For Jupiter and Saturn - that's to make sure they don't impact into the potentially habitable moons such as Enceladus or Europa. There are other possibilities. The Europa orbiter will finish its mission in a semi-permanent orbit around Europa which they have calculated will mean that it will receive enough radiation from Jupiter to sterilize it completely before it has a chance of an impact into Europa (I hope they are right there!).
Cassini is going to impact Saturn at the end of mission, but they had other options also - a permanent orbit around Saturn far from any moons - or a permanent orbit around Titan (they'd have done that except they didn't have enough fuel for it) - or for that matter, by using the "Interplanetary super highway" - repeated flybys first of Saturn's moons, then out to a special place from which they could send the spacecraft anywhere in our solar system pretty much - all the way from the inner asteroid belt all the way out to the Kelper belt. So that's an option also. With Cassini they chose not to do that because they didn't feel they could guarantee a still functioning spacecraft at the end of it. They could have directed it to impact into Jupiter instead of Saturn - but it would take so long to get there - the Saturn one was much more interesting and the Jupiter impact might not have returned any science data at all depending on its condition when it got there.
For Mars, then the aim is to keep the satellites in orbit for as long as possible, the object is not to impact them into Mars itself, because they are not sufficently sterilized for that. Unless sterilized to Viking levels or better, then the current planetary protection requirements are that they have to stay in orbit for at least 20 years after arrival at Mars with probability of 99% and at least 50 years with probability at least 95%. The National Academies Press
For Earth then satellites in low orbits especially close to LEO - they need a way to de-orbit at the end of their mission. If they are in Geostationary orbit, then at the end of their mission, similar thing, they have to be moved to a "graveyard orbit" just a few hundred kilometers higher, far enough away to have negligible risk of impacting a working satellite (the delta v to get to geotransfer orbit to impact Earth atmosphere is too much, well over 1 km / sec. so that's not an option for them).
I'm not sure how much either of these is an absolute requirement and how much it is just good practice. Older satellites were launched before it became clear how important it is to sort out an end of mission scenario for them.
For satellites in between those two, there are no special requirements - as they can stay up in orbit for centuries anyway, with no air resistance to speak of - and unlike GEO or LEO - both of which are getting very crowded now - there is plenty of orbital space to put them into. GEO is crowded because although it is a huge orbit around Earth, there's only one altitude with the orbital period exactly one day - and for most of them they want to be equatorial also, so they are all crowded into quite a small amount of orbital space. LEO is much more flexible by way of altitude, also can be launched at different inclinations, but a smaller orbit and many more satellites there. Both risk the Kessler syndrome, where two satellites that crash into each other create more debris leading to more satellites breaking up in a chain reaction. But a disintegrating satellite in LEO is not going to send significant amounts of debris to GEO or vice versa. And the Kessler syndrome, unlike in movies, would play out over years and decades, it's not like one year there's a break up of two satellites that hit each other, and next year or even week or day, they are all broken down to dust. Indeed in LEO some would say it has started already and has to be stopped before it gets worse.
Cassini is going to impact Saturn at the end of mission, but they had other options also - a permanent orbit around Saturn far from any moons - or a permanent orbit around Titan (they'd have done that except they didn't have enough fuel for it) - or for that matter, by using the "Interplanetary super highway" - repeated flybys first of Saturn's moons, then out to a special place from which they could send the spacecraft anywhere in our solar system pretty much - all the way from the inner asteroid belt all the way out to the Kelper belt. So that's an option also. With Cassini they chose not to do that because they didn't feel they could guarantee a still functioning spacecraft at the end of it. They could have directed it to impact into Jupiter instead of Saturn - but it would take so long to get there - the Saturn one was much more interesting and the Jupiter impact might not have returned any science data at all depending on its condition when it got there.
For Mars, then the aim is to keep the satellites in orbit for as long as possible, the object is not to impact them into Mars itself, because they are not sufficently sterilized for that. Unless sterilized to Viking levels or better, then the current planetary protection requirements are that they have to stay in orbit for at least 20 years after arrival at Mars with probability of 99% and at least 50 years with probability at least 95%. The National Academies Press
For Earth then satellites in low orbits especially close to LEO - they need a way to de-orbit at the end of their mission. If they are in Geostationary orbit, then at the end of their mission, similar thing, they have to be moved to a "graveyard orbit" just a few hundred kilometers higher, far enough away to have negligible risk of impacting a working satellite (the delta v to get to geotransfer orbit to impact Earth atmosphere is too much, well over 1 km / sec. so that's not an option for them).
I'm not sure how much either of these is an absolute requirement and how much it is just good practice. Older satellites were launched before it became clear how important it is to sort out an end of mission scenario for them.
For satellites in between those two, there are no special requirements - as they can stay up in orbit for centuries anyway, with no air resistance to speak of - and unlike GEO or LEO - both of which are getting very crowded now - there is plenty of orbital space to put them into. GEO is crowded because although it is a huge orbit around Earth, there's only one altitude with the orbital period exactly one day - and for most of them they want to be equatorial also, so they are all crowded into quite a small amount of orbital space. LEO is much more flexible by way of altitude, also can be launched at different inclinations, but a smaller orbit and many more satellites there. Both risk the Kessler syndrome, where two satellites that crash into each other create more debris leading to more satellites breaking up in a chain reaction. But a disintegrating satellite in LEO is not going to send significant amounts of debris to GEO or vice versa. And the Kessler syndrome, unlike in movies, would play out over years and decades, it's not like one year there's a break up of two satellites that hit each other, and next year or even week or day, they are all broken down to dust. Indeed in LEO some would say it has started already and has to be stopped before it gets worse.
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
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