A three-ton U.S. military satellite released and electronically observed a series of mock Soviet missiles, warheads and warhead decoys in space yesterday, completing the most costly experiment yet by the Strategic Defense Initiative (SDI) research program.

The satellite ejected the objects over several hours as it orbited roughly 195 miles above the Earth, shortly after launch from Cape Canaveral, Fla., late Monday afternoon. The satellite then slowly danced around the objects to obtain different views through its sensors.

The goal of the $250 million experiment was to try to track such objects and tell them apart, an essential task of any space-based "Star Wars" defense against Soviet ballistic missiles.

"Right now, we believe we've got a very successful mission," said Army Maj. Andrew Green, who commanded the Delta 181 mission.

Green and others at a Florida news conference yesterday said this assessment was based on a quick review of 5 percent of the copious data collected by the satellite. The remainder is to be transmitted to ground stations in bursts during the next nine days.

Green said one of the satellite's seven sensors failed at the outset of the mission, preventing it from viewing the space objects in a particular wavelength of infrared light. Green said that although this was "not any big problem . . . we would have liked to have it."

Tracking and identifying objects in space is considered one of the most difficult missions of the SDI program.

Studies have shown that up to a million Soviet warheads and warhead decoys could be launched in a massive attack on the United States, far more than U.S. space-based weapons could destroy.

As a result, SDI officials must develop space-based sensors that can weed out a comparatively few nuclear warheads in a large field of decoys, so U.S. space-based weapons can concentrate their fire. One approach is to scan with infrared sensors for the heat generated by a nuclear warhead's electronics in the extreme cold of space; another is to distinguish warheads and decoys by shape or weight.

"One of the major things that SDI doesn't know is what it is we are trying to attack -- what are the objects, what do they look like, exactly what arena are we working in?" said Col. Leonard J. Otten, an assistant director of SDI's kinetic energy weapons office.

The space-based sensors were prepared to collect data on certain characteristics of the space environment, such as a brightly lit region of the Earth's atmosphere known as the "Earth limb," which can potentially confuse space-based warhead sensors.

Another goal was to track four sounding rockets, ejected from the satellite and ignited in space to generate "plumes" of fire similar to those generated by Soviet missiles. A fifth sounding rocket was launched from Hawaii and observed by sensors in space and aboard two nearby military aircraft.

Ten other objects, designed to look like Soviet warheads and decoys, were released and observed by the satellite sensors for 10 to 15 minutes during a 12-hour period.

Some of the sensors were said by SDI officials to be the most advanced developed by the United States.

But SDI officials acknowledged they were less sophisticated than those needed for a space-based missile defense system.

The experiment "will help us to design sensors in the future to distinguish types of objects in space," said Gordon Smith, deputy director of the SDI organization.

Unlike the last major SDI space experiment, Delta 181 did not include any simulated attacks on the space objects.

SDI officials and independent experts agreed the experiment fell within the constraints of the "narrow," or traditional, interpretation of 1972 Antiballistic Missile (ABM) Treaty, which bars realistic space tests of missile defense technologies.

Former secretary of defense Caspar W. Weinberger and others pressed at one point to include a simulated missile attack in the experiment, which would have breached the "narrow" ABM Treaty interpretation. But it was not included due to the potential high economic and political cost.