When the space shuttle Challenger lifted off on Jan. 28, it had 740 components that each had to function perfectly to prevent disaster, the National Aeronautics and Space Administration announced yesterday.

Failure of one of these components -- the O-ring seals in the booster joints -- is widely suspected as causing the Challenger explosion.

Late last month NASA announced that it would "re-review" all "Criticality 1" components -- so called because they have no backup systems if they fail -- to see whether backups or new designs can be provided.

Although the number came as no surprise to NASA, officials said it was the first time all such items had been totaled. Failure of any one of the 740 shuttle components, plus eight more in ground support equipment at Kennedy Space Center, Cape Canaveral, Fla., automatically means that the shuttle and its crew would be destroyed.

Of the total, 131 are items for which no backup is practical. This includes the vehicle's "primary structure," or the fuselage as a whole, its heat-shielding tiles, pressure vessels such as the fuel tanks and the explosive bolts that allow various parts of the vehicle to separate when needed.

The other 617 items, however, are shuttle parts for which a backup is theoretically possible but is not provided because NASA engineers have assured themselves that a failure is unlikely, according to Marion E. Merrell of the safety, reliability and quality assurance office at Johnson Space Center in Houston.

The booster joints were originally designed with a second O-ring to seal the joint if the primary O-ring failed. It was originally designated as a Criticality 1-R, which meant there was redundancy in case of failure. But, more than three years ago, it was found that the joints can be warped while the engine is firing, causing the backup O-ring to come out of sealing position.

As a result, the joints were reclassified as Criticality 1 and NASA officials obtained an internal waiver from NASA's normal fail-safe rules that allowed them to continue flying. Although the waiver was granted in the belief that a failure of the primary O-ring was unlikely, NASA and Morton Thiokol Inc., the booster manufacturer, began searching for a better way to seal the joints.

Of the 748 Criticality 1 items, 335 are on the orbiter, 94 on its main engines, 114 on the boosters, 133 on the external tank and 64 on other "government-furnished equipment."

"We are reviewing in detail all the items on the critical items list," Arnold D. Aldrich, manager of the shuttle systems office, said in an interview last week. "The point is to completely re-review them and identify any safety concerns."

Richard Colonna, director of orbiter projects, said his office is conducting two independent reviews of the items. Contractors and NASA engineers are independently reviewing each item, Colonna said.

"We're looking for anything the sensitivity or importance of which we may have misconstrued," Colonna said.

Aldrich and Colonna were less specific about what the eventual results of the massive review might be.

"Most of the things we're looking at, probably all of the things, have been looked at previously and we decided not to take additional action then," Aldrich said.

"Some we'll take some corrective action on, some things we've been working on all along," Colonna said. "Some things we'll just live with."

Lawrence B. Mulloy, director of the solid-rocket booster project, has asked his engineers at Marshall Space Flight Center in Huntsville, Ala., to begin redesigning the critical joint that may have failed. Late last week, Mulloy said, he reviewed preliminary redesigns.

Meanwhile, more remains of Challenger's crew apparently were brought ashore last night near Cape Canaveral. Strong currents delayed other salvage operations.