Space-based missile defenses systems could produce dangerous space debris that would make low-Earth orbits permanently unusable, according to Joel Primack of the University of California at Santa Cruz. The orbiting anti-missile battle stations proposed by the U.S. could result in so much space junk that other satellites wouldn?t have room to operate.

Most critics of the U.S. military’s missile defense program focus on the imperfect trial results of their ground-based interceptors, which have destroyed dummy warheads in four out of six trials. But in tests, neither the target or the interceptor reach orbital velocity, so most of the debris they produce falls to the ground.

Primack’s concerns focus on space-based interceptors and high-energy lasers that the Missile Defense Agency is developing to target missiles in their vulnerable boost phase, and on measures other countries might take to block their deployment.

The most dangerous type of space debris are particles smaller than about 4 inches, which are too small to track from the ground but can carry as much destructive energy as a safe dropped from a three-story building. NASA estimates low-Earth orbit already contains 100,000 particles 4 inches or less, which are a threat to orbiting satellites. Most of these were produced by explosions of the spent upper stages of rockets.

The problem with the space-based missile defense systems is that thousands of lasers or interceptors would have to be deployed, in order to ensure adequate coverage and protection. Deploying that many would mean increasing current launch rates by at least a factor of 10. The Missile Defense Agency plans to begin testing a space-based “kinetic energy kill vehicle” by 2006; tests of a space-based laser would not start until at least 2012. These tests will produce debris, although most of it may fall back to the ground, depending on the trajectory.

The worst scenario would be an attempt to disable U.S. orbital defenses. “It’s easy to attack satellites – basically you put gravel into orbit,” says Primack. A multitude of small, untrackable objects would hit existing satellites and spent boosters, destroying them and producing an increasing cascade of debris.

NASA says debris in low orbits will fall to Earth within several years, but objects in higher orbits will survive for a century or more. Primack warns that, “Once we fill space with debris, it’s permanent.”

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