Space-faring nations: Clean up low Earth orbit or you're grounded.
- By Tony Reichhardt
- Air & Space magazine, March 2008
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Two years later, that's still the case: No one knows how to begin removing orbital debris. "No easy or cheap solutions have yet been identified," Johnson says flatly. It isn't for lack of ideas. Well-meaning inventors have come forward with all kinds of schemes for clearing out space junk: space flypapers, sweepers, robot garbage scows. Take, for example, U.S. patent no. 4,991,799, filed in 1990, for a propeller-like sweeper that would ram into small particles and knock them from a threatening orbit. Or patent no. 6,655,637, filed in 2002, for a robot that could grab space junk with "inflatable fingers."
"Some of the ideas are technically outlandish, some are technically feasible," says Johnson. The problem, almost always, is cost. "If you want to spend tens of millions to retrieve a single rocket body, you can do it," he says. "But it doesn't make any sense economically."
So Johnson and other debris experts from Europe, the United States, and Japan are working on a comprehensive study for the International Academy of Astronautics that will evaluate cleanup options. Results are due next year.
The ideas tend to come in two sizes: systems for clearing out particles smaller than 10 centimeters (four inches), and schemes for "de-orbiting" large objects like whole rocket bodies, usually by pushing them down to lower orbits, where they burn up due to increased atmospheric drag.
Fragments between 1 and 10 centimeters in size will penetrate most spacecraft, according to the Aerospace Corporation's Center for Orbital and Reentry Debris Studies, and more than 100,000 are estimated to be circling Earth. (Pieces even smaller than a centimeter can cause damage, as NASA space shuttle managers know; they've had to replace more than 60 shuttle windows, dinged by tiny particles.) In the 1990s, NASA and the U.S. Air Force Space Command studied a concept called Orion, sometimes called a "laser broom," designed to eliminate small debris. A ground-based laser would be aimed at each object until pressure from the beam, coupled with the reaction force from material ablating away from the target, sends it into a lower orbit.
Orion, though, "turned out to be not all that easy technically," says Johnson. And with an estimated cost of $500 million, "it was certainly not within anybody's budget." The system would have required its own tracking network, since current space surveillance cameras track objects only down to 10 centimeters. Engineers would have to work out a system that imparted enough momentum to move a chunk of debris and that would be sure to lower instead of raise its orbit. "There are lots of little gotchas in the Orion final report," Johnson says. "There's a reason why it's been sitting on the shelf for more than a decade."
Okay; can something be done about the bigger pieces? Over the long term, removing large objects like empty rocket stages is the most effective way to reduce the likelihood of collisions, since such objects account for most of the total surface area that could be hit by smaller pieces. According to Johnson and Liou's calculations, de-orbiting just five large objects a year could reduce the overall collision risk significantly, and help stem the proliferation of junk. Removing 20 objects a year would reduce orbiting debris from 65,000 pieces by the year 2200 (if we do nothing) to just 18,000—not much more than what exists today.
One way to bring down a large piece of junk is to attach a small rocket engine and fire it toward Earth. Rocket engines are expensive, though, so space junk junkies are turning their attention toward tethers—among the cheapest methods of space propulsion.
Tethers sound impossibly great, like perpetual motion machines. Simply by attaching a single thin wire, which extends downward from a satellite for several miles, you can lower its orbit. No fuel required. As the wire drags through Earth's magnetic field, it generates a current, which acts as a natural brake against the orbital motion. Or, attach a "momentum tether" to a spacecraft, and it can be flung from one orbit to another, even from one tether to another, like Tarzan swinging from vine to vine. Again, no fuel required. Several experiments have already flown, on both U.S. and European spacecraft.