Space debris experts have focused their attention on low Earth orbit, where the population of debris is greatest, and on the higher geostationary orbit, where most commercial telecommunications satellites operate. But given the current and planned activity in medium Earth orbit around 20,000 kilometers in altitude, the focus should include satellite-disposal options there as well, according to Rene Oosterlinck, director of navigation at the European Space Agency (ESA).

“Satellites in a 20,000-kilometer orbit are there pretty much forever — thousands of years,” Oosterlinck said. “We need to take measures to dispose of retired satellites there. The problem is not simple because a disposal orbit today may be very useful in the future. So the question is, ‘Where should we put these satellites?’”

Satellite navigation constellations operate in medium Earth orbit. The United States and Russia currently operate satellite navigation constellations that require at least 24 satellites each, plus in-orbit spares. Europe and China are planning similar constellations to be in service in the coming years — Europe with 27 satellites plus spares, China with 30 satellites plus spares.

That means more than 100 operational satellites across the four systems at any given time. Moreover, all of these nations intend to keep their constellations operational in perpetuity — they view positioning, timing and navigation as strategic priorities — meaning the clutter of operational and spent satellites will only increase over time.

International, nonbinding guidelines ask operators of geostationary-orbiting satellites to save enough fuel at the end of their satellites’ lives to raise the spacecraft to a graveyard orbit 200-300 kilometers above the geostationary arc. In low orbit, operators are asked to push their satellites into lower orbits so that they are gradually pulled in by Earth’s gravity and destroyed within 25 years on re-entry into Earth’s atmosphere.

For medium-orbit spacecraft there is no such formula. Oosterlinck said that in addition to the navigation satellites, certain launch systems for medium Earth orbit, including Russia’s Soyuz rocket that Europe will be using for its Galileo constellation, leave their upper stages in the operational orbit after dropping off their payload.