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" Close calls in orbit happen all the time--scientists estimate that launch vehicles and other objects come within striking distance of one other over 1000 times a day. So when tracking reports on Feb. 10, 2009, predicted that Iridium 33, a 12-foot-long, 1200-pound communications satellite, and a 1-ton Russian military sat, Kosmos 2251, would pass within less than half a mile of each other, no one was alarmed. It wasn&amp;#039;t the closest call predicted for that day, or even the closest pass for any of the 66 Iridium satellites that coming week. But at the time of the predicted approach, Iridium 33 fell silent.  The two satellites had collided 500 miles above Siberia at 26,000 mph, generating a debris cloud that spread around the Earth in just a few hours. The junk was in the orbital path of the Hubble Space Telescope and just 250 miles higher than the orbit of the International Space Station. It was the first time two satellites were known to have collided, which is remarkable considering there are about 3000 of them circling the Earth. And orbiting with the sats are an estimated 16,000 other man-made objects, mostly junk. Depending on the height of orbit--the higher the debris, the longer it takes to decay--fragments may linger in space for centuries.  If enough debris accumulates, we risk setting off a chain reaction called the Kessler syndrome, in which satellite fragments damage other satellites, producing more debris and so on, until low Earth orbit (LEO) is so cluttered that satellite use and space travel become impossible. With LEO getting messier by the day, the February collision raises a thorny legal question: Who is responsible for damages caused by collisions in space? Contrary to widespread opinion, outer space is not a lawless region. In fact, it&amp;#039;s governed by international law--and, in the case of U.S. spacecraft, or the U.S. parts of the International Space Station, by American law. Damage to and by spacecraft is covered by the 1972 Convention on International Liability for Damage Caused by Space Objects. Under that treaty, liability for damage caused to people or property on the ground is &amp;quot;absolute&amp;quot;--meaning that the country that launched the spacecraft is liable for damages even if there was no negligence. The same is true if a space object strikes an aircraft. If you operate a spacecraft, you must realize it can do damage wherever it crashes. Expecting people on the ground to be on guard against crashing satellites is asking a bit much.  When it comes to sats in space, however, things aren&amp;#039;t so straightforward. When one spacecraft collides with another, there&amp;#039;s only liability if the spacecraft operator is negligent. But what constitutes negligence in this context is not clear. Operating a spacecraft in a way that poses a foreseeable risk to others probably qualifies, but in this case the Russian satellite was described as defunct, meaning that it had either broken down or run out of maneuvering fuel. There are many such satellites in orbit, and although good practice calls for their operators to either deorbit them or boost them into harmless parking orbits, this is not always possible. There are no standards in place that require space operators to ensure that satellites don&amp;#039;t remain in high-traffic orbits at the end of their lives, so it would be hard to argue that failure to do so constitutes negligence. One way to prevent space collisions is better traffic control. Right now, satellite operators don&amp;#039;t have precise orbital parameters for satellites that might pose a risk of collision. Only a few countries--chiefly the United States, and to some degree Russia and China, as well as Europe--have access to precise tracking data, and that information is kept secret lest it reveal details about the sophisticated instrumentation used to gather it. Military planners have long talked about the need for &amp;quot;space situational awareness,&amp;quot; and the military tracks satellites for its own purposes, but the increasingly crowded nature of near-Earth orbits, plus the threat posed by collision debris, has led many to argue that we need similar tracking capability for the civilian sector. The cheap way to do this is to make some of the military data public; the expensive way would be to build a separate civilian system.  Planners in the U.S. and other militaries have good reason to want to keep adversaries guessing about their surveillance capabilities. But they also have good reason not to want to see space filled with debris that is as deadly to their own satellites as it is to civilian craft. The European Space Agency is working on a plan to track 1000 satellites in order to prevent collisions, and all nations that send craft to space should ensure that initial orbits are carefully chosen with an eye toward reduced risk of collision.  But even if these good practices don&amp;#039;t come to pass, there&amp;#039;s another way to deal with space junk: Clean it up. Specialized spacecraft are designed to rendezvous with intact, nonoperational satellites and take them back to Earth or place them in safer orbits. Cleaning up fragments is much harder. Even small debris traveling at tremendous velocities can wreck a spacecraft, but it&amp;#039;s impossible to rendezvous with a paint speck or a lug nut. Some have proposed incinerating debris with lasers, or capturing it with big blobs of orbiting aerogel. But these and other solutions are expensive and may produce more debris from collisions. Complicating an already complex situation is the fact that debris fragments are often from unidentifiable sources, and who should be responsible for getting rid of the mess is a gray area. Some scholars, including me, have proposed that countries be taxed for debris cleanup based on the amount of material left in orbit, but such proposals remain purely academic. Unfortunately, current space law doesn&amp;#039;t allow for the best possible space-junk solution: salvage. Under the 1967 Outer Space Treaty, nations retain &amp;quot;jurisdiction and control&amp;quot; over their spacecraft even when they are inoperable--so a salvage company wouldn&amp;#039;t be able to take title or claim an award for recovering a defunct craft, as is done on Earth. A space salvage law might even help commercial space efforts by providing yet another moneymaking option; governments, or insurance companies, could pay private operators a bounty for eliminating space junk. Though space lawyers have been arguing for years that the proliferation of space junk makes some sort of salvage law necessary, and despite the obvious commercial benefits, there has been little progress.  What&amp;#039;s clear is that space is getting crowded, and the problem urgently needs attention from all spacefaring nations, lest we find ourselves earthbound under a shroud of orbiting trash. Glenn Harlan Reynolds, a PM contributing editor, teaches space law at the University of Tennessee.  RELATED STORIES \u2022 PLUS: Protecting Spacecraft From Orbiting Junk  \u2022 EARLIER: Space Debris a Growing Problem  \u2022 VISUAL: Anatomy of a Destroyed Satellite  \u2022 FOLLOW US: Popular Mechanics is Now on Twitter!  \u2022 FRIEND US: Find PM on Facebook   (Illustration by John Hendrix) "^^<http://www.w3.org/2001/XMLSchema#string>
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