An 875-pound piece of wreckage pulled from the Atlantic two weeks ago is part of the space shuttle Challenger's right solid-rocket booster, the head of the salvage operation said today.

The segment contains part of the joint suspected in the explosion that destroyed the shuttle, although it comes from the opposite side of the area where flames were observed shortly after the Jan. 28 liftoff, Air Force Col. Edward A. O'Connor said. Nevertheless, the section is "quite valuable" to the investigation, he added.

O'Connor, appearing at a Cape Canaveral news conference broadcast at the Johnson Space Center here, said salvagers were unlikely to recover the actual point of rupture because it would have been destroyed by the nearly 6,000-degree plume of flame.

The shuttle's two cylindrical boosters consist of four segments attached by metal joints designed to prevent hot gas from leaking.

NASA officials believe that the bottom joint on Challenger's right booster leaked as its rocket was ignited, and that about a minute later the leak triggered the explosion that destroyed the craft and killed its crew of seven.

In another development today, the presidential commission investigating the Challenger disaster confirmed reports that it will hold a public hearing in Washington Thursday to question National Aeronautics and Space Administration officials and astronauts about the role of the astronaut office in flight safety and operation of the shuttle.

The commission said it will call eight witnesses, including five astronauts or former astronauts. Among them will be John W. Young, head of the astronaut office, and George W.S. Abbey, director of flight crew operations.

Recovery of portions of Challenger's suspect booster joint has been a priority in the Navy's vast salvage operation, which as of March 14 cost $6.8 million, O'Connor said. Officials hope to find more large pieces from the right booster in the same area where other large sections have been found. No other pieces of the critical joint have been recovered, he said.

O'Connor said it took two weeks to positively identify the 875-pound segment because it bore no identifying numbers or marks and showed no signs of burning.