Alastair G. W. Cameron, a Harvard astrophysicist who helped develop a revolutionary theory to explain how the Moon was formed, died Oct. 3 at his home in Tucson. He was 80.
The cause was heart failure, his family said.
Dr. Cameron's famous work, known as the giant impact theory, holds that a planet roughly the size of Mars struck Earth, sending fragments of Earth's mantle spinning into the atmosphere. The ring of space debris that resulted may have ultimately come together to form the Moon.
Dr. Cameron was a former associate director for planetary sciences at Harvard and was chairman of its astronomy department from 1976 to 1982.
He and others, principally William K. Hartmann of the Planetary Science Institute in Tucson, proposed the impact theory in the 1970's and developed it in later decades. The two scientists had been working independently on the idea when Dr. Hartmann presented his research at a meeting at Cornell in 1974.
The theory accounts, in part, for the Moon's lack of water and its few volatile elements, which would have been burned away in the planetary collision.
Michael J. Drake, a planetary scientist who is director of the Lunar and Planetary Laboratory at the University of Arizona, said Dr. Cameron made "clever calculations using advances in computing power and realized that such debris could possibly coalesce."
Dr. Drake observed, "The giant impact theory, which since the 80's has become largely accepted in the scientific mainstream, became a brilliant synthesis of earlier, flawed hypotheses about the origin of the Moon."
Dr. Cameron was also interested in how chemical elements are formed inside stars, a field known as nucleosynthesis. He continued to study the origins of the solar system and in particular the importance of the explosions of large stars, or supernovas, that appear to create new stars in their destruction.
The son of a Canadian biochemist, Alastair Graham Walter Cameron was born in Winnipeg. He earned his undergraduate degree from the University of Manitoba, and a doctorate from the University of Saskatchewan.
He taught at Iowa State and conducted atomic research in Canada before joining the National Aeronautics and Space Administration, serving as a senior scientist from 1961 to 1966. He became an American citizen in the 1960's.
Dr. Cameron later became a professor of space physics at Yeshiva University, where he taught from 1966 to 1973. He then taught at Harvard, remaining there until 1999, when he moved to Tucson. He continued to work as a senior research scientist at the Arizona lunar laboratory until last month.
In September, the American Physical Society's division of nuclear physics awarded Dr. Cameron its Hans A. Bethe Prize.
Dr. Cameron's wife, the former Elizabeth MacMillan, died in 2001. The couple lived in Belmont, Mass.
He is survived by a sister, Janet Matthews, and a niece, Valerie Matthews Lemieux, both of Winnipeg, and by two nephews.