The 19th mission of the space shuttle was aborted at Cape Canaveral yesterday when onboard computers shut down Challenger's three main engines after ignition but three seconds before liftoff. The crew was safe.

The abort was the second for a shuttle mission in less than 13 months, a mishap the National Aeronautics and Space Administration has not suffered since October 1965 in the 24 years NASA has flown manned spacecraft.

The first shuttle abort occurred June 26, 1984, when onboard computers shut down two of Discovery's three engines four seconds before liftoff.

"We're back about a week behind in our launch schedule right now," launch operations manager Thomas Utsman said yesterday at Kennedy Space Center in Cape Canaveral. "My expectations are that we will be able to fly again in seven to 10 days."

Utsman said launch directors have no immediate indication of what caused the three hydrogen-fueled engines to shut down three seconds before two solid-fuel rocket engines would have been fired and taken the 100-ton spaceliner into space.

The three engines normally ignite about 6.6 seconds before liftoff so they can attain full power before the two solid rockets fire and explode bolts holding the shuttle to the pad.

Utsman said onboard computers sensed that a valve on engine No. 2 did not close, triggering an automatic shutdown of the three engines.

"If we lose one of our valves on any of our three main engines prior to liftoff, we shut down," Utsman said. "We don't want to leave the launch pad knowing we might have a potential problem in flight."

The incident chilled what had again begun to look like a series of routine shuttle launches. The last seven had been virtually flawless, with each encountering fewer problems.

In what was to be a rare afternoon liftoff, Challenger's rear starboard engine ignited on time at 4:30, leaving a cloud of steam beneath it. A fraction of a second later, the port engine fired ahead of the third engine, and all three blasted at full strength for almost three seconds. Then came the abrupt halt.

If the three had continued firing, the solid-fuel rockets would have fired automatically and started liftoff. The seven-man crew almost surely would have faced what shuttle engineers call a "return-to-launch-site abort."

Mission rules call for crews to bring the shuttle back to Cape Canaveral if one main engine malfunctions in the first minutes of flight.

The abort technique involves burning the solid-fuel engines until their fuel is depleted about 30 miles east of and 150,000 feet above the cape. The technique has been rehearsed only in computer simulations.

"We all have mixed emotions here, but we're thankful the system worked the way it should," Challenger Commander C. Gordon Fullerton told reporters two hours after the abort. "It was the longest three seconds I've ever experienced."

Utsman said that crews would strip parts of the malfunctioned engine and that engineers have identified four different parts that could have failed when they should have been pumping fuel from the hydrogen tank into the engine chamber.

When liquid-fuel engines ignite, valves that feed fuel into the chambers are fully open so engines can attain speed instantly. Just before liftoff, the valves are partially closed to cool engines slightly and prevent too much fuel from entering the chamber.

The valve on one engine failed to close partially yesterday.

"Our computers picked this up immediately and shut the system down," shuttle program manager Robert Lindstrom said. "We don't think the engine valve itself failed the way a valve failed the other time we had a launch abort."

The first abort triggered an investigation that led to complete overhaul of the shuttle Discovery's main engines. It had to be moved back to the giant Vehicle Assembly Building where its cargo was removed and placed aboard another shuttle that flew into space two months later.

"We don't see that happening this time," Utsman said. "I really believe there's a fair likelihood we'll get off again in another week."

Any delay will have an impact on the mission's objectives.

Instruments in Challenger's cargo bay include an infrared telescope that can be operated successfully only in a completely dark sky. Had the flight left yesterday, the sky would have been dark for most of the next seven days until a new moon.

The next shuttle launch is scheduled Aug. 24 when Discovery is to lift off for an attempted rescue of a satellite stranded in space last April. It is not immediately clear how yesterday's abort may affect that mission.