In October 2006, the military affairs journal Jane's Intelligence Weekly reported that U.S. Army officials detected a “sudden decline” in the effectiveness of some its spy satellites throughout the fall as they passed over China.
Before receiving an explanation from the Chinese military, initial fears were that the country had intentionally aimed ground-based lasers at the U.S. satellites to temporally disrupt their ability to collect information, or worse, materially damage them.
Was this a hostile attempt by the Chinese to test its laser anti-satellite (ASAT) weapons, obstructing U.S. military reconnaissance missions in Low Earth Orbit?
Probably not, said a pair of military affairs experts at a recent AAAS co-sponsored briefing. More likely, they said, the Chinese may have been trying to glean details of the satellites orbital path or, perhaps, conducting a test to see if we could detect their laser against our spy satellites. But the case does raise questions about protecting satellites in orbit.
Held 27 August at the Fairmont Hotel in Washington, D.C., the event was sponsored by the AAAS Center for Science, Technology, and Security Policy and the Center for Media and Security.
Yousaf Butt, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, and Brian Weeden, a technical adviser for the Secure World Foundation, said that the U.S. military noticed unexpected features in some of its satellite imagery of China taken in fall 2006.
According to the 16 October 2006 article in Jane's, previous tests by the Chinese had been reported, but this newest round of “laser attacks were at least partially effective.”
Soon after that report appeared, U.S. Army Command Sergeant Major David Lady, who was quoted in the Janes's article, said that that sensors at the Reagan Test Site on Kwajalein atoll in the South Pacific “sensed the projection of beams against the spacecraft (spy satellite).” The sensors, used to detect unusual activity surrounding U.S. satellites, indicated that photons, or a laser beam, had “illuminated” the satellite.
Some news outlets identified the events as China testing a new type of warfare. Defense Daily reported that China had “fired high-power lasers at U.S. spy satellites flying over its territory in what experts see as a test of Chinese ability to blind the spacecraft.” But Butt and Weeden have a less alarmist explanation—satellite laser ranging (SLR) technology.
SLR stations aim ultra-short laser pulses at the satellites and time how long it takes the light to return to Earth. By documenting the round-trip time of the pulsed light, scientists can gather information about the Earth's gravity, calibrate sensitive instruments, and also obtain satellites' orbital parameters. Butt said that there are about 40 SLR stations around the world including five in China
“Not all lasing in space is malicious,” said Butt. At Chinese SLR stations, he added, the lasers are typically pulsed at 10 times per second and relatively weak, around one watt average power—though the power of a given pulse may be much higher. If Chinese SLR activities are the explanation, then they need not even have directly illuminated the satellite optics, he explained, but the scattered light may still have caused the reconstructed image to be overexposed in a pulsed fashion.
Butt said that if the Chinese were trying to use this laser to damage satellites optics, it would have to know exactly when the mirror was aimed directly at the SLR site. And even then, he puts the chance of permanent optical damage at 1 in a 1000.
Without a formal explanation from the Chinese, Butt and Weeden speculated that the most likely explanations were either that the Chinese were interested in obtaining the orbital parameters of the satellites or that the Chinese intentionally fired the laser at the satellite from their SLR station to test if the U.S. could detect it.
“The Chinese most likely knew that they wouldn't be able to damage our satellite,” he said. “But it does point out the U.S. must do more to ensure the safety and stability of its satellites.”
Added Butt: “We can't ban lasers in space, but we can regulate them.”
Benn Tannenbaum, associate program director for the AAAS Center for Science, Technology and Security Policy, stressed the importance of developing international regulations for what is, and what is not, allowed in space.
“While the SRL lasers do not pose much of an immediate threat to U.S. satellites, the incidents exposed a potential vulnerability of our space surveillance programs,” said Tannenbaum. “And now it's important for policy officials and military experts to develop strategies to mitigate the threat.”
Weeden, who served nine years as an officer in the U.S. Air Force working in space and intercontinental ballistic missile operations, said that while space law is a “developing entity,” there are some guidelines already in place to regulate space lasing.
In 2000, the U.S. Department of Defense established the Laser Clearinghouse Program, which was designed to protect satellites against accidental lasing which could damage or disrupt satellites. Based at Vandenberg Air Force Base in California, the program requires all U.S. government organizations, including the U.S. military, to register their lasers which might damage satellites. A similar program could be considered for implementation world-wide to safely manage all peaceful laser firings into space.
Those lasers deemed incapable of damaging satellites in orbit are waived from firing restrictions. Weeden said that laser operators sending beams potentially powerful enough to damage a satellite are given “safe windows” in which they can be fired. These safe windows are determined by looking at the positions of the estimated 900 active satellites in orbit, of which about 430 are owned by the U.S., according to the Union of Concerned Scientists.
“There are a lot of peaceful uses for lasers in space, so we need to be careful not to overly restrict their use,” said Weeden. For example, using lasers to communicate between the ground and orbiting satellites is very secure and potentially a way to alleviate the spectrum crunch.