You may get annoyed if a passing driver on the interstate litters your path with a soda can, but it’s unlikely to cause any serious danger to your automobile. Now, imagine that the can is flying toward you at over 3,000 meters per second (over 6,700 miles per hour) — and you’re flying a spacecraft instead of driving a car.
This is the danger posed to future astronauts by orbital “space junk,” the detritus left from earlier space missions that now circles the Earth without human control. NASA tracks over 500,000 pieces of debris, 20,000 of which are larger than a softball, including a number of full-sized satellites and rocket boosters. Currently, astronauts have few possible responses to encountering this junk: those in the International Space Station either perform a “debris avoidance maneuver” or, if they lack sufficient warning, hunker down in an attached Soyuz capsule that can serve as a lifeboat if a collision should occur. But aerospace engineers are investigating proactive methods that could eventually clean up the mess mankind has left in orbit.
Two of the most seriously considered technologies seem like they’ve leapt off the screen of “Star Wars.” The first, proposed by James Mason and his colleagues at the NASA Ames Research Center, involves turning the beam of a ground-based laser onto troublesome pieces of debris. A laser powerful enough to actually destroy an object would be prohibitively expensive (sorry, wannabe Vaders), so Mason instead plans to use the beam to push junk objects out of dangerous paths. Photons, the “particles” that make up light, carry momentum that is transferred to whatever they run into; this force is relatively tiny, but applied over the course of several hours, a stream of photons could nudge debris into harmlessly burning up on reentry into Earth’s atmosphere. It’s worth noting that the same principle could also be used to propel spacecraft, as has been explored by designers of “solar sails.”
The second proposal, offered by Hanspeter Schaub of the University of Colorado Boulder, is eerily similar to the tractor beam seen in many works of science fiction. A satellite, which Schaub playfully acronymed GLiDer (Geosynchronous Large Debris Reorbiter), would emit a stream of electrons towards a piece of space junk, giving the debris a negative charge while the satellite gained a positive charge. This charge difference would cause the object to become attracted to the satellite and trail behind it; after the junk gained speed, the satellite would release the beam, launching it into deeper space. Although the process is relatively slow, an estimated two to three months per object, a collection of GLiDer satellites could make short work of the larger obstructions in geosynchronous orbit.
Space, in the words of science fiction author Douglas Adams, “is big — really big.” But in our neighborhood, useful orbits are limited, and carelessly cluttering them up may have severe consequences for future space exploration and development. These cleanup methods are a good place to start in solving the problem, but the message for space agencies should be clear — don’t litter.