far too much fuel to push themselves down into the atmosphere, and otherwise they’d stay in orbit for thousands of years.
Most of our satellites are communications satellites. Think about how many millions of cell phone towers you have on earth. Well, all of these communications satellites are doing the same job as a cell phone towers, but because they’re higher up they can cover a wider area, so you need thousands rather than millions to get planet wide coverage.
There’s also a lot of GPS satellites. A gps network needs a minimum of 24 satellites. Once a decade or so, you’ll want to send up a new set with all the upgraded modern tech for more precision. Then there’s more because china and Russia don’t want to have to piggyback off of anyone else’s satellites, so they send up their own set of GPS satellites. There’s a few hundred of these things in orbit now.
All the big countries have a couple of spy satellites with really big cameras to see what the neighbors are getting up to. Official numbers on how many of these everyone has are of course going to be kept secret, but there ought to be a few dozen.
Weather satellites are very important to have, but you don’t need a lot of them. These give us advance warning of hurricanes and other large storms.
There are lots of satellites because we use them for many things like internet, GPS, weather, and TV, and when they stop working, it’s usually too hard and expensive to bring them back, so we either leave them where they are or move them to a parking orbit instead of pulling them down.
There are so many satellites in space because we keep putting more up there.
We aren't really keeping the dead, unused satellites up there so much as nobody made any plans to deorbit them before they stopped working. Once something is in orbit it stays in orbit until something else causes the orbit to decay or change. For satellites in low Earth orbit atmospheric drag causes the orbit to decay and the satellite to eventually reenter the atmosphere, but this process can be slow and unpredictable. For satellites in higher orbits (such as geosynchronous orbit) gravitational interactions from the Moon and planets may eventually perturb the orbit but that can have timescales of years or longer. Deliberately deorbiting a satellite in a planned way would require keeping some reserve propellant in the satellite to slow it down enough to cause its orbit to enter the atmosphere, and also not having the satellite malfunction badly enough that we could no longer control it.
Because there's so much stuff up there now, it's now more common for satellites to have planned lifetimes and to be intentionally deorbited rather than just leaving them up until they fail and letting whatever happens happen after that.
There really aren't that many in space. Right now there are about 15,000 total orbiting the earth. Even if they were exactly at the same altitude (they aren't), that isn't that many compared to cars on the roads.
Right now, satellites have to file a deorbit plan with the government of their launch state, as well as any state that issues them a spectrum licence. It the operators responsibility throughout the lifecycle of the rocket.
If it's not in use, it's probably is GEO orbit and it's probably a country. But even in LEO satellites do NOT touch each other except in some rare circumstances; like the ISS. The technologies to refurbish satellites in situ is just now being developed. (Yes, yes, I'm sure theres some random Intel agency that does it. But it isn't available in the commercial market). So that means they must deorbit at the end of their usable life.
What happens if they don't comply and just abandon the satellite? It's never come up (in Canada/United States at least), but technically they could take control of the spectrum that controls the downlink and lawfully force it to deorbit. But someone has to get it out before it gets smashed, and causes runaway chain reactions, Kessler Syndrome
The point is that sustainability of orbital slots is a focus of NASA, ESA, CSA. It's incredibly important that everyone act responsibly to prevent the tragedy of the Commons. That means noone gets to pollute space with debris under theOuter Space Treaty
Because ... we are human! At one point our pop bottles were recyclable. Then we started making plastic bottles, and recycling became optional. Now, although these plastic bottles made there own island in the Pacific, it's too costly to retrieve and recycle them. So what do we do ... screw it just make more cheaper plastic bottles so we can fulfill the needs of a capitalistic society.
So if we make cheaper, better satellites, who cares what happens to the EOLN satellites.
Why we need many satellites:
Think of the "twilight bark" or the "beacons" from Lord of the Rings. We can only talk directly to satellites that we can "see" right now. Since they move around the sky and go around (relative to) the Earth, just like the moon and the sun do, we need a long line of similar ones travelling in a row (like cars on the highway) so that we can always see and talk to at least one at a time. And if we want to send a message to the other side of the world, we can do that because one satellite can pass a message on to the next one, and the next one, and so on, until the satellite on top of Australia (or wherever) sends it back down to the ground.
Why we need to replace them:
Think of things that wear out and need to be replaced, like shoes. Sometimes your shoes wear down the soles, or get holes in the top, and you need new shoes. Sometimes your needs change, feet grow, and you need new shoes. Also, think about your XBOX or your PlayStation. For a while you can play new games on the same machine, but every few years when new games start to require better graphics, sometimes you need to get an entirely new game console in order to handle those games.
Why we leave them up there:
When you lose a ball or a frisbee on your neighbour's roof, you have to ask the neighbour to go up and get it back down for you. That is a lot to ask and the neighbour can't always do this (public/private space program resource limits). Worse, if no neighbour lives in that house (no active agency responsible for junk sat), you have no choice but to just wait until it falls down on its own, which it might never do (or at least take a really, really long time). The only alternative is to build a three-story ladder and climb up yourself, which is too complicated and dangerous for a child to do on their own (satellite retrieval and de-orbiting tech is in infancy and prohibitively expensive right now).