T STARTS with a handful of missiles taking out orbiting satellites. The shrapnel destroys more satellites and triggers a chain reaction, generating yet more high-speed debris. Before long, we're trapped on Earth in the nightmare scenario: space is a no-go zone, made too dangerous to use for generations by countless chunks of junk.
That is the risk of "space war" that the US White House seeks to avoid with its National Space Policy, released on 28 June. The policy reopens the door to international arms-control measures against space weapons, reversing the 2006 policy of George W. Bush's administration, which ruled out such talks.
Crafting such an agreement is easier said than done, thanks to issues created by technologies that are dual-use, in that they can be used for peaceful or defensive purposes as well as for attacking other countries' satellites.
"Dual-use technology will hugely complicate the issue of agreements," says Joan Johnson-Freese of the US Naval War College in Newport, Rhode Island. For example, missiles that can shoot down other missiles to shield a country from attack could also be used to destroy a satellite in space.Indeed, there is "no fundamental difference" between the missiles used in each application, says Ray Williamson of the Secure World Foundation (SWF) in Washington DC.
The US demonstrated this in 2008 when it shot down one of its own defunct satellites, called USA 193, using an interceptor fired from a ship equipped with the US's Aegis ballistic missile defence system. Officials said this was to prevent the satellite, which was on a decaying orbit, from falling to Earth in a populated area and causing harm by releasing its toxic hydrazine fuel.
Some analysts speculated that it was in fact intended to demonstrate the US's anti-satellite capabilities, in response to a 2007 test in which China destroyed one of its own satellites with a missile.
USA 193 was destroyed in an orbit low enough that the debris quickly entered the atmosphere and burned up, but the Chinese test created thousands of pieces of long-lived debris, many of which remain in orbit.
Russia, too, has tested anti-satellite weapons, though not since the cold war. It also has a missile defence system that could in theory be used to knock out satellites, though this has not been put to the test. India may soon have the ability to wage space war too, having recently announced that it is developing anti-satellite and missile-defence interceptors.
Eliminating all missiles that can hit satellites would require getting rid of these missile-defence systems, something countries that have them will almost certainly refuse to do, particularly the US. Any treaty that attempts to ban such missile defence would "stir up a hornet's nest on Capitol Hill", says Michael Krepon of the Henry L. Stimson Center in Washington DC.
On top of that, the technology is still spreading. The US has sold or is selling the Aegis system (pictured, below) to countries including Norway, Spain, South Korea and Japan. Though the current system uses relatively short-range missiles with limited ability to reach the altitudes where satellites orbit - USA 193 was exceptionally low, on the verge of re-entering the atmosphere - a planned system upgrade "would have much longer range and would be able to reach probably nearly all low-Earth orbiting satellites, should one want to use it that way", says Laura Grego of the Union of Concerned Scientists in Cambridge, Massachusetts.
Other double-edged swords are satellites designed to autonomously navigate their way to the vicinity of another satellite in space, a technology that the US demonstrated by flying a mission called XSS-11 in 2005.
A country could use such technology to inspect and repair one of its own malfunctioning satellites or to grab it and drag it into the atmosphere to dispose of it without adding to space junk. But the technology could also be used to interfere with or damage another country's satellite, says Brian Weeden of SWF. "If you can remove a piece of debris from orbit, then if you really wanted to you could probably remove an active satellite maliciously," he says. "The rendezvous technology is spreading to a lot of places, because people are seeing economic incentive in on-orbit servicing."
If you can remove a piece of debris from orbit, you can maliciously remove an active satellite
On 16 June, for example, the Swedish Space Corporation launched a couple of satellites called Mango and Tango to demonstrate rendezvous technology in a joint project called Prisma, with the German Aerospace Center, France's National Centre for Space Studies and the Technical University of Denmark. Even as early as the cold war, a Soviet satellite was designed to closely approach a target satellite, then release pellets to act like a shotgun blast, colliding with the satellite at high speed and destroying it.
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