Ground-based lasers designed to knock out Soviet ballistic missiles during a nuclear attack would be so vulnerable to atmospheric interference that they could not be used on a cloudy day, the chief scientist for the "Star Wars" defense program said Monday.
Gerold Yonas of the Defense Department's Strategic Defense Initiative Organization told several hundred scientists during a Los Angeles symposium that ground-based lasers could compensate for mild atmospheric conditions. But when asked by a member of the audience how lasers could offset the kind of distortion that would be caused by a heavy cloud layer, Yonas responded:
"A ground-based laser cannot operate with cloud cover."
Yonas did not elaborate, and he could not be reached for comment after the session, which was held as part of the national meeting of the American Assn. for the Advancement of Science.
His statement, however, was particularly significant in that he had said a few minutes earlier that it now appears unlikely that giant lasers will be placed in orbit, and a more likely application of laser technology would be through ground-based devices that would bounce their high energy laser beams off orbiting mirrors.
Yonas' comments would seem to indicate that the Soviet Union could get around ground-based American lasers simply by launching a nuclear attack on a cloudy day, but such reasoning does not eliminate the need for research in that area, according to the philosophy governing "Star Wars" research. Since the objective at this point is research, Yonas has indicated in the past, proposals that might seem absurd on the surface should be pursued if there is any chance of a technological breakthrough.
Yonas said that one of the major problems with a ground-based system is that even under normal atmospheric conditions, including air turbulence, there would be a distortion of the light waves and blurring of the focus of the beam, much the same as a scene is distorted when viewed through a window of poor quality.
However, he said, "we could predistort the wave front so it will be corrected by atmospheric interference," in effect using the distorting factor of the atmosphere to refocus the wave correctly.
The greater the air turbulence, the greater the potential for distorting the laser beam, and moisture in the air can have an even larger impact because it scatters the beam as each water droplet bends the rays of light that pass through it.
Although moisture would rule out use of ground lasers on cloudy days, Yonas said earlier in his presentation that lasers would be only part of any anti-missile defense system. Other elements would be of a more traditional nature, including so-called "smart rockets," missiles that would be fired toward incoming rockets to destroy them on impact.
Yonas shared the speaker's platform Monday with several other scientists, including Richard Garwin, a national defense expert who has emerged as probably the most authoritative critic of the "Star Wars" program.
About the only thing the opponents agreed on was that a space-based defense program will be extremely difficult and expensive to develop, and the plan as it is seen today is far different from that suggested by President Reagan in his so-called "Star Wars" speech more than two years ago.
"There has been a progression of goals--from protecting the cities to 'it's a bargaining chip'--in arms negotiations," said Garwin, who helped develop the hydrogen bomb and has served for many years as a consultant to the government on strategic defense systems.
Yonas conceded that the goal is no longer to protect the cities, which Reagan implied when he said the program should remove the threat of nuclear arms to all citizens. The objective today, Yonas said, is to determine if an anti-missile system could be built in which "the leakage (the number of enemy missiles that would get through to deliver their payloads) would be of such a low military value that it would discourage a first strike.
"If it wouldn't work," Yonas said, "then clearly you wouldn't have the basis to continue beyond the research stage."
Garwin scoffed at the term "military value." The purpose of a strike, he said, is to destroy the enemy, and it doesn't take very many nuclear warheads to do that, so any leakage would have military value.
Yonas repeatedly emphasized that the purpose of his agency is to determine the feasibility of developing an anti-missile system. That information, he said, should permit a President and the Congress in the future to decide if they want to build it.
He said the Strategic Defense Initiative is seeking $3.7 billion in next year's budget to continue the research.