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.
There's just one trouble with space tethers. "They all fail!" Johnson laughs.
Despite the well-understood physics and the "no reason this shouldn't work" assurances from proponents, something always seems to go wrong. Last year alone, two space tether experiments returned the kind of mixed results that have become frustratingly common for the technology. In April, the Multi-Application Survivable Tether, built by Tethers
Unlimited, Inc. of Bothell, Washington, got hung up while one tether-deploying satellite was separating from a companion satellite. In September, a tether called YES2, built by European students, apparently extended to its full length of 18 miles in orbit, a new world record. But only apparently. The YES2 team had to piece together what happened, because the satellite attached to the end of the tether disappeared and hasn't been heard from since.
Rob Hoyt, president and chief scientist at Tethers Unlimited, admits that the glitches and half-successes haven't exactly inspired confidence. Yet he is not alone in believing that tethers will, after more flight experiments, eventually be certified for real work, including debris removal. The Japanese space agency JAXA is working on designs for a small satellite that could attach a tether to a piece of space junk to remove it from orbit. A tether test is planned in space, says project engineer Satomi Kawamoto, although no date has been set.
Johnson agrees that tethers should be able to handle the de-orbiting job, and says their first use may be as kits attached to new satellites as a means to dispose of them safely at mission's end. Hoyt's company has just such a system in mind—the Terminator Tether—which he hopes could be priced at under half a million dollars.