Although the International Space Station has a pair of self-repairing robots, most artificial objects in orbit aren't nearly as lucky. Sooner or later, satellites reach the end of their useful life. When this happens, they either drop down and burn up during re-entry, or are sent to the satellite graveyard.
The satellite graveyard is sadly lacking in large engraved stones. Remixed image by NASA/Mika McKinnon
The satellite graveyard is a supersynchronous orbit around the Earth, so high that it takes longer than a full rotational day of the Earth for an object to complete a single orbit. At the end of their operational lives, satellites do one last hard burn, enough to boost them up from the busy lower-altitude orbits.
But it wasn't always this way. In the early days of space exploration, it was a free-for-all of satellites and space-junk everywhere. Even after international agreement was reached to reduce the amount of orbital garbage to control the growing space-junk problem, we still didn't have planetary consensus on what to do with defunct satellites.
Finally, in 1993 space agencies co-ordinated that satellites should be sent crashing through the atmosphere to burn in a final blaze of glory, or, failing that, boosted up into a higher orbit for their retirement. This would free up space in the busy lower orbits stuffed full of still-functional satellites, reducing the odds of devastating and debris-generating collisions.
The Globalstar satellites were shifted into higher orbits at the end of their useful lives. Image credit: NASA
For geostationary satellites, it requires a big burst of fuel to deorbit and crash back to Earth, roughly 1,500 meters per second. In contrast, boosting that same satellite up to the graveyard orbit only takes a change of velocity of 11 meters per second. Alas, a satellite needs to do more than go fast to make it to the satellite graveyard — changing orbits also requires functional attitude controls, which not every aging satellite can pull off with grace.
By now, it's standard practice to retire old satellites in stable storage orbits 300 kilometers above the functional high geosynchronous orbits. It's a retirement home for elderly spacecraft, where they can admire the view and shuffle around peacefully without constantly having young whippersnapper satellites dodging around them. Best of all, it has a nice, healthy buffer between it and the functional satellites, so even if a retired satellite gets a bit wonky in its orbit, it won't go staggering into active traffic without warning.
At the moment we have over a hundred satellites in the graveyard orbit. For now, it's working. But eventually, with more satellites retiring upwards and onwards, sooner or later we're going to get collisions in the graveyard orbit. A high-orbit collision would produce space junk with a long residency time, endangering active satellites.
Of course, not every satellite gets to retire. After all, ISEE-3 was launched in the 1970s, retired in the 1990s, yet is being reclaimed this year to go back out and do even more science!