In space, a 15,000-mph collision between a missile and an interceptor rocket makes no noise. But down on Earth, a scientific and political debate is getting very loud.

This 08 March 2006 US Department of Defense handout image shows a Standard Missile - 3 (SM-3) being launched from the USS Lake Erie (CG 70) in a Missile Defense Agency and Japan Defense Agency joint test in the Pacific.  [Agencies]

Barring low clouds, a test target missile will launch Thursday from Alaska's Kodiak Island and should encounter an interceptor rocket from Vandenberg Air Force Base in California, says US Missile Defense Agency spokesman Richard Lehner.

It's a technology that scientists say won't work effectively.

Such midcourse interceptors, designed to destroy their targets in space at their trajectory's peak, already are in place in Alaska and California.

A plan to place a $3.5 billion interceptor base in Poland by 2012 has generated friction with Russian President Vladimir Putin. Missile defense will be discussed at the G-8 summit June 6-8 in Germany.

The interceptor test is part of the agency's plan to spend $49 billion over the next five years to deploy and test missile defense technologies.

But physicists such as Frederic Lamb of the University of Illinois Urbana-Champaign, who co-headed a 2003 American Institute of Physics report on missile defense, are saying that midcourse interceptors are losers from a physics standpoint.

According to Lamb: Aside from the difficulty of the interceptor connecting with the missile when both are free-falling outside of the Earth's atmosphere, it will be impossible to detect a decoy missile. Atmospheric drag identifies the qualities of a missile, such as size, speed and heat. Without those signatures, a balloon, for example, would register on radar the same way a rocket would.

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The test will take place about 100 miles above the Pacific Ocean, where the atmosphere is thin. Analysts consider anything above 62 miles altitude to be a region of diminished or non-existent air drag, says physicist David Wright of the Union of Concerned Scientists, a science advocacy group in Cambridge, Mass.

"Outside the Earth's atmosphere, the drag is zero on a warhead and a decoy alike," Lamb says. "This is an intractable problem. There is no signature that distinguishes a warhead that can't be cheaply counterfeited."

Lehner says by e-mail: "Critics forget that we have a variety of sensors that are extremely capable of discriminating countermeasures and decoys from lethal warheads."

Rep. Ellen Tauscher, D-Calif., chairwoman of the House Armed Services Subcommittee on Strategic Forces, says, "We've invested billions in missile defense since the 1980s." She favors more proven shorter-range missile defense systems: "It's got to be better than a science project."