The Solwind satellite destroyed last Friday during the first test of a U.S. antisatellite (ASAT) weapon was providing "very useful data" on solar activity until the moment it was hit, according to astrophysicists who were surprised and upset at seeing a fruitful experiment being used as a military target.

Defense Secretary Caspar W. Weinberger as recently as yesterday referred to the target as a "burned-out satellite." But physicist Robert M. MacQueen, director of the high-altitude observatory of the National Center for Atmospheric Research in Colorado, said yesterday that it was "deplorable" that the Pentagon "had taken a scientifically useful thing and sacrificed it in this way."

The satellite carried seven experiments into space six years ago for the Naval Research Laboratory (NRL) and other government agencies. One NRL experiment used a coronagraph that sent images to Earth of activity on the surface of the sun during each of the satellite's orbits, or roughly 15 times a day, several astrophysicists said.

Several months ago, NRL scientists were asked to draft what one source said "they thought was a routine paper to justify continued operation of their coronagraph." The scientists acknowledged problems with the spacecraft system, the source said, but wrote that it should continue.

In July, however, the NRL scientists were told "the satellite would be turned off sometime after Aug. 1, but they weren't told how," the source said.

On Sept. 6, The Washington Post reported that Solwind was the likely target for the ASAT test because the original target, an instrumented balloon, was plagued with technical problems. The NRL scientists learned what was in store for their experiment when they were told, "unofficially," to note The Post article, the source said.

"The satellite was transmitting normal data last Friday until impact," he added.

NRL officials were first asked about Solwind's functions and choice as a target on Monday by The Post; yesterday, an Air Force spokesman said the Pentagon was not ready to provide complete answers to those queries. He said the satellite was originally intended to operate for three years at most after launching in 1979 and that recent data from the satellite had "marginal value." In announcing the success of the test, Lt. Gen. Bernard P. Randolph, Air Force deputy chief of staff for research, described the target satellite as one that had outlived its usefulness.

The Air Force has another satellite, Solar Max, that was launched in 1980; it has a coronagraph that is considered more sensitive than the one destroyed Friday. But Solar Max failed six months after launch and did not resume transmitting data on solar activity until 1984, after being repaired by space shuttle astronauts.

Solar Max imagery covers a smaller area of the sun's surface, one scientist said, and lacks the historic data base associated with the Solwind experiment.

Yesterday, several scientists familiar with Solwind and Solar Max said they were "different, but complementary."

"It would be wrong to say they were redundant," one scientist added. Another criticized Pentagon assertions that Solwind was defunct as "hardly trustworthy."

MacQueen, whose organization designed Solar Max and runs it for the Air Force, said the "continuous observations" of the Solwind satellite, stretching from a period of maximum solar activity in 1980 through minimal activity recently, were "very valuable."

He said the data has been sought by the Air Force to determine the effect of the sun's surface activities on the upper atmosphere and particularly on telecommunications.

The Air Force has been asked to explain how it chose the Solwind satellite as a target. Under the Air Force's original ASAT test plans, the first two tests of the weapon against targets in space were to use an instrumented space vehicle that carried two balloons as targets. Tests against satellites were to have been held later in the 10-test program.

In June, the weapon and the target developed problems requiring service by their manufacturers. At the time, Air Force sources said it would be a few months before the tests could take place, suggesting that the controversial first test would occur after the planned Nov. 19-20 summit meeting between President Reagan and Soviet leader Mikhail Gorbachev in Geneva.

In early August, when the weapon became available but the instrumented target did not, Weinberger pushed for the test against a satellite "to show resolve," according to a Pentagon spokesman. At the time, Soviet spokesmen and members of Congress critical of the program were calling for the administration to join in Moscow's self-imposed, two-year moratorium on testing antisatellite weapons.

Reagan agreed with Weinberger and approved the test against a satellite. On Monday, according to sources, the NRL scientists were told not to discuss their project with journalists and to pass all inquiries through the laboratory's public affairs staff. On Tuesday, Donna McKinney, an NRL spokeswoman, said questions that had been submitted the day before would be answered by the public affairs office of the chief of naval operations.

Cmdr. Kenneth Pease, of the Navy information office in the Pentagon, said yesterday that the service would respond to questions about their experiments "as soon as they were cleared."