The controversial new U.S. antisatellite weapon passed its toughest test yesterday by destroying an old Air Force scientific satellite in a collision 345 miles above the Pacific Ocean.

Jubilant Pentagon officials said they confirmed the success of the shot, the first against a target in space, through radar monitoring and the sudden halt of radio signals from both the antisatellite weapon and its target.

Lt. Gen. Bernard P. Randolph, Air Force deputy chief of staff for research, told newsmen, "The satellite was in fact destroyed . . . . From everything that we can tell, it went absolutely flawless." He refused to give operational details.

Yesterday's test, undertaken despite Soviet protests and critics' warnings that it could accelerate a space arms race, indicates that past technical problems plaguing the seven-year development of the weapon have been overcome. The testing program had been delayed almost two years.

It also may give President Reagan a new bargaining chip in his summit meeting with Soviet leader Mikhail Gorbachev Nov. 19-20. The Soviet Union has an older operational antisatellite system that is considered relatively crude.

Two years ago, the Soviet Union declared a moratorium on testing of its weapon and challenged the United States to follow suit.

One of the Reagan administration's prime military goals in the past 4 1/2 years has been to correct what a Pentagon spokesman yesterday termed the "serious imbalance" between the two superpowers' antisatellite capabilities.

Repercussions from Capitol Hill and Moscow are likely.

Congressional critics of the weapon have attempted to limit the number of tests in the coming year, contending that the U.S. device will lead to a new form of space arms race. Under language approved by a House-Senate conference in July on next year's defense spending bill, the Air Force would be permitted only two more tests of the weapon over the next 12 months.

Last week, when it was apparent the U.S. test was imminent, the Soviet Union declared that if the United States conducts "tests" of "antisatellite weapons" against targets in space, Moscow would feel free to end its self-imposed moratorium by resuming development and deployment of its space-based weapon.

Randolph said yesterday that the next U.S. test would come "in the next few months," after data from yesterday's shot had been analyzed.

An instrumented target, unavailable for yesterday's test because of technical problems, will be returned to the Air Force from its manufacturer in the next two weeks, Pentagon officials said yesterday. Because of lengthy preparations necessary, it appeared doubtful that the next shot will take place before the Reagan-Gorbachev meeting.

Under the present program, the Air Force has said there will be nine more tests against space targets, with at least six more needed before the weapon can be declared operational.

In yesterday's test, an F15 from Edwards Air Force Base, Calif., flying at about 35,000 feet and directed by a radar command center at Vandenberg Air Force Base, launched the two-stage rocket carrying the antisatellite homing vehicle. It was aimed at the oncoming satellite, which was traveling over the Pacific in a north-south polar orbit.

The homing vehicle then separated from the rocket and guided itself by a telescope tracking heat emissions from the target satellite. The weapon's tiny jet engines maneuvered into the path of the satellite and at 4:42 p.m., Washington time, it struck the target.

"When the warhead hit the test satellite, both telemetry signals ceased at exactly the same time," a Pentagon spokesman said.

Officials said debris from the collision would either remain in orbit or burn up as it falls through the earth's atmosphere, posing no threat on the ground.

Defense Secretary Caspar W. Weinberger, who pushed aside some Air Force concerns at using a satellite target, was described by Pentagon spokesman Fred S. Hoffman as "absolutely delighted by the result." Part of that enthusiasm stemmed from the fact that yesterday's satellite was a smaller target than the one originally scheduled for the first target test.

Weinberger was "looking forward to a second test of this promising system in the months ahead," Hoffman said.

Democratic Reps. George E. Brown Jr. (Calif.) and John Joseph Moakley (Mass.), who unsuccessfully tried to halt the test through legislation and court action, called yesterday's test a blow to arms control.

After being informed of the successful shot, Brown said he would "do my darndest" to prevent any more tests from taking place.

Howard Ris, the executive director of the Union of Concerned Scientists, said the test meant "the U.S. really shot itself in the foot today." He added that the antisatellite test "could ignite an arms race in space which ultimately will weaken U.S. security.