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Image above: The International Space Station's length and width is about the size of a football field.
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On your street, littering is an annoyance. But in space, it could be deadly. Bits of junk from past missions, dead satellites and old rocket boosters now amount to some 30,000 tons of debris in orbit, ranging from lost nuts and bolts to entire defunct satellites. That's a problem for astronauts and working spacecraft, because even a BB-sized piece of material moving at orbital speeds can pack a punch. According to NASA, small pieces of debris can move as fast as 17,500 miles per hour—nearly eight times the speed of a bullet fired from a military rifle.
Proposals to get rid of space junk include nets in space and solar sails. The problem with those concepts is the need to match orbits with the debris—maneuvering to do that requires lots of fuel or, in the case of solar sails, a lot of pirouettes in space. So some scientists and engineers are proposing something simpler: shoot the stuff down.
This week, a team led by Toshikazu Ebisuzaki of Japan's RIKEN research institute proposed using a space-based ultraviolet laser aimed with a telescope. To demonstrate the concept, they want to launch a fiber-optic laser to the International Space Station and pair it with an already-budgeted and approved telescope, the Extreme Universe Space Observatory (EUSO), due to be mounted on the ISS and scheduled for launch in 2017.
"We want to use the ISS as a platform and test bed," says Ebisuzaki, whose team described their idea last month in Acta Astronautica. EUSO was originally designed as a cosmic ray detector. When the high-energy rays hit Earth's atmosphere, they create a UV glow, and the telescope is able to pick up such bursts. Ebisuzaki's team thinks it could do a kind of double duty, because the instrument has a wide field of view.
The plan is to for the telescope to look for debris when the ISS is on the night side of Earth but is still able to look over the horizon at stuff that's lit by the sun. This happens for about 5 minutes of every 90-minute orbit. Once the telescope sees something, a relatively weak laser pulse can be fired to light up the object. The beam would reflect off it and allow the system to get a better read on how far away it is and how fast it is moving—basically, a UV version of radar.
At that point, the fiber-optic laser could fire more pulses, this time with more power and a tighter beam. Each pulse would last only about a tenth of a nanosecond, but thousands would be fired. The laser wouldn't disintegrate the debris outright, but would instead vaporize a tiny portion of it. The vapor then acts like a tiny thruster burn, slowing the pieces down enough that they fall into the atmosphere and burn up.
Even if the laser on board the ISS proves its potential, the team won't get cocky. Their next step would be to launch a small independent satellite with a similar set-up to further test the concept. This "mini-EUSO," as Ebisuzaki calls it, would orbit at the same approximate altitude as the ISS and would serve as a more "real world" demonstration. If that stage is successful, the team would finally send up a full-size debris-clearing satellite, which would go higher than the ISS to about 500 miles, just above where the debris density tends to peak.
Some scientists, though, have expressed skepticism about details of Ebisuzaki's plan. Claude Phipps, managing partner at Photonics Associates, a Santa Fe company that studies laser propulsion, proposed a similar system called L'ADROIT in 2014. Phipps' proposal involves a visible-light telescope that would travel in a polar orbit (perpendicular to the Equator), because that's where most of the debris tends to be.
Some of the problems with the EUSO plan involve the physics of where the ISS orbits, which will limit the project's effectiveness, Phipps says. The ISS is below much of the problematic debris, and the telescope is designed to point toward the Earth, so catching targets at the right angle will be hard. Also, the push from vaporization might be in the wrong direction to efficiently de-orbit space junk—the beam wouldn't always be hitting it head-on, which is what you want to send the pieces into the atmosphere.
Another issue is how well the fiber-optic lasers would work outside of the lab. Such lasers are made from multiple fibers bundled together, and the light traveling through them will have to be precisely timed. "If you have say, ten fibers, or ten thousand, in some kind of circular array, the wave fronts have to come out at the same moment," Phipps says. That requires a lot of fine-tuning. Phipps' idea is to use a more conventional single-beam laser, although Ebisuzaki says the fiber-optic system allows for better heat dissipation.
Still, a good part of Ebisuzaki's idea is the staged approach, which allows them to show what works as they scale up, Phipps says. "[Ebisuzaki] also has a pretty clever design for the telescope," he adds.
Removing space debris with such systems is garnering more interest—Phipps is going to a meeting of experts in France next week to discuss that very topic. Besides lessening the danger to astronauts, getting old satellites out of valuable orbital real estate would be an economic boon, since those nonfunctioning hulks get in the way. And the danger that a low-Earth orbit satellite like those in the GPS system will be shattered by a stray hunk of metal or even another satellite is very real. It happened once in 2009, when an Iridium communications satellite and an older Russian probe collided, spreading even more debris far and wide.
With ideas and proposals piling up, Phipps hopes that space agencies will start to move faster and implement a plan before space junk takes a deadly toll. "I'm afraid it's going to take a fatality," Phipps says. "But I hope not."
Quelle: Smithsonian
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