Full deployment of a "Star Wars" laser shield to shoot down Soviet ballistic missiles in space is at least 20 years away, a senior analyst for Congress' Office of Technology Assessment (OTA) told a scientific conference yesterday.

The conclusion runs counter to claims by independent experts that a space-based laser shield could be deployed by the end of the next decade, and it challenges the Reagan administration's proposed timetable for making a decision on missile defense deployments in the early 1990s.

OTA analyst Anthony Fainberg said that due to the enormous technical challenges, it might be "at least 10 years" before a decision could be expected on building a laser missile defense in space.

"Deployment of an entire system of a hundred lasers would likely require at least 10 more years" after such a decision, Fainberg said at a Boston meeting of the American Association for the Advancement of Science. "This is probably an optimistic schedule."

Fainberg's comments came as Pentagon officials confirmed a published report that a costly missile defense experiment in space by the Strategic Defense Initiative (SDI) on Monday was not as successful as Defense Department officials had first described it.

Pentagon officials had earlier said the $250 million experiment, aimed at tracking and identifying mock Soviet missiles and warheads ejected by a U.S. satellite, was marred only by the failure of one of seven sensors aboard the Delta 181 vehicle orbiting payload.

But yesterday, they said an apparent malfunction of other equipment further degraded results of the experiment. And they disclosed that the sensor that had failed was one of the most important in the detection array.

"We've got a very successful mission, and we learned an awful lot," said Army Maj. Andrew Green, commander of the space mission. He added, however, that an apparent sensor or computer defect aboard the satellite resulted in loss of a portion of the tracking data expected from the mission. Green said the significance of the lost data is not yet known.

The malfunction was revealed in an article in Aviation Week and Space Technology, a trade publication, to be published next week.

SDI officials said in a January report that space-based lasers and other directed-energy weapons will be an important part of the so-called "second phase" of U.S. antimissile deployments in the 1990s.

Fainberg's report called into question SDI's proposed timetable for fielding missile defense weaponry. The first phase, ostensibly to be deployed in the early-to-mid 1990s, would be primarily space-based rockets that attempt to ram Soviet missiles shortly after launch, before they dispense multiple, independently-targeted warheads.

The second phase, to be composed partly of space-based lasers, will be deployed when the Soviet Union develops extremely fast missiles capable of dispensing warheads before the U.S. rockets can arrive, according to SDI plans.

SDI contractors have reported that these directed-energy weapons should be ready in space by the end of the decade to destroy speedy Soviet missiles. But Fainberg's report suggests the lasers probably will not be available that quickly.

SDI officials have promised that before deploying the first phase of rockets, they will have achieved full understanding of the capabilities of lasers and other directed-energy antimissile weapons. But Fainberg's report states that "it is difficult to imagine" key laser experiments occurring "before the mid- to late-1990s," well after the proposed SDI timetable for deployment of rockets.

SDI officials declined to comment on Fainberg's assessment yesterday. Last April, however, they criticized as "subjective and unduly pessimistic" a report by the American Physical Society, the premier group of U.S. physicists, that reached similar conclusions.

Fainberg received classified briefings by SDI officials to help prepare a secret, 900-page OTA study on the SDI research program. The OTA study was completed last August but has been withheld by Pentagon officials who said it contained sensitive information.

Fainberg said yesterday "we hope to reach agreement . . . on an unclassified version {of the OTA report} within a month at the outside."