SAN JOSE, Calif. (AP) _ NASA researcher James B. Pollack, who worked with Carl Sagan on the theory that atomic war would result in a ''nuclear winter'' of soot blocking the sun's rays, has died. He was 55.

He died at home Monday of spinal cancer.

Pollack joined the National Aeronautics and Space Administration's Ames Research Center in 1970 and worked at the center in Mountain View for 24 years.

Despite his inoperable cancer, Pollack continued to work at home, publishing papers and communicating with colleagues by phone and computer, said his sister, Ginny Breslauer. ''He always used to say to me, 'We've got to go as far as we can go.' ''

The 1971 orbit of Mariner 9 around Mars provided information for Sagan, Pollack and three other researchers in coming up with the nuclear winter theory. They said nuclear war would result in airborne soot that would block the sun's rays and drop temperatures to catastrophic levels.

The team later conceded they overestimated the severity of the freezing.

Pollack and two other scientists at the Ames center studied Venus to come up with a new theory that could prove helpful in explaining how the greenhouse effect may play out on Earth.

The research indicated that for hundreds of millions of years, most of the water on Venus remained in liquid form near the boiling point before drying up. The older theory held that water existed on Venus, but only as vapor.

Pollack also discovered that Saturn's rings are made of ice chunks up to 12 inches thick.

Pollack earned a bachelor's degree in physics from Princeton University. He received his master's degree from the University of California, Berkeley, and his doctorate from Harvard University.