Ones that are in low earth orbit eventually decay and end up in the atmosphere. If you can find a way to increase the drag they will decay more rapidly, which can be a way to get rid of them if needed. The smaller ones will just burn up in the atmosphere (e.g. the Planetary.org lightsail cubesat). Or if anything does survive all the way to the ground, it's small and has low terminal velocities. Really big satellites like the ISS can have big chunks that survive all the way to the ground with high terminal velocities.
So, ideally any satellites likely to leave large bits of debris that survive all the way to the ground should be de-orbited in a controlled way using rocket propulsion so you can choose where they will re-enter the Earth's atmosphere. That's what is planned for the ISS at the end of its life whenever that is.
There have been several uncontrolled re-entries. The largest of these was Skylab (they set it spinning to increase the chance that it would break up but couldn't choose the re-entry point because they hadn't built that capability into the space station). It crashed near to Perth in Australia. The Day Skylab Crashed to Earth: Facts About the First U.S. Space Station’s Re-Entry
The Soyuz MIR space station however had a controlled re-entry - with a lot of debris spread over a pre-planned corridor across the oceans. That shows it can be done, and that's what will happen with ISS whenever it reaches the end of its useful life.
So far nobody has been hurt by satellite re-entry debris. Some details here - it includes some of the history of previous re-entries and their debris: TRMM spacecraft completes destructive re-entry over South Indian Ocean. See also 6 Biggest Spacecraft to Fall Uncontrolled From Space
Satellites that are in Medium Earth orbit stay where they are for many decades and as there is a lot of space in between LEO and GEO then they aren't a great concern. Ditto for High Earth orbit above geostationary orbit.
Ones in Geostationary Orbit are moved to a "Graveyard orbit" a couple of hundred kilometers above GEO - because the space there is crowded - all the satellites need to fit into a thin band at a particular height above the equator - if you left dead satellites there they would easily bump into each other when their fuel is exhausted. And from GEO it would take over 1 km / second to move them into a geostationary transfer orbit so they could burn up in the atmosphere - not very practical - so they are moved into a slightly higher orbit instead.
That's the ideal situation anyway. But we also have many satellites that are in low orbits, but high enough so they won't be brought down into the atmosphere quickly - and bits of debris there also - there's a risk that the debris, hitting other satellites could cause a build up of more and more smaller pieces as the satellites break up into small pieces.
That's called the Kessler syndrome and in worst case could end up with a runaway "chain reaction" with so many small pieces of broken satellites in orbit that you can no longer use LEO (say) or whichever orbit is affected. This is of especial concern in the crowded space of low Earth orbit and also to some extent in Geostationary orbit.
By a runaway reaction though it doesn't mean that the satellites all blow up into tiny fragments in a few hours - that's a bit of movie hyperbole. Think decades not days. Rather - that the population of smaller fragments increases far more quickly than they are removed from orbit. That's actually happening so in a way we are already into a Kessler Syndrome type event, but it is happening really slowly, and we need to make sure it doesn't accelerate and then take actions to reverse it.
And we have dead satellites in Geostationary orbit that couldn't be moved to the graveyard orbit. This could lead to a separate Kessler type event in GEO.
So - some time in the near future we'll probably start deliberately capturing and de-orbiting dead satellites (or moving them to graveyard orbits) mainly to avoid the Kessler syndrome. And also catch and de-orbit the many fragments of debris from LEO as well.
There are many ideas and proposals and also methods being develop to deal with this issue. And with new satellites then they think about how they can safely dispose of the satellite at the end of mission when designing the mission, if necessary.
So, ideally any satellites likely to leave large bits of debris that survive all the way to the ground should be de-orbited in a controlled way using rocket propulsion so you can choose where they will re-enter the Earth's atmosphere. That's what is planned for the ISS at the end of its life whenever that is.
There have been several uncontrolled re-entries. The largest of these was Skylab (they set it spinning to increase the chance that it would break up but couldn't choose the re-entry point because they hadn't built that capability into the space station). It crashed near to Perth in Australia. The Day Skylab Crashed to Earth: Facts About the First U.S. Space Station’s Re-Entry
The Soyuz MIR space station however had a controlled re-entry - with a lot of debris spread over a pre-planned corridor across the oceans. That shows it can be done, and that's what will happen with ISS whenever it reaches the end of its useful life.
So far nobody has been hurt by satellite re-entry debris. Some details here - it includes some of the history of previous re-entries and their debris: TRMM spacecraft completes destructive re-entry over South Indian Ocean. See also 6 Biggest Spacecraft to Fall Uncontrolled From Space
Satellites that are in Medium Earth orbit stay where they are for many decades and as there is a lot of space in between LEO and GEO then they aren't a great concern. Ditto for High Earth orbit above geostationary orbit.
Ones in Geostationary Orbit are moved to a "Graveyard orbit" a couple of hundred kilometers above GEO - because the space there is crowded - all the satellites need to fit into a thin band at a particular height above the equator - if you left dead satellites there they would easily bump into each other when their fuel is exhausted. And from GEO it would take over 1 km / second to move them into a geostationary transfer orbit so they could burn up in the atmosphere - not very practical - so they are moved into a slightly higher orbit instead.
That's the ideal situation anyway. But we also have many satellites that are in low orbits, but high enough so they won't be brought down into the atmosphere quickly - and bits of debris there also - there's a risk that the debris, hitting other satellites could cause a build up of more and more smaller pieces as the satellites break up into small pieces.
That's called the Kessler syndrome and in worst case could end up with a runaway "chain reaction" with so many small pieces of broken satellites in orbit that you can no longer use LEO (say) or whichever orbit is affected. This is of especial concern in the crowded space of low Earth orbit and also to some extent in Geostationary orbit.
By a runaway reaction though it doesn't mean that the satellites all blow up into tiny fragments in a few hours - that's a bit of movie hyperbole. Think decades not days. Rather - that the population of smaller fragments increases far more quickly than they are removed from orbit. That's actually happening so in a way we are already into a Kessler Syndrome type event, but it is happening really slowly, and we need to make sure it doesn't accelerate and then take actions to reverse it.
And we have dead satellites in Geostationary orbit that couldn't be moved to the graveyard orbit. This could lead to a separate Kessler type event in GEO.
So - some time in the near future we'll probably start deliberately capturing and de-orbiting dead satellites (or moving them to graveyard orbits) mainly to avoid the Kessler syndrome. And also catch and de-orbit the many fragments of debris from LEO as well.
There are many ideas and proposals and also methods being develop to deal with this issue. And with new satellites then they think about how they can safely dispose of the satellite at the end of mission when designing the mission, if necessary.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
4.8m answer views110.3k this month
Top Writer2017, 2016, and 2015
Published WriterHuffPost, Slate, and 4 moreHuffPost, Slate, Forbes, Newsweek, Business Insider, and Science 2.0