Reagan administration budget austerity will prolong the space agency's 1985 flight to Jupiter by two years, causing it to reach the big planet in 1989 instead of 1987 as planned.

"Galileo has been saved in the new Reagan budget but at a price," a Capitol Hill source said yesterday. "Under the new plan, the spacecraft will arrive at Jupiter in the second quarter of 1989, which means the Galileo science team will have to be kept at work for an extra two years before the mission is accomplished."

There had been widespread fear among space scientists that the Galileo mission would be lost in the new Reagan budgets, but sources on Capitol Hill and inside the administration insisted yesterday that it is still alive.

Prolongation of the flight results from a budget decision to postpone development of the hydrogen-fueled high-energy Centaur engine, which would have enabled Galileo to fly a direct path to Jupiter that would have taken only two years.

Instead, Galileo will be fired out of Earth orbit by a conventional upper-stage engine on a path around the sun and back to the environs of Earth in 1987. Earth's gravity will give it a boost that will speed it toward Jupiter as if it had been fired by a high-energy Centaur engine. It will just take two years longer.

Also still alive is the Voyager mission to Uranus and Neptune, which the administration had considered killing even though the Voyager spacecraft is already on its way to Uranus.

"This administration has no intention of throwing away such a rich mass of data for a relatively modest savings," White House science adviser George A. Keyworth II told the annual meeting of the American Association for the Advancement of Science. "We will continue to fund Voyager through its flights by Uranus and Neptune."

As recently as last October, the National Aeronautics and Space Administration had begun to weigh a plan that would have abandoned Voyager once it flew by Saturn last summer. This would have saved $222 million over the next eight years.

Even though Galileo and Voyager were saved from the Reagan budget ax, other programs started by the Carter administration were not saved. A mission to orbit Venus with a huge radar dish has been postponed indefinitely, sources said. Killed also is any mission to Halley's comet when it flies around the sun in 1986.

"Halley is a comet that will not be visited by an American spacecraft," an administration source said yesterday. "We will leave Halley up to the Russians, the Europeans and the Japanese."