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Catching Satellite Debris With Smart Nets

© Norman Sperling, February 6, 2011

Thanks to Dennis Normile, the Science Insider of Science Magazine, we've learned that last week's flap over satellite-catching nets began with mistranslation and ballooned as journalists and bloggers skipped fact-checking and blundered directly into copying and embroidery.

As far as I know, space nets have not yet been tried. I think they ought to be.

DARPA described the problem this way in Solicitation Number DARPA-SN-09-68, September 17, 2009: "Since the advent of the space-age over 5 decades ago, more than 35,000 man-made objects have been cataloged by the US Space Surveillance Network. Nearly 20,000 of those objects remain in orbit today, 94% of which are non-functioning orbital debris. These figures do not include the hundreds-of-thousands of objects too small to be cataloged, but still large enough to pose a threat to approximately 900 operational satellites in orbit around the Earth."

The zone needing the most cleanup is Clarke orbit, where dead geosynchronous satellites clutter longitude stations at which new live ones could accomplish something. Geosynchronous orbit is full of hulks, defectives, and need-fixins. Clarke orbit is high and therefore expensive to reach.

Several Low-Earth Orbits (LEO) also beg for cleanup:
* the planes littered with fragments of the February 10, 2009, collision between Iridium 33 and Kosmos 2251;
* P78, the Solwind satellite, which the Reagan Administration attacked on September 13, 1985, while it was still producing good scientific information;
* and FY-1C, which China used similarly on January 11, 2007, to demonstrate that it could be as malicious and dumb as the US.

Derelicts beg for removal from certain other orbits, too.

Since at least the 1980s, satellite managers have warned that space debris is growing to the point that LEO will be too hazardous to be worth placing satellites in. I attended a conference on that in 1988 and that wasn't the first. At orbital speed, a teensy paint chip has enough impact to destroy a big expensive satellite. That's from good old Newton's second law: f = ma. Even if the mass is tiny, acceleration is so huge that the resulting force can blast a satellite into a whole lot more fragments, each of which can wreck something else the same way. Studies indicate that the effect cascades to make LEO unusable. Weeding out hulks greatly truncates the risk of cascading shrapnel later.

But catching a satellite is hard. Experts pinpoint satellites' positions to a kilometer or better. Most predictions are not likely to be accurate to 300 meters, much less the actual size of the satellite. So a catcher/killer has to be able to steer to actually capture its prey. Astronauts do this all the time, but I don't remember hearing that capture has been achieved by remote robotics yet. The targets are too small, often tumbling, and hard to attach to.

For satellites that we simply want to de-orbit and burn up by re-entering Earth's atmosphere, I've long thought that nets are the most-likely answer.

The spacing between strands should take the target into account. If the target is 5 meters across, a weave with gaps of 20 to 50 cm should hang onto virtually all parts. Wide gaps give the net light weight and compact storage.

Several different fibers can be interwoven to optimize strength. I would suppose that Kevlar fibers and titanium wires would be good to start with. Elasticity would probably help, and since latex wouldn't survive solar ultraviolet light, perhaps springy coils might work. Several different weaves, with and without knots, can be explored to optimize strength-to-weight ratio.

A net that closely matches its prey's orbit can capture its target because the difference in velocity is small and therefore unlikely to rip or punch through the net. With a slow approach velocity, too, the relative positions can be optimized, so that encounter occurs within the net's spread. Except for rare urgent cases, several attempts could be made per satellite, refining the orbital targeting from each miss to achieve eventual capture.

A "dumb" net probably would not suffice. But a net doesn't have to be dumb. Pack it in a bus that has, on its back, a lot of small rocket engines that can swivel. Include some video cameras. When the net approaches its prey, unfurl it from the bus. Each of those little rocket engines is attached to a part of the net - most along the edge, one in the center, perhaps a few more between the center and edge. These can fire independently, to shape and cast the net for greatest effect (flat? a bowl like a parachute or jellyfish? something else?). Technicians on the ground can steer and operate the rockets.

When the satellite punches into the net, that forces most of the net to trail behind. Various edge-rockets could make the net wrap around the satellite, bundling it tight, like a spider secures a fly. Several rockets will end up on the outermost layer. Several more will find themselves not obstructed by the wide-woven layers above them. Properly aimed and timed, outermost and unobstructed rockets can fire to lower the perigee into the atmosphere to burn up on re-entry. If the bundle is rotating, it may be possible to use thrust from many, most, or even all of the rockets ... just not all at once. There should be plenty of time so that should be OK.

Placing too many rockets around the net greatly increases their risk of banging into one another instantly after the target plunges into the net. Too few could reduce the possible firing geometries for later steering.

Even-tougher nets might be deployed to capture satellites crossing the nets' orbital plane. Encounter velocity will be much larger. But this greatly increases the number of satellites that can be captured from a single deployment of a bundle of net kits.

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