Space agency officials have provided investigators with new evidence challenging the prevailing view that the effect of cold weather on rocket booster seals was the primary cause of the Jan. 28 shuttle explosion and suggesting that other factors -- including a defective or improperly assembled seal -- were responsible, according to space agency officials and other sources.

The National Aeronautics and Space Administration evidence on the blast that killed seven astronauts includes:

*Prelaunch photographs reportedly showing "something funny," perhaps a kink or other flaw, in one of the O-rings sealing the suspected joint on the right-hand booster rocket.

*The fact that workers encountered unusual difficulties and delays in assembling the booster because of a misshapen rocket segment.

*New tests reportedly demonstrating that the O-rings will seal properly down to 10 degrees below zero. Engineers for the rocket manufacturer testified that they warned the agency that pre-launch temperatures -- in the 20s and 30s -- were too low for proper sealing.

The presidential commission investigating the disaster is expected to review the new information, as well as data on inspections of the shuttle, in public hearings Friday at the Kennedy Space Center in Florida. A source close to the investigation said today that NASA's probe is focusing on other scenarios.

In another development today, President Reagan announced the nomination of James C. Fletcher as new NASA administrator. Fletcher, who held the job from 1971 to 1977, had been described as the leading candidate if he would accept. Fletcher would succeed James M. Beggs, who left the job following his indictment on charges unrelated to the agency. William R. Graham has been NASA's acting administrator.

Astronaut Sally Ride, a veteran of two shuttle flights, told ABC News today that she is "not ready to fly again now" and that she thinks other astronauts share her view.

Ride, a member of the Challenger investigation team, also said that the risks might not have been fully explained to Christa McAuliffe, the teacher killed in the explosion. "I think that we may have been misleading people into thinking that this is a routine operation, that it's just like getting on an airliner . . . and that it's that safe. And it's not," Ride said.

The effect of the cold on the O-ring seals has been a prime focus of the investigation so far, largely as a result of testimony from engineers at Morton Thiokol Inc. that they warned NASA in prelaunch conferences that the low temperatures could significantly impair the O-rings, allowing hot exhaust gases to leak from the booster and set off an explosion.

Photos of the lower section of the right booster, taken just before it was attached, show "something funny . . . something strange" about the backup O-ring, according to Lawrence Mulloy, who heads the solid rocket booster program at the Marshall Space Flight Center here. He said it could be as innocuous as grease or as dangerous as "a kink in the O-ring, like in a twist of licorice."

Mulloy, according to the Thiokol engineers, was among the NASA officials who "pressured" Thiokol management into approving the launch over their objections.

"You couldn't tell whether it was a shadow, or a distressed area in the O-ring, or what," Horace Lamberth, a NASA engineer at Cape Canaveral, said of the photos. Mulloy said the photos were not examined before the shuttle launch, but were routinely taken for later use. "They are not a substitute for inspection," he said. Both men spoke before viewing enhanced photos.

Officials also said today that the lower section of the right booster required time and effort to be reshaped for assembly.

The six cylindrical segments, 12 feet in diameter, join to form the 14-story booster rockets. They become slightly "out-of-round" from lying on their sides while being transported to Cape Canaveral for reuse after each launch, officials said.

Normally, Lamberth said, the segments return to their circular shape when they are hung upright prior to assembly. The misshapen segment in question was one of only "five or six" in the history of the shuttle program that had to be coaxed back into its shape by means of a rounding tool, Lamberth said.

Mulloy said that dozens of tests done since the explosion "show that the O-ring will function normally down to minus 10 degrees." The tests, he said, were conducted at Morton Thiokol's plant in Utah.

The new test evidence has not been reviewed by commission members, according to sources. But it is being cited by Thiokol management and NASA officials as a vindication of their position.

In addition, it was learned today that the commission was told that the shuttle underwent a violent wind shear of 125 to 150 miles an hour shortly before the explosion. The evidence was calculated by meteorologist Irving P. Krick, who said the wind shear would have been "like flying into a tornado."

According to a report scheduled for publication in next week's Aviation Week magazine, investigators also found photographic evidence that a puff of steam jetted out of the booster joint a fraction of a second before the cloud of black smoke.

The finding suggests that, during the 37 days Challenger stood on the pad, rain water collected in the U-shaped part of the booster joint. According to the industry magazine, there is suspicion that low temperatures turned the water to ice and the expansion pushed the lower O-ring out of position.

After ignition, gases rushed past the two O-rings and turned the ice to steam, the magazine says, and black smoke emerged as the heat burned the O-rings and other seals.