SPACE CENTER, Houston (AP) _ Space shuttle Challenger pilot Michael J. Smith exclaimed ''Uh-oh 3/8'' at the moment the spacecraft exploded, and some of the crew apparently lived long enough to turn on emergency air packs, NASA said Monday.

Smith's remark, heard on a tape of the shuttle's intercom system, was the first indication that any of the seven astronauts killed may have been aware of the Jan. 28 disaster, the worst in the history of space exploration.

The astronauts probably survived the explosion and breakup of the shuttle orbiter and could have had 6 to 15 seconds of ''useful consciousness'' inside the crew compartment after the blast, said Dr. Joseph Kerwin, an astronaut- physician who investigated the cause of death for the crew.

The force of the crew compartment hitting the ocean was so destructive, however, that the precise cause of death for the crew could not be determined, he said.

The intercom tapes, which include enthusiastic chatter among the crew about the moments after liftoff, were recovered from the wreckage of the Challenger and analyzed by National Aeronautics and Space Administration and IBM engineers.

The tape, a transcript of which was released by NASA on Monday, offered no evidence that any crew members other than Smith knew anything was abnormal prior to his single exclamation 73 seconds after launch - the very second that ground controllers lost all communication with the craft.

Previously, the last known words from the Challenger were those heard from Commander Dick Scobee to ground controllers, when he responded ''Roger, go at throttle up,'' confirming that the shuttle's main engines had been raised to full power.

School teacher Christa McAuliffe and mission specialists Ronald McNair and Gregory B. Jarvis are not heard on the recording.

NASA said the three ''could monitor all voice activity but did not make any ... comments.''

Truly said it was not unusual for there to be no comment from crew members not on the flight deck during a launch because they usually don't say anything unless there is an emergency bit of information that the flight deck needs to be aware of.

As recently as July 17, NASA said its reviews of the voice tape indicated that the crew was unaware of the events preceding the breakup of the orbiter.

NASA said Monday, however, that further analysis showed that Smith's final comment offered the first potential indication of crew awareness of the accident.

Smith's exclamation could indicate ''there was a moment of awareness,'' said Admiral Richard H. Truly, associate administrator for space flight.

''There could have been something in the cockpit, some valve or something, that caused that remark, or it could have been awareness of the explosion,'' he said.

Four of the air packs were recovered and three had been turned on, with an analysis of gauges on two of the air packs showing three-fourths to seven- eighths of the air had been breathed, Kerwin said.

One of the air packs that had been turned on belonged to Smith, Truly said. It was not known who the other two activated air packs belonged to, and the recovered air pack that was not turned on belonged to Scobee, NASA said.

Smith and Scobee could not easily have turned on their air packs without getting out of their launch couches, Truly said.

Astronauts Judy Resnick and Ellison Onizuka were riding just behind Smith and Scobee, and they may have assisted Smith, he said.

''The most plausible explanation is that one of the two reached over and turned on Mike's air pack,'' Truly said.

The emergency air tanks would continue to provide air to a crew member whether conscious or not, as long as the face place of the crew helmet was in place, Kerwin said.

Normal conversation without the intercom system would have required the face plates to be raised, he said. This would indicate that the crew members made no attempt to communicate verbally with each other after the intercom failed, he added.

Kerwin said that it was possible the crew members lost consciousness due to a loss of pressure inside the crew compartment. The loss of pressure could be deadly, even if the crew members were breathing from the air packs, he said.

The packs, designed to be used for a shuttle emergency on the ground, normally hold about five minutes worth of air, NASA said.

Investigators say the accident occurred when a joint in a solid rocket booster failed and caused superheated gases to burn through the external fuel tank containing liquid hydrogen and liquid oxygen, triggering the explosion.

The crew cabin tore loose at 45,000 feet, arced upward to about 65,000 feet, and then began a 2-minute, 45-second plunge to the Atlantic Ocean, Kerwin said.

Acceleration forces at the time of the breakup were estimated at 12 to 20 Gs for about 2 seconds, the report said.

''Medical analysis indicates that these accelerations are survivable, and that the probability of major injury to crew members is low,'' it said.

An analysis showed that if the crew members lost consciousness due to a loss of pressure, they would not have had time to revive as the crew compartment fell into denser air at lower altitudes, Kerwin said.

He said the damage to the compartment from the impact was so great that ''no positive evidence for or against in-flight pressure loss could be found.''

The cabin would have hit the water at about 207 mph, creating a force equal to about 200 times that of gravity, he said.

The force of the crew compartment hitting the ocean, which was ''far in excess of the structural limits of the crew compartment or crew survivability levels,'' caused such destruction that the experts were unable to determine the precise cause of death, Kerwin said.

''The findings are inconclusive,'' said Kerwin, who said his investigative team concluded the forces of the Challenger breakup ''were probably not sufficient to cause death or serious injury.''

Truly also said NASA has been ''unable to determine positively'' exactly what killed the Challenger astronauts, but he said ''it is possible, but not certain, that loss of consciousness did occur in the seconds following the orbiter breakup.''

''Many dedicated people, both from within NASA and from other agencies, have devoted long hours and many months, first to recover the Challenger crew module from the ocean floor, and then to examine all available evidence to establish the cause of death of the crew,'' Truly said.

''I believe their efforts have now closed this chapter of the Challenger loss. We have now turned our full efforts to the future, but will never forget our seven friends who gave their lives to America's space frontier.''

Asked when the Challenger families were told of the findings, Truly replied only ''over the past several days.''