Discovery serves as target for low-power `star wars' laser test

The Pentagon is trying to take a small step forward in its quest to develop weapons for a shield against nuclear missiles. Scientists Wednesday shot a laser from a mountaintop in Hawaii and tried to hit a reflector on the orbiting shuttle Discovery. But the test failed. Experimenters will try again Saturday.

The experiment, carried on board the shuttle as part of President Reagan's Strategic Defense Initiative (SDI), popularly known as ``star wars,'' is the latest attempt by scientists to overcome the distoring effects of the earth's atmosphere on laser light.

Such a feat is considered crucial in determining if any ground-based laser system could be used to knock out enemy missiles or satellites in space.

Under one envisioned scheme, high-powered lasers would be placed on mountaintops. The lasers could be fired into space and bounced off gigantic mirrors, which would redirect the beams to hit incoming missiles. This won't work, however, unless the beams can be shot through the turbulence in the air -- the same phenomenon that makes stars appear to twinkle -- without scattering.

Scientists have already made some headway in this area. In one experiment early last year, a low-power laser was successfully bounced between a defense facility in Hawaii and an aircraft flying 20,000 feet overhead.

The shuttle experiment, however, is designed to carry ability of lasers to compensate for atmospheric distortions and track moving objects another step farther. In this test, a low-power (four watt) beam of green laser light was to be flashed from a mountain site on Maui, Hawaii, to a special 8-inch diameter mirror on a window of Discovery.

The mirror bounces the beam back to the point of origin, where the distortion in the air is recorded. Normally, a second beam would then be sent, pre-distorted to compensate for the turbulence. But in this case, scientists only want to gather data on how much the light spread and whether corrections could be made.

The test is also meant to help determine if such a laser could be used to track a fast-moving missile warhead, but not to demonstrate the ability to destroy one.

If the experiment succeeds, the Pentagon's next move, probably later this summer, will be to bounce a laser beam off an unmanned sounding rocket that will fly higher than the shuttle. The idea will be to test the technology over an even greater distance, as well as track a more elusive object, as opposed to the shuttle, which moves in a fairly predictable orbit.

The the next big hurdle after that, presumably, would be to test the shooting of high-powered lasers through the atmosphere similar to what eventually might be used in a weapons system.

All this, however, is a long way from a practical ground-based laser defense system. Critics like to point out -- and Strategic Defense Initiative Organization (SDIO) scientists generally concede -- that a ground-based laser system could be hampered by cloud cover.

Moreover, such a system would require a lot of power: By some estimates, powering enough lasers to hit 2,000 targets from space would take at least as much electrical energy in a few minutes as New York City consumes in six average hours. Another snag is to come up with protective coatings for the space mirrors themselves. The slightest pitting or warping could cause a laser beam to scatter.

``Coatings research is probably the go, no-go factor for a lot of this,'' says John Bosma, editor of ``Military Space,'' a newsletter.

The experiment aboard the shuttle is the most visible in a series of recent advances in various technologies that might someday be applied to a space defense system.

For instance, breakthroughs have been reported in X-ray lasers (exotic weapons powered by the explosion of a nuclear bomb), particle beams, sensors that could be used to track enemy missles, and other types of lasers.

Taken together, SDI proponents contend the advances have given the program some momentum.

``Many of the results we are getting now are coming earlier than we had expected,'' says Air Force Lt. Col. Tom Meyer, head of SDIO's ground-based laser program.

But critics counter that these isolated advances are a long way from a realistic defense system that could overcome whatever countermeasures the Soviets might develop.

``I think it is certainly possible to build these individual gadgets,'' says John Pike, head of space policy for the Wasington-based Federation of American Scientists. ``Whether you can string them all together in a system that will work the first time is another question.''