RECENTLY, the European Space Agency (ESA) conducted a neighborhood cleanup in space, a new kind of cleanup that is becoming increasingly important. It moved a decommissioned satellite out of the highly prized geosynchronous orbit where most communications satellites travel.
This orbit - one-tenth of the way to the moon - has become increasingly crowded. As a result, there is a small but growing risk of collision between satellites and of damage from satellite debris.
Thus when ESA's GEOS-2 scientific satellite finished its task of measuring electric and magnetic fields and particles last December, its owners decided to remove it from the 36,000-kilometer-high orbit. The satellite's maneuvering rocket had enough fuel to boost it 270 km higher, where it now resides. ESA expects soon to repeat the process with another defunct spacecraft - OTS-2.
These craft will continue to clutter space near Earth, along with hundreds of other pieces of orbital junk. But they will be far enough removed so that they no longer threaten users of the geosynchronous orbit.
Satellites in this orbit move at the same speed at which Earth rotates. They remain more or less fixed with respect to a given location on the ground. This makes the orbit ideal for weather observation and for communications relays. But as usage has expanded so has the clutter of disused spacecraft.
No one knows exactly how many spacecraft there are along the geosynchronous path. Military craft are not always listed publicly. Some users have reserved space but do not yet have an actual satellite aloft. Defunct vehicles are not listed at all. A widely quoted rough estimate is that there are 150 satellites in the geosynchronous orbit, of which perhaps 60 or so are defunct.
So far, there have been no reports of any actual collisions. However, with demand for space along the orbit continuing to intensify, trash management has to share attention competition for a seat at the orbital table.
This competition is forcing an increased physical crowding along favored arcs of the orbit, such as those from which Europe and the Americas are served. Typical spacing for US communications satellites, for example, is narrowing from typically 4 degrees to 2 degrees, a separation of about 800 miles. That is just the separation between assigned orbital slots. Several different satellites operating in different radio frequency ranges are usually clustered around each slot.
Up to now, there has been more concern about radio interference among satellites than about collision. But as ESA points out, a collision would not only damage satellites, it would create a cloud of debris. There would be a growing and uncontrollable risk of further collisions.
Thus it seems that development of the space frontier has already reached a level where neighborhood trash removal has become a necessity.