That seems a bit counter-intuitive, because in an orbit above the belt, the satellite would eventually lose gain velocity, so its orbit would intersect the orbital plane of the geostationary satellites possibly creating a collision hazard. To be re-orbited, a satellite has to have functional attitude control [Wikipedia], which would also allow to lower increase the velocity to decrease the altitude instead. So, why isn't the so-called graveyard orbit below the geostationary altitude?
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$\begingroup$ Losing velocity happens when you raise an orbit. Lower orbits are faster. The moon is slowing down and therefore leaving Earth's orbit some day. You are probably thinking of low Earth orbit (LEO) where there is atmospheric drag. Despite the counterintuitive feeling it gives, drag causes LEO satellites to orbit faster as they fall to Earth. Give them a boost, they raise to higher orbit, but orbit more slowly. $\endgroup$– uhohCommented Apr 7, 2017 at 17:00
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$\begingroup$ @uhoh: The Moon orbit growing is caused by tidal bulging of Earth due to Moon gravity. The satellites are not nearly massive enough to cause such effect. (never mind the effect is exactly zero at GEO, reversing direction there.) $\endgroup$– SF.Commented Apr 7, 2017 at 17:17
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1$\begingroup$ Yes. Indeed - Lower orbit = higher velocity. My bad. Thanks @uhoh. $\endgroup$– MarkCommented Apr 7, 2017 at 17:47
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1$\begingroup$ @uhoh, I knew that, yet made this silly mistake :) $\endgroup$– MarkCommented Apr 7, 2017 at 18:02
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5$\begingroup$ Lower orbit has higher velocity but to go from a high orbit to a low orbit is done with a pair of deceleration burns. Welcome to the strange world of orbital mechanics. $\endgroup$– Peter GreenCommented Apr 7, 2017 at 18:50
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2 Answers
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Orbits at the altitude of GEO are stable for very long times (millions of years). There is no significant decay of the orbital height due to some kind of drag, so the risk of these satellites interfering with working ones is close to zero. On the other hand, there are good reasons to store them above the belt and not below:
The region below is used for maneuvers, e.g. to move a satellite from one longitude to another (ok, not a really good argument, because you could do this above as well).
One very important aspect: If satellites are in a lower orbit, they might interfere with communication links between GEO and Earth if the satellite passes by directly in front of another.
There are two ways how satellites typically approach GEO: from a high GTO using conventional thrusters, so they move rather quickly and chance of collisions are low. On the other hand, there are some satellites using ion thrusters to reach their final orbit rather slowly and from lower orbits. Having to move them through the graveyard orbit would be a great risk due to the long travel time.
Additional information: Have a look to this question: Why is the ribbon of decommissioned geosynchronous satellites skewed? - the graveyard orbits are not strictly above GEO, but also tilted by several degrees.
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1$\begingroup$ Technically the first entry maneuver will only happen from below... but again, chances of collision there is tiny. $\endgroup$ Commented Apr 7, 2017 at 20:23
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2$\begingroup$ There are a whole host of earth observing satellites in addition to the comm sats you mention. A satellite parked just below Geo orbit would occupy a sizable solid angle of an earth observing satellites FOV. $\endgroup$– caseyCommented Apr 7, 2017 at 23:20
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1$\begingroup$ “to move a satellite from one latitude to another” — I'm guessing you mean ‘longitude’. $\endgroup$ Commented Apr 9, 2017 at 5:46
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Actually, it makes a lot of sense to raise the orbit of end-of-life geostationary satellites: Coming from Earth you have to cross through a lower orbit to transfer from low earth orbit to a geostationary orbit but you don't have to go farther out than that (some transfer orbits do, but it's not a requirement). That means that a higher orbit has less risk of collision than a lower orbit with regard to the continuing use of the geostationary orbit belt.
At the altitude of geostationary satellites (~36,000 km) there is basically nothing to cause orbits to lower on a reasonable time scale. The atmosphere doesn't stop at a set altitude (the exosphere is apparently measurable up to 10,000+ km and estimated to reach half way to the moon), but the effect of drag is minuscule that far out. I didn't find a good reference for the longevity of geostationary orbits specifically, but LAGEOS is only at 6,000 km and is estimated to not reenter for 8.4 million years.
The intended graveyard orbit, as required by US regulations for geostationary satellites, is 300 km above the geo belt. That's not very far compared to the distance of the geo belt from Earth, but it's more than far enough to ensure that maneuvers in the geo belt of active satellites and perturbations from the sun and moon on the dead satellites don't cause them to cross paths.
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1$\begingroup$ the applicable NASA standard for orbital debris also calls for a graveyard orbit lower than GEO, but I do not know how frequently it is used, in practice $\endgroup$– costromCommented Apr 8, 2017 at 17:06