The United States has embarked on an ambitious effort to launch nuclear reactors into space, despite virtually abandoning a similar program more than a decade ago.

The impetus to the billion-dollar program, which has aroused safety and cost concerns, is President Reagan's Strategic Defense Initiative (SDI), or "Star Wars," a proposed space-based defense against nuclear missiles.

SDI might use an amount of energy equal to "a substantial fraction of the output of the eastern U.S. power grid," SDI official James Ionson said recently, and nuclear reactors appear to be the only possible source of the energy.

The U.S. nuclear industry, with few terrestrial customers these days, has welcomed SDI as a potential customer; an industry newsletter summed up the reaction by saying "it's about time nuclear power had something going for it."

Even if SDI is never built, Air Force officials say they need nuclear power for their next generation of space-based weapons, and the National Aeronautics and Space Administration says that nuclear reactors could power space stations, lunar bases, interplanetary explorers and satellite traffic control.

"Space nuclear power is more than the stuff of which Buck Rogers fantasies are made," Rep. Sid Morrison (R-Wash.), ranking minority member of the Science and Technology subcommittee on energy research, said at a hearing last fall. "It is a technology whose time has come."

The new program, coinciding with the Jan. 28 Challenger shuttle disaster, has rekindled fears of release of radioactive material in the event of an accident, or of "one of these reactors falling on our heads one of these days," as Rep. Manuel Lujan Jr. (R-N.M.) said at a recent hearing. Government officials respond that safety will have priority as they design a new generation of compact, powerful reactors for use in space.

The United States spent more than $1.4 billion on space nuclear reactors and then abandoned the program in 1973, mostly because no one could find a mission to justify the cost.

The Soviet Union never gave up on space reactors and has launched at least 20 nuclear-powered satellites, mostly ocean surveillance craft that track U.S. Navy ships. One such satellite crashed in 1978, scattering thousands of pieces of radioactive debris over northwest Canada. The Soviets eventually paid half of a $6 million cleanup bill the Canadians submitted.

The United States has fueled about two dozen satellites with radioactive isotopes which, while less powerful than a reactor, emit heat that can be converted to electricity. After the Challenger explosion, James J. Lombard, an Energy Department official, acknowledged to Congress that if a shuttle exploded on the pad while carrying such a cargo, there could be "a release of the plutonium in the launch pad area."

Lombard said the likely number of casualties was classified. But Rep. Edward J. Markey (D-Mass.) later obtained Energy Department data showing that such an accident could, at worst, contaminate 930 square miles around Cape Canaveral and cause 386 cases of cancer.

For the most part, the United States has relied on solar energy for its space activities. But on Star Wars weapons, solar panels would be too vulnerable to attack by space mines, SDI officials said, while nuclear reactors could be more compact and shielded.

In addition, solar cells could not provide the power SDI would need to keep thousands of space weapons and sensors working. According to NASA and Defense Department officials, the space shuttle can generate 36 kilowatts of power for several weeks, "comparable to an average American home," Ionson said. The proposed space station would need about 75 kilowatts, while Star Wars platforms would need hundreds of kilowatts as they spin in orbit, and bursts of hundreds of megawatts to fire lasers and other directed-energy weapons.

Lt. Gen. James A. Abrahamson, director of the Pentagon's SDI office, said last fall that the system's energy needs could be "in excess of four orders of magnitude above current space-based systems," meaning 10,000 times more powerful. In order to remain in space for years without repair, they will also need "unprecedented levels of reliability," he said, adding that "an appreciation of the challenge facing our scientists begins to emerge."

To meet that challenge, the Energy Department has shifted millions of dollars from civilian reactor programs to Star Wars, and the SDI office has budgeted millions more. For two chief programs, the SP100 and the multimegawatt reactor, the two agencies plan to spend more than $900 million during the next five years, according to figures submitted to Markey.

The SP100 is a nuclear reactor being built at Hanford Reservation in Richland, Wash. About a yard in diameter but able to generate 300 kilowatts, the reactor is supposed to be ready for a test space flight in 1993. The multimegawatt reactor requires even more of a technological leap and is only a concept so far.

U.S. officials have said it is too soon to estimate how many reactors SDI would require or how much they would cost.

James W. Vaughan Jr., acting assistant secretary of energy for nuclear energy, said that the "driving factor" behind the rebirth of space nuclear power is SDI.

"I think if it were not for that, we would be hard-pressed to have a sufficient number of defined missions to sustain it at the levels we're talking about today," he said.

But others are rushing to piggyback on SDI's efforts. A January conference in New Mexico, attended by scientists from Westinghouse Electric Corp., General Electric Co. and other giants of nuclear industry, highlighted the need for nuclear power to mine the moons of Mars.

Similarly, Air Force Col. James H. Heilman told Congress last month that space nuclear power will be essential to meet "today's stressing threat and tomorrow's ever-evolving demands" even if the SDI is never deployed. Nuclear reactors will fuel powerful space-based surveillance radars, satellites that can maneuver in space and unmanned "orbital transfer vehicles" that could transfer cargo from one satellite to another or back to Earth, he said.

Officials said they will protect public safety by launching the reactors in a "cold" state and start them after they reach orbit. In addition, they said, the reactors will orbit high enough to allow radioactive materials to decay before the spacecraft tumbles back to Earth.

But at the conference in New Mexico, government and industry scientists agreed that considerable work will be necessary to ensure that no reactors accidentally poison the atmosphere or ocean, either at launch or on return to Earth.