Space-faring nations: Clean up low Earth orbit or you're grounded.
- By Tony Reichhardt
- Air & Space magazine, March 2008
(Page 3 of 4)
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.
Installing a tether on the ground before launch is one thing. Attaching a tether, or any kind of de-orbit package, to a crumbling, tumbling rocket stage in orbit is another—particularly if the target has nothing for the package to hold on to, because it wasn't designed to be touched ever again by human or robot after being put in space.
JAXA engineers are looking at several options for wrangling such uncooperative targets. In one scheme, the junk removal satellite would dampen any tumbling motion of a large object by shooting small projectiles at it—ice pellets work nicely—before moving in to attach a tether.
In 2003, Tethers Unlimited designed a system called GRASP (Grapple, Retrieve, And Secure Payload), which used a net made of Kevlar yarn to snare a small object and steady it enough for a tether to be attached. With funding from the Defense Advanced Research Projects Agency, the company got as far as testing a prototype during short stretches of weightlessness on a zero-G airplane flying parabolic arcs. It worked, says Hoyt, but DARPA hasn't come through with money for a follow-on test.
And that, as usual, is the rub. No public or private entity has volunteered to research, let alone build, an operational junk removal system.