The trouble-ridden space shuttle Challenger lost the use of one of its three main engines as it blasted away from Earth yesterday afternoon, but its seven-man crew flew the 100-ton spacecraft safely into orbit.

On-board computers shut down Challenger's center engine six minutes into the 19th shuttle mission and 50th U.S. manned flight.

Challenger was 70 miles above Earth at the time, almost 50 miles below the orbit considered safe for a manned spacecraft.

"We are safely in orbit. The crew is doing fine. And we're optimistic about fulfilling most of the goals of our seven-day mission," NASA associate administrator Jesse W. Moore said at Florida's Kennedy Space Center.

"We were lucky today because the engine shutdown occurred at a point in our trajectory where we could achieve orbit," Moore said.

Computers ordered the center engine shut down when heat-sensing devices signaled that a high-pressure pump feeding fuel to the engine was overheating. Moore said the cause could have been an overheated fuel pump whose bearings or turbine blades had worn out in flight, or the sensors could have been picking up false signals from the fuel pump.

"We won't know until Challenger returns to Earth and we get a good look at that engine," Moore said. "Right now, I can't say whether we've got an engine problem or a sensor problem. All I know is that the crew is safely in orbit."

The shuttle's three main engines, including the one that lost power, are not used after liftoff.

It was the first time in more than 20 years of manned space flight that a rocket engine quit before the astronauts reached orbit. Had the engine been shut down sooner, the crew would have had to attempt a landing at the closest airfield along the flight path.

"I don't even want to think about that," Moore said. "That's why we've got red lines in all our systems, to keep things like that from happening."

The shaky 5 p.m. liftoff followed an aborted mission 12 days ago when Challenger's computers shut down its main engines three seconds before the scheduled liftoff.

Moore said he had no idea whether yesterday's mishap would affect the shuttle program's launch schedule.

He said that, as far as he knew, the shuttle Discovery will be moved to its launch pad Monday night to get it ready for an Aug. 24 liftoff on a mission that will attempt to rescue a Navy communications satellite drifting in orbit.

"Anybody who operates in this launch business is going to see things like the one that happened today," he said.

"I sure hope it has no impact on our reputation. In fact, I believe that it shows that we have built-in design capabilities to overcome difficulties like this one."

Moore said the on-board sensors first indicated that the center engine might be overheating four minutes after liftoff. Computers then ordered the center engine fuel pump to bypass one valve and use another to feed fuel into the combustion chamber.

Two minutes later, computers again sensed that the fuel pump was overheating and automatically shut down the center engine.

The two remaining main engines continued to run smoothly and burned about a minute and 10 seconds longer than originally planned, putting the shuttle into the lower-than-scheduled orbit.

Two auxiliary engines were used to raise the orbit, in steps, from an initial 122 miles to 194 miles by 10:30 last night. Space agency officials had planned a 400-mile in order to achieve the mission's scientific goals.

The crew dumped as much as 4,400 pounds of fuel so that the spacecraft could get into as high an orbit as possible, though Moore conceded that Challenger probably will not reach the orbit its crew would like.

He said several scientific experiments on board might lose data because the altitude was not high enough. One case in point is an experiment to measure the helium blown off the sun by the solar wind.

"It's too early to assess the scientific impact of what went wrong with our engine," Moore said. "I can only say we hope to get as much out of this mission as we can."

The spacecraft is carrying $80 million worth of scientific experiments.

Astronaut Dick Richards at the Mission Control Center in Houston told the shuttle's commander, Air Force Col. C. Gordon Fullerton: "As you can imagine, there's a lot of replanning going on around here. The good news is: We think we'll be able to pull most of it off."

By 7:30 last night, Challenger had used more than 5,000 pounds of the 24,000 pounds of fuel it carries to burn in its two auxiliary engines, located on either side of the shuttle's towering tail and used to maneuver the craft in orbit and slow its descent to Earth.

"We have 20,000 pounds of fuel left," Moore said. "That's not as much as we'd like to have at the start of the mission, and one of the things we'll be doing is to outline a rationing program to make the best use of that fuel."

The biggest concern flight directors had yesterday after the unprecedented engine shutdown was whether a bearing, turbine blade or pump seal had overheated so badly that it caused the turbine to run hot.

The high-speed pumps used to feed fuel to the shuttle's main engines turn about 60,000 times a minute, making them the most powerful pumps in use anywhere. If allowed to run hot, an overheated fuel pump could explode.

In addition to Fullerton, the crew includes the oldest person to fly in space, mission specialist Karl G. Henize, 58. Cleon Lacefield, ascent flight director, said the Mission Control Center in Houston also received indications of dangerously high readings on a second engine, and the crew was instructed to disconnect a backup sensor to prevent the second engine from shutting down before the spacecraft reached orbit.

If another engine had failed at that point, Lacefield said, "We would have been in the water." He said there was a chance the shuttle could have landed in Crete.