The National Aeronautics and Space Administration has identified the remains of at least four of the seven astronauts killed in the Jan. 28 Challenger space shuttle explosion, according to members of the astronauts' families.

Family members said this week that they had been informed by NASA that partial body identifications had been made by military pathologists working in a special medical laboratory at the Kennedy Space Center. Almost all of the astronauts' remains have been recovered from the bottom of the ocean, and there have been reports, so far unconfirmed, that the pathologists may have identified remains of six of the astronauts.

"They've made some identification . . . but they're being very cautious," Mary McNair, the sister-in-law of astronaut Ronald E. McNair, said in a telephone interview. "It's a long process. They've said it could be another two or three weeks."

In another development, two Senate subcommittees were told by the Air Force's top space official that the Pentagon will exercise its "bumping rights" to take the first spaces available on shuttles, when they are cleared to resume flights. Such bumping would cause even greater delays for commercial and scientific payloads.

Edward C. Aldridge Jr., the Air Force's top space official, said the loss of the Challenger posed a national emergency because it is causing a severe backup of plans to launch military and civilian payloads. Although NASA has said it hopes to resume shuttle flights in 12 to 18 months, it may be closer to two years. If so, the Air Force undersecretary said, there will be 21 backed-up military payloads.

Aldridge and officials from NASA urged the construction of a new shuttle orbiter, at a cost of $2.8 billion, and a step-up in construction and use of unmanned rockets to meet the demand for launchings into space.

Rear Adm. Richard H. Truly, a former astronaut who is NASA's new head of the shuttle program, told the panels that the space agency now favors using a mixture of shuttles and unmanned rockets to meet the demand. Until the Challenger accident, NASA had opposed the military's plans to rely on unmanned rockets because they would take commercial launching business away from the shuttle.

Meanwhile yesterday, acting NASA administrator William R. Graham praised the presidential commission investigating the explosion of the space shuttle for its technical expertise and its conduct of the inquiry.

Graham's views, offered in an interview with reporters, contrasts with the sometimes sharp criticism of the commission, headed by former secretary of state William P. Rogers, voiced in recent days by some in the space agency and the aerospace industry.

Earlier this month, for example, Richard G. Smith, director of the Kennedy Space Center, criticized the Rogers commission for declaring NASA's decision-making process "flawed" before it determined what caused the accident. Smith said the commission's approach needlessly damaged the reputations of space agency officials and could "cripple the agency" by triggering a "mass exodus" of top officials.

"They've got a very tough job," Graham said of the commissioners, "and I think they're doing a very good job of handling it. I think they're being very constructive in their approach to it. I wouldn't try to second-guess them."

Graham also attributed signs of internal dissension at NASA -- most notably chief astronaut John Young's angry memo criticizing NASA management for compromising flight safety -- to the tensions that the agency has been under since the accident.

"This has been a very trying time," Graham said. "It's been hard on everyone and people let their feelings and their frustrations be known in a variety of ways. I think we have to take that into account when we look at the discussion that's going on."

Once the military pathologists have completed their tests and the remains have been turned over to the astronauts' families, NASA is likely to conduct a special memorial service, an agency official said yesterday.

"There are going to be some kind of memorial services," said Hugh Harris, NASA's chief public affairs officer at the Kennedy Space Center. "I'm sure the crew will leave here with due respect and the sort of handling that will make the families feel good and show the great regard we all hold for them."