The growing cloud of space junk surrounding the Earth is a hazard to spaceflight, and will only get worse as large pieces of debris collide and fragment. NASA space scientists have hit on a new way to manage the mess: Use mid-powered lasers to nudge space junk off collision courses.

The U.S. military currently tracks about 20,000 pieces of junk in low-Earth orbit, most of which are discarded bits of spacecraft or debris from collisions in orbit.

The atmosphere naturally drags a portion of this refuse down to Earth every year. But in 1978, NASA astronomer Don Kessler predicted a doomsday scenario: As collisions drive up the debris, we'll hit a point where the amount of trash is growing faster than it can fall out of the sky. The Earth will end up with a permanent junk belt that could make space too dangerous to fly in, a situation now called "Kessler syndrome."

Low-Earth orbit has already seen some scary smashes and near-misses, including the collision of two communications satellites in 2009. Fragments from that collision nearly hit the International Space Station a few months later. Some models found that the runaway Kessler syndrome is probably already underway at certain orbit elevations.

"There’s not a lot of argument that this is going to screw us if we don't do something," said NASA engineer Creon Levit. "Right now it's at the tipping point ... and it just keeps getting worse."

In a paper submitted to Advances in Space Research and posted to the preprint server arXiv.org, a team led by NASA space scientist James Mason suggests a novel way to cope: Instead of dragging space junk down to Earth, just make sure the collisions stop.

"If you stop that cascade, the beauty of that is that natural atmospheric drag can take its natural course and start taking things down," said William Marshall, a space scientist at NASA and coauthor of the new study. "It gives the environment an opportunity to clean itself up."

Simply keeping new fragments from forming can make a big difference for orbital safety, Levit said. Because objects with more surface area feel more drag, the atmosphere pulls down the lightest, flattest fragments of space junk first. When big pieces of debris break up into smaller ones, the pieces become harder and harder to remove.

Worse, the pieces left behind are often the most dangerous: small, dense things like bolts.

"If one collides with a satellite or another piece of debris at the not-unreasonable relative velocity of, say 5 miles per second, it will blow it to smithereens," Levit said.

In the new study, the researchers suggest focusing a mid-powered laser through a telescope to shine on pieces of orbital debris that look like they're on a collision course. Each photon of laser light carries a tiny amount of momentum. Together, all the photons in the beam can nudge an object in space and slow it down by about .04 inches per second.

Shining the laser on bits of space litter for an hour or two a day should be enough to move the whole object by about 650 feet per day, the researchers show. That might not be enough to pull the object out of orbit altogether, but preliminary simulations suggest it could be enough to avoid more than half of all debris collisions.

NASA scientists have suggested shooting space junk with lasers before. But earlier plans relied on military-class lasers that would either destroy an object altogether, or vaporize part of its surface and create little plasma plumes that would rocket the piece of litter away. Those lasers would be prohibitively expensive, the team says, not to mention make other space-faring nations nervous about what exactly that military-grade laser is pointing at.

The laser to be used in the new system is the kind used for welding and cutting in car factories and other industrial processes. They're commercially available for about $0.8 million. The rest of the system could cost between a few and a few tens of millions of dollars, depending on whether the researchers build it from scratch or modify an existing telescope, perhaps a telescope at the Air Force Maui Optical Station in Hawaii or at Mt. Stromlo in Australia.

"This system solves technological problems, makes them cheaper, and makes it less of a threat that these will be used for nefarious things," said space security expert Brian Weeden, a technical adviser for the Secure World Foundation who was not involved in the new study. "It’s certainly very interesting."

However, "I don't think this is a long-term solution," Weeden said. "It might be useful to buy some time. But I don't think it would replace the need to remove debris, or stop creating new junk."

Don Kessler, from whom the Kessler syndrome takes its name, agrees, and points out that laser light isn't forceful enough to divert the biggest pieces of junk.

"The only complete solution to is to prevent collisions involving the most massive objects in Earth orbit," he said.

Image: ESA

Citation:

"Orbital Debris-Debris Collision Avoidance." James Mason, Jan Stupl, William Marshall and Creon Levit. Submitted to Advances in Space Research.

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