CAPE CANAVERAL, Fla. (AP) _ A Delta Star missile-hunting satellite thundered into orbit Friday and quickly spotted a target as it began a key months-long test to develop a split-second ''Star Wars'' defense against nuclear rockets.

The payload's sensors passed their first trial within two hours after launch when they successfully tracked the thrusting second stage of the Delta booster rocket after separation and during its fiery destruction in the atmosphere over the Indian Ocean.

''We watched that burn with the sensors; we not only got the second stage burning, but we also got the re-entry and breakup,'' Air Force Col. Michael Rendine, program manager for the Strategic Defense Initiative Office, told a news conference.

''We had a perfect ride into space, and the spacecraft is operating just like we expected it to,'' he said.

It was a good beginning for the $190 million mission.

Over the next several months Delta Star is to aim its sensors at a series of ground-based missile and space launches to help perfect the technology for detecting and destroying enemy boosters within minutes after they leave their launch pads.

Researchers especially want to gather rocket exhaust data against the background of the North Pole region - an area through which attacking Soviet missiles would travel.

The 11-story-tall Delta vaulted off its launch pad at 4:51 p.m. EST after a secret countdown. News media representatives received information about the launch in advance and were escorted to a viewing area, but, for security reasons, were not told the exact time of liftoff.

Air Force officials said the Delta Star experiment is part of the research being done for the Strategic Defense Initiative, or Star Wars, program to develop a space-based missile defense system.

The three-ton satellite carried a lasar radar, seven video imaging cameras, and an infrared imager. A Laser Illumination Detection System was designed to spot ground-based laser firings, a capability that would permit future satellites to take evasive action.

Using a 48-jet thruster aiming system, Delta Star had the capability to maneuver into position to spot the blazing plume of missiles and rockets to be launched over the next several months from Cape Canaveral; Wallops Island, Va.; White Sands, N.M.; Poker Flats, Alaska; Barking Sands, Hawaii, and perhaps from Soviet launch sites.

In addition to quickly locating a rocket plume, researchers want the sensors to gather information to help distinguish the exhaust signature from various backgrounds such as land, ocean, horizon, space, atmospheric effects and the bright aurora borealis in the north polar region.

The polar background information is critical because Soviet missiles would travel through that region in any attack on the United States.

The sensors also were to study rising rockets from different angles and under varying environmental conditions.

The launch was the third in a series. In a 1986 test, two satellites tracked each other and one destroyed the other by crashing into it. Last September, a satellite tracked 15 simulated nuclear missiles released by the same rocket in a test aimed at detecting missiles in midcourse flight, after their motors have burned out.

As envisioned by former President Reagan when he proposed it in 1983, the Stars Wars defense shield would include several hundred large orbiting platforms equipped with killer rockets and hundreds of supporting surveillance and tracking satellites.

But the program, on which $16 billion has been spent so far, has met with increasing resistence from a budget-minded Congress, and officials have spoken in recent months of a modified or scaled-down system. President Bush has said he supports SDI, but it and other strategic systems currently are under review in his administration.

Vice President Dan Quayle earlier this week endorsed a new version of the SDI program known as ''brilliant pebbles,'' citing its small size, comparatively low cost and fast schedule for development.

The ''brilliant pebbles'' technology would involve thousands of small satellites that could home in on enemy missiles and destroy them by colliding with them. Each satellite, about three feet long and weighing 88 pounds, would have sensors and navigation equipment to track the missiles.