Space science suffered a major blow yesterday as NASA terminated its costly and controversial plan to use the shuttle to launch the Centaur Upper Stage, a rocket propelled by highly volatile liquid fuels, which had been scheduled to boost two high-priority planetary missions from Earth orbit last month.

James C. Fletcher, chief of the National Aeronautics and Space Administration, said the Centaur "would not meet safety criteria" now being applied to the shuttle and its cargo, even though the booster has been modified as a result of continuing concerns about it.

Fletcher's decision was prompted in part by congressional studies done, in the wake of the Jan. 28 Challenger disaster, for Reps. Edward P. Boland (D-Mass.), chairman of the House subcommittee that appropriates money for NASA, and Bill Green (R-N.Y.), the panel's ranking minority member, according to a NASA statement.

"You're seeing for the first time the true impact of the accident," said a House aide familiar with the concerns about the Centaur. "Now, safety is the yardstick by which everything will be measured. NASA and the country will have to come to grips with the question of what risk is acceptable."

NASA's statement said the agency "will provide assistance" to the Pentagon "as it examines alternatives for those national security missions which had planned to use Centaur." The Defense Department had ordered a handful of smaller Centaurs for classified missions.

The leading alternative to shuttle launches for Centaur missions, including two scheduled for Jupiter and the sun, is to use unmanned, expendable Titan 34D-7 rockets, a NASA spokesman said.

Another possibility, according to a Capitol Hill source, is to use the shuttle in combination with a safer, less powerful solid-fuel rocket -- known as the Inertial Upper Stage (IUS) -- to boost the planetary probes out of orbit.

In either case, the deep space probes -- Ulysses and Galileo -- will be delayed for years. The Titan 34D-7s, a new class of expendable rockets, are on order by the military but will not be available until the early 1990s. It is unclear when the shuttle will fly again.

Ulysses is designed to whip around Jupiter, using the boost of the planet's gravity to sling the probe into a polar orbit of the sun, which has never been seen from that angle. Galileo is to orbit Jupiter and study its atmosphere.

These two craft plus a third, a radar mapper called Magellan, were designed to fit the shuttle's cargo bay. Expensive modifications would be required to redesign them for the unmanned Titan rockets -- more than $1 billion to pay for both the new rockets and the redesign, by one estimate.

The Centaur has been used on other unmanned rockets but has never been launched on the shuttle. NASA's decision does not affect Centaur programs not designed for the shuttle.

NASA's chief engineer, its astronaut office and others inside the agency, as well as independent experts, had raised alarms about the safety of the Centaur before the Challenger accident.

In addition to the danger of the propellants on the Centaur, the Ulysses and Galileo deep space probes carry a nuclear source of electricity because they will be too far from the sun to use solar cells. This has raised the specter of radiation release in addition to an explosion.

Even before the Challenger accident, the military's version of the Centaur, which is not ready to fly, was being modified for added safety. But "because of the rush to get the 1986 Ulysses and Galileo missions launched, these improvements were not approved" for them, the presidential commission that investigated Challenger said in its report. A number of waivers to safety requirements had been granted and others were pending. After the Challenger accident, NASA allotted $75 million to make the safety improvements. But even that was not enough in the post-accident era.