The space shuttle Challenger did not explode 73 seconds into its Jan. 28 launch, and some parts -- including the crew cabin -- remained essentially intact until they smashed into the Atlantic Ocean minutes later, new evidence on the accident indicated today.

Wreckage from the shuttle orbiter, displayed to reporters here for the first time, showed virtually no heat or blast damage from the accident, a National Transportation Safety Board (NTSB) investigator reported.

"We've all used the word 'explosion' for a long time . . . but explosion of the entire shuttle is not something we're seeing," said Terry J. Armentrout, director of the NTSB's bureau of accident investigation, who has been assisting with the Challenger probe.

The disclosure leaves open the question of how Challenger's crew of seven astronauts died. Armentrout refused to speculate on that or the exact state of the crew cabin at the time of impact.

Experts have said they believe that the astronauts could have been rendered unconscious or killed by the shock of the initial fire or by rapid loss of cabin pressure.

Armentrout also emphasized that the shuttle orbiter was under stress from what he called "aerodynamic breakup" as it plunged toward the water at speeds estimated at 140 to 180 mph.

Armentrout's account, based on chemical and other analysis of shuttle wreckage retrieved by Navy salvage ships, clashed significantly with most accounts of Challenger's doomed mission.

Many experts have thought that Challenger was destroyed in an explosion after hot gas erupted from a joint in its right solid-rocket booster about 73 seconds into the mission. The booster then broke loose from a connecting strut and crashed into Challenger's huge external fuel tank, igniting what millions of television viewers saw as a massive, fiery blast.

In fact, Armentrout suggested, the apparent fireball was smoke and flame produced when the fuel tank's liquid oxygen and hydrogen propellants mixed and burned after being exposed to rocket-booster exhaust.

"The optics were deceiving in some regard because it appeared to all of us initially that there was a huge explosion and everything came down in tiny fragments," Armentrout said. "But . . . we don't have any extremely high physical evidence of any blast or thermal distress.

"This was hard for all of us to accept, that there is no burn damage," he said, adding that the orbiter's tail section does show flame damage, suggesting that it struck the fuel tank.

Armentrout said the orbiter's nose section containing the cabin and adjacent react control system broke off during the fire and apparently fell essentially in one piece.

When the nose section "struck the water, it had to have some mass inside; obviously, that mass is the crew module," he said.

Armentrout, who is to present his findings Thursday to members of the presidential commission headed by former secretary of state William P. Rogers that is investigating the accident, briefed reporters in a large hangar at the Kennedy Space Center where the wreckage is being painstakingly reassembled.

Inside are large chunks of the orbiter's right wing, its payload bay and fuselage. The most compelling debris is a fractured but well-preserved segment of the right outer casing that surrounded the crew module and, as Armentrout noted, showed no signs of scorching.

Serial numbers on many of the shell's individual heat tiles are plainly visible, as is a yellow "Rescue" sign denoting an escape hatch in the event of an emergency water landing.

Cabin wreckage, being assembled separately, was first located by Navy divers March 8 in 100 feet of water about 18 miles from the Florida coast. Remains of the crew have been retrieved and are being examined by pathologists.

The National Aeronautics and Space Administration, which sought the safety board's help after the Challenger accident, has steadfastly refused to comment about the remains until the testing process is complete.

Astronauts Ronald E. McNair, Gregory B. Jarvis and teacher Christa McAuliffe, wearing helmets, were seated in the cabin's lower mid-deck and restrained by two shoulder harnesses and a lap belt.

Above them, similarly equipped and strapped into seats in the flight deck, were Michael J. Smith, Judith A. Resnik, Ellison S. Onizuka and commander Francis R. (Dick) Scobee.

According to interviews with members of the astronauts' families, who have been briefed by NASA, the upper flight deck was found to have been crushed by the collision with the water, and all of the astronauts' bodies were found in the hull's lower portion.

At the Marshall Space Center in Huntsville, Ala., today, Lawrence B. Mulloy, head of the space agency's solid-rocket booster program, said redesign of the booster's joint is under way.

In a preliminary report Tuesday, an internal NASA task force identified four problems with the joint in the solid-rocket booster and said they combined to trigger the Challenger fireball.