The space shuttle Challenger probably aborted its launch Friday when a mechanical device called an actuator failed to move a valve that allows cooling fluid into an engine chamber, NASA said yesterday.

"We won't know we have the culprit until Sunday when technicians get into the engine and remove suspect components," James Ball of the National Aeronautics and Space Administration said yesterday from Florida's Kennedy Space Center, "but engineers feel strongly enough that they want to get their hands on that actuator to verify that it was the problem."

The actuator moves a valve to regulate the amount of supercold liquid hydrogen that chills the engine chamber before hydrogen fuel floods the chamber and the engine burns at full throttle. The valve on the No. 2 engine failed to close Friday, and Challenger's computers sensed the problem immediately and ordered the three shuttle engines to shut down three seconds before liftoff.

Launch pad workers were due to strip down parts of the engine today and remove at least four components that could have triggered the launch abort, including the suspected actuator.

"A second launch attempt is at least seven to 10 days off or even longer," Ball said. "We'll know a great deal more on Monday when launch directors have a full-scale management meeting about what to do next."

Adding to the uncertainty about a new launch date is the scheduled arrival in Florida today of the spaceliner Columbia, which has just undergone an 18-month overhaul at the Rockwell International plant in California. When Columbia arrives at Kennedy Space Center, all four spaceliners will be at the Florida spaceport for the first time.

Atlantis, the only one of the four yet to fly, is due to be moved on Monday to the Vehicle Assembly Building to be mated to its two solid rocket boosters and its huge external fuel tank. It had been scheduled for a July 30 engine firing, a test that may have to be postponed, depending on how long Challenger must sit on the same launch pad.