Engineers worked through the night to replace the first of three suspect turbopumps in the shuttle Discovery's cramped engine compartment with the ship's crew on hand for the start of a practice countdown.

The pump replacement, ordered last week, forced NASA to delay the year's first shuttle flight from Feb. 23 to mid-March, a target that must be met if the space agency is to avoid delays for a high-priority science mission planned for April.Main engine No. 1's high-pressure liquid oxygen turbopump was to be removed overnight, and all three "LOX" pumps are scheduled to be out of the orbiter by the end of the week. If all goes well, replacements will be in place by the end of the month.

Hoping for the best, Discovery's crew flew to the Kennedy Space Center from Houston Sunday to participate in the traditional countdown test, a dress-rehearsal of launch-day procedures. The countdown began early Monday.

"We'd like to take this opportunity to express our appreciation to everybody . . . doing heroic work trying to get the turbopumps built and replaced so we can get off sometime in the middle of March," said commander Michael Coats. "We certainly appreciate all the work that's being done."

Coats, 43, and his crewmates - co-pilot John Blaha, 46, James Bagian, 36, Robert Springer, 46 and James Buchli, 43 - planned to practice emergency launch pad escape procedures Monday before climbing aboard Discovery Tuesday for the final hours of the practice countdown.

Discovery's launch processing has proceeded in fits and starts with numerous glitches and technical problems causing repeated delays.

The spaceship was hauled to launch pad 39B Friday - five days late - but its $100 million payload, a NASA Tracking and Data Relay Satellite, was not installed in the ship's 60-foot cargo bay Saturday as planned.

NASA spokeswoman Lisa Malone said interference between the satellite and another payload already in the cargo bay forced engineers to put off the installation until Monday.

While a firm launch date has not been determined, sources say March 10 is the earliest possible target based on a "success oriented" processing schedule that calls for around-the-clock work at the launch pad without any contingency time for unexpected problems.

All three oxygen turbopumps are being replaced because of concern about cracks found in a bearing assembly inside a pump used by the shuttle Atlantis in December.