By Jon Cartwright
Scientists in the United States have devised a new way to avoid collisions among space debris, and possibly even reduce the amount of debris in orbit. The method uses a medium-powered, ground-based laser to nudge the debris off course -- but some are concerned that the laser could be used as a weapon.
Debris orbiting Earth is a mounting problem. Two years ago, a satellite owned by the communications provider Iridium, based in McLean, Virginia, smashed into a defunct Russian satellite at ten times the speed of a rifle bullet, putting an end to the 'big sky' theory that assumed space was too vast for chance collisions. That incident alone created more than 1,700 pieces of debris, raising the total amount by nearly 20%.
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Space analysts are particularly concerned about the possible onset of Kessler syndrome, when enough debris is present to make collisions so likely there would be an avalanche effect that would leave the Earth's orbit uninhabitable for satellites.
Sweeping up the mess
Scientists at NASA have considered using a ground-based laser to mitigate debris collisions before. However, in their 'laser broom' concept, a powerful, megawatt-class laser would vaporize the surface of a piece of debris that is heading for another, causing the debris to recoil out of harm's way. But critics argued that the laser could be used as a weapon, as it could easily damage an enemy's active satellites. Indeed, both the United States and China have in the past 15 years been accused of testing the ability of ground-based lasers to 'dazzle' satellites and render them inoperable.
Now, James Mason, a NASA contractor at the Universities Space Research Association in Moffett Field, California, and his colleagues have come up with a variation on the laser broom concept that they claim is unlikely to be useful as a weapon. In a paper uploaded to the arXiv preprint server, Mason and colleagues suggest using a medium-powered laser of 5-10 kilowatts to illuminate debris with light a few times more intense than sunlight, imparting just enough momentum to nudge the debris off course. "We think this scheme is potentially one of the least-threatening ways to solve a problem that has to be addressed," says Mason.
In the researchers' proposal, a piece of debris that has a high risk of collision would be tracked by another laser and a telescope. As the debris comes over the horizon, technicians would switch on the main laser and illuminate the debris until it reaches its highest point. If the debris isn't nudged far enough to avoid a collision the first time, the technicians would repeat the procedure for several days until the collision risk becomes negligible.
Risk reduction
With just one laser facility, Mason's group says, the number of debris collisions could be almost halved. What's more, by mitigating the number of collisions, the amount of debris would lessen as it slowly burns up in Earth's atmosphere. And that would avoid the onset of Kessler syndrome, the researchers say.
All the experts in space debris contacted by Nature said that the new proposal is feasible, but still has problems. "It'll be ineffective against dense objects that are too heavy to move," says William Priedhorsky of Los Alamos National Laboratory in New Mexico. "To use a medical analogy, they propose not to cure the disease, but to manage it."
And some are concerned that the laser could still be used to push enemy satellites out of orbit. Christophe Bonnal, a debris expert at the French space agency CNES, doesn't buy the researchers' claim that the laser's power would be too low for anti-satellite uses. "Let's be logical," he says. "If the power is low, you'll have no effect on the debris."
But Hugh Lewis, an engineer at the Southampton University, UK, "cautiously" welcomes the idea. "Any method that aims to address the growing debris problem should be taken seriously, I think," he says.
