The first of a series of tests of sensors designed to support the Pentagon's "Brilliant Pebbles" space-based missile defense system achieved only partial success, the Defense Department reported yesterday.

The department said electronic data being sent back to earth on the functioning of the experiment was cut off in mid-flight.

But the Pentagon said the 13-minute experiment, launched Saturday from the Wallops Island, Va., facility was a beneficial first step in the scheduled series of 12 experimental flights.

The Brilliant Pebbles component of the Strategic Defense Initiative, or "Star Wars," would place a cluster of small interceptor rockets into earth orbit, each capable of detecting the launch of hostile strategic missiles.

The interceptor rockets could be ordered to speed toward the incoming missiles over thousands of miles and destroy them by high-velocity impact.

The Brilliant Pebbles experimental package was launched aboard a Black Brant X rocket at 3:04 a.m. Saturday. The Pentagon said the expendable rocket and its payload landed as planned in the Atlantic.

The payload was to have been deployed at an altitude of 124 miles but Pentagon officials said they do not yet know if that goal was achieved.

"Only a partial set of objectives was achieved due to a premature termination of telemetry," the Pentagon said, referring to the interrupted flow of electronic data.

A Pentagon official, who commented on condition he not be identified, said program managers will now try to learn how much of the experiment was completed, using backup sources from an aircraft, a satellite and radars at Wallops Island.

"A number of the parts of the launch were very successful and validated other parts of the experiment," the official said.