Is the Airborne Laser Allergic to Dust?

Dan Drollette

America's $11.3 billion Airborne Laser missile defense program could be defeated by dust, said researcher Natan (Norman) S. Kopeika of Ben-Gurion University.

The laser is intended to shoot down enemy Scud-type missiles moments after they leave their launch pads. The system under research is designed to target the missiles via a series of three infrared lasers firing in a precise minuet from a flying jumbo jet. The trio of lasers successively locates each missile's position, tracks it and corrects for any optical distortions via the use of adaptive optics. Once the target is pinpointed, a high-energy chemical laser burns holes in the missile's fuel tanks, causing the missile to crash to the ground (see "Future-Proofing Defense," Photonics Spectra, May 2001).

An artist's rendition of the Airborne Laser system in action shows the jumbo jets targeting and destroying enemy missiles. An Israeli researcher suggests that atmospheric aerosols will ground the project.

But tiny particles of dust and ash in the atmosphere, collectively known as aerosols, may throw a monkey wrench into this plan, Kopeika said. Based on his experiments, the presence of the aerosols will cause small-angle and large-angle scattering of the lasers' beams, making them broaden and attenuate, respectively. Under this scenario, the chemical laser, for instance, would still hit its target. But at distances of 100 km or more, it would be too weak to destroy any missiles.

He also said that while corrective technologies, such as adaptive optics, can compensate for wavefront distortion caused by optical turbulence in the atmosphere, they cannot handle the beam broadening caused by aerosols. However, Ken Englade, a spokesman for the US Air Force's Airborne Laser research program, said that aerosol concentrations are minimal at the altitude of 40,000 feet where the lasers would operate. He cited studies conducted by NASA's stratospheric aerosol and gas experiment satellites, adding that aerosols have a relatively minor effect on the laser system, "except under the most extreme (rarely observed) atmospheric conditions."

In addition, he asserted that the success of some preliminary outdoor field tests of the laser system disproved the theory of excessive beam broadening caused by aerosols. Any broadening that did occur was dealt with by the adaptive optics system.

But Kopeika is not convinced. He maintains that aerosols are to blame for the unsuccessful results of the remaining field tests.