Active Debris Removal
A number of studies have concluded that the debris environment in low Earth orbit will grow over time even if strict end-of-life disposal measures are followed. The biggest source of this debris is from collisions between large objects such as spent rocket upper stages and unused satellites. Since these objects are large they can produce more fragments which are also large enough to produce additional fragments in future collisions.
A way to prevent this growth is to remove large existing objects from the parts of low Earth orbit where collisions are most likely to occur. This is called active debris removal. By removing large objects there is less material in orbit from which new debris can be created.
Removing large objects from orbit, usually by causing them to re-enter the Earth’s atmosphere, is difficult. Almost no spacecraft are designed to be physically grappled once they are in orbit, and they may have antennas, solar arrays, or other fragile projections. They may be tumbling or spinning, making them difficult to grapple and control. Many of these old satellites and rocket upper stages weight thousands of pounds, making them difficult to move. Some of the objects have been in orbit for decades and so may not be as sturdy as when launched or may contain fuel that could be triggered to explode.
A number of concepts have been proposed to remove large space objects from orbit, including flying up to them and grabbing them to pull them out of orbit, or attaching a device, such as a balloon, that will increase the speed with which the Earth’s atmosphere causes their orbits to decay and re-enter the atmosphere on their own. Regardless of the approach, a number of technologies must be developed before these concepts can be made to work.
Another challenge for active debris removal is international treaties. Currently ownership of a satellite or rocket upper stage remains with the country that launched it even after the satellite or upper stage is no longer used. This means that one country cannot remove the debris launched from a second country without that second country’s permission.