17 January 2018
There are two reactions to a paper regarding space junk that was published this week by researchers at China’s Air Force Engineering University. One, it seems good that the spacefaring nations of the world are considering ways to clean up an increasingly cluttered low Earth orbit. And the Chinese are putting forward a new plan, one that proposes to use a laser to zap the debris. Then there’s the other reaction—maybe it's not just about space junk.
There’s been a lot of talk in Washington D.C. and Asia about China’s increasing military posture when it comes to space. The 2017 U.S.-China Economic and Security Review Commission, which is appointed by Congress, reported that China is very interested in shooting down U.S. satellites. There are many ways to do this: launch a suicide sat to collide with the target, shoot sats with lasers from the ground or jam them with dazzling light and electronic noise. China, the commission says, is working on all of these methods—and more.
“China has pursued a robust and comprehensive array of counterspace weapons,” the report reads. “Including ground-launched anti-satellite (ASAT) missiles, ground-based directed energy weapons, ground-based satellite jammers, computer network operations, and co-orbital ASAT systems.”
What are co-orbital anti-satellite systems? Just a wonky way of saying the Chinese are researching and testing ways to use armed spacecraft to disable satellites in space in space. Could a laser be the weapon of choice?
Lasers in Space
The non-military application that the Chinese paper is advocating is certainly feasible. In fact, NASA scientists have studied the idea for a long while, publishing one comprehensive study as far back as 1996. Using a space-based system to clean space junk is smart, and studying up on it answers a lot of questions on how a less friendly ASAT version might work.
Forget every space explosion you’ve seen in Star Wars. Lasers are not good for blowing up things in orbit. Besides, why would you want to? The whole idea is to remove space junk, not reduce it to a bunch of smaller, harder-to-track bits of debris.
Rather, the idea is to use a laser to heat up just one portion of the space junk, burning off some material that then ejects into space. The force generated by that ejection could nudge the space debris toward earth and a fiery reentry that destroys it.
If you're going to attempt this, it would be nice to shoot lasers from the Earth that could clean junk from orbit. Ground based lasers already track satellites as they zip by overhead as part of the International Laser Ranging Service, headquartered in Maryland.
China operates ground-based lasers that track satellites as part of ILRS. Those trackers have a single watt of power behind them, but the Chinese also use 40-watt lasers to track smaller bits of space debris. Both of these power levels are too low, especially when applied for nanoseconds required to track them, to influence the orbits of their targets.
That is, if the system is used as intended. A recent RAND report cast some doubt over even this sanctioned laser activity. The low power lasers in the existing tracking stations could linger on some satellites could fry or temporary dazzle sensors, the report says, adding that the stations have already repeatedly illuminated U.S. reconnaissance satellites, worryingly but without effect.
“Although the energy that its lasers emit is currently very low, scaling the power up would not present a difficult technical challenge, and each station might have that capability even now,” the report says. The same can be said about U.S. ground-based satellite tracking stations, by the way.
Many of the satellites that are ripe military targets are in geosynchronous orbits. In other words, they park just far away enough from the planet that they stay over one spot on the spinning globe all the time. Targets in geosynchronous orbits include satellites that conduct reconnaissance, relay communications, and spot the heat plumes of missile launches. “We can not let that satellite be dazzled for 10 or 15 minutes,” Rep. Mike Rogers of Alabama, a member of the House Armed Services Committee, recently fretted about the missile-warning satellites. “It would be too late.”
Staying over one spot makes geosynchronous satellites sitting ducks but also protects them. They are father out than satellites in speedier low Earth orbits and that extra distance makes them harder to hit and saps the laser’s power. But putting a laser in space removes distance, atmosphere and attenuation from the equation.
So when the Chinese researchers propose putting a satellite in orbit that can hunt space debris, professionally suspicious minds will naturally drift to weaponization. But not all targets are created equally.
When it comes to space debris, anything between 1 and 10 centimeters is a threat to satellites in low Earth orbit, since these chunks are big enough to cause damage but too small to track easily. The international space surveillance system tracks anything larger than 30 cm at geosynchronous orbit.
Those bits are moving at 12km a second and therefore pack a lot of energy. Larger objects are more rare and can be avoided—the threat they pose is usually put in terms of collisions that cause more small pieces. Space debris laser systems are built to heat up parts of these small pieces and drive them into the atmosphere.
The prey of any self-respecting military ASAT laser, meanwhile, would be much larger. For example, the Space Based Infrared System Geosynchronous Earth Orbit satellites used to detect missiles weigh 5 tons. These are not rocket stages, lens covers, or pieces from orbital explosions spinning aimlessly in low Earth orbit. These are large, functional spacecraft.
But a satellite could still do damage even if its only weapon is a small laser fit for nothing more than cleaning up debris. Small, individual sensors could be vulnerable to laser attacks. If one piece of the target can be heated to the point where it burns off pieces, the force of that expulsion could make the satellite spin. That could be enough tampering to make the attack a success.
At the end of the day, the question of a Chinese space junk laser is a lesson in what experts call “dual use technology.” The lessons learned by creating a space junk hunter could be adapted to militaristic ends. It joins many other space-related developments that fall into both realms.
Eventually, the question of whether or not a proposed
system is a threat or not falls away from engineers and scientists and
enters the realm of diplomats and politicians. And in those disciplines,
solid equations are hard to come by.