George A. Keyworth II, Reagan Science Adviser, Dies at 77

George A. Keyworth II, a science adviser to President Ronald Reagan who strongly backed the antimissile plan known as Star Wars and came to see it as an important factor in the Soviet collapse, died on Wednesday in Monterey, Calif. He was 77.

Bruce Abell, a longtime aide, said the cause was prostate cancer.

Reagan’s proposed antimissile system “offers a way to stop an attack,” Dr. Keyworth said in a 1983 talk. Two years later, he told military contractors that federal research might soon offer “unequivocal proof” of the nation’s ability to destroy enemy missiles.

The administration’s vision of weapons in space — a projected five-year, $26 billion project (almost $60 billion today) — met stiff opposition from specialists and Congress, and the plan never got beyond the research stage.

Even so, conservative historians say Moscow became so unnerved at the prospect of a daunting new rivalry that the threat helped end the Cold War and decades of nuclear buildups.

Dr. Keyworth, a physicist, came to this watershed of history not from social prominence, political experience or high technical standing, but rather from an unassuming role he had played between warring factions of the scientific community.

In the 1970s he worked at the Los Alamos weapons lab in New Mexico, where J. Robert Oppenheimer had presided over the birth of the atom bomb during World War II. One of Oppenheimer’s rivals, Edward Teller, had lobbied for the hydrogen bomb — roughly a thousand times more powerful — and had clashed bitterly with Dr. Oppenheimer over his initial opposition to the H-bomb’s development.