Space agency officials confirmed today that they believe an object spotted under 1,100 feet of water off Cape Canaveral is the right-hand booster that flared irregularly seconds before the space shuttle Challenger exploded Jan. 28.

The site is consistent with projections of the right-hand rocket's trajectory after it separated from the shuttle following the explosion, officials said.

Recovery of the booster would be invaluable to the investigation of the Challenger disaster, since some rocket experts believe that a flaw, perhaps in the seals between the rocket's fuel segments or at the point where rocket segments are joined, may have allowed a flame to leak out and cause the explosion.

The National Aeronautics and Space Administration board that has been investigating the explosion is expected to detail its findings publicly for the first time Thursday in Washington at a briefing for the new presidential commission that is taking over the probe.

The daylong session is designed to aid the 12-member panel, headed by former secretary of state William P. Rogers, that President Reagan named Monday to examine the shuttle disaster and issue findings within 120 days.

In a related development today, NASA officials in Washington said at a budget briefing that the destruction of Challenger has left the agency's proposed $7.69 billion fiscal 1987 budget shrouded in uncertainty.

"At the moment, we have the entire NASA budget under review," said NASA's acting administrator, William R. Graham. "We have to look across the range of our budget and the range of our programs.

"The response to the Challenger accident is far from complete," Graham said.

Rescue crews have concentrated their search for Challenger's suspect solid rocket booster on a deep-water site about 35 miles offshore, it was announced today.

NASA officials said a specially equipped recovery vessel, the Liberty Star, was maintaining position, but had been forced to suspend operations this morning because of choppy seas and winds of 20 to 25 knots.

"Strong sonar 'hits' which may indicate the presence on the ocean floor of a large object in an area of suspected SRB impact have been received," officials said.

A companion ship, the Independence, was reported on its way to the location.

Officials denied persistent reports that the solid-fuel booster had not only been positively identified but raised and brought to shore.

NASA contract crews have been using sophisticated side-scanning sonar devices, the type that found the long-lost Titanic, as well as powerful sonar devices aboard robot submarines that can take three-dimensional pictures. The submarines are also equipped with special cameras.

Asked why positive identification had yet to be made in light of all the available equipment, NASA's chief spokesman here, Hugh Harris, said, "I don't know the answer."

Another "shallow water" team of ships, the Freedom Star and a U.S. Navy vessel, the Variety November, was conducting sonar searches 15 miles nearly due east of the launch site. The ocean there is relatively shallow with depths ranging to 150 feet.

Officials refused to say what they think might be in this area. But there have been reports that a portion of the crew compartment has been located.

More than 12 tons of surface debris have been found and brought to shore at a Navy Trident nuclear submarine dock south of the Kennedy Space Center complex here.

Pieces belonging to Challenger are being cleaned as they arrive and laid out on a giant, crossword-puzzle-like grid in a space center warehouse. The aim is to reconstruct as much as possible of the orbiter.

Clearly visible in a NASA videotape shown to reporters were the large underbody flaps from the Challenger superstructure that were used to slow the craft on landing. Some control flaps from the right wing have also been laid out.

The launch, which resulted in the death of all seven crew members, took place at 11:38 a.m. Jan. 28 in abnormally cold weather, a factor that has become one of the major issues of the inquiry into what went wrong.

According to a spokesman for Morton Thiokol Inc., the company that produces and assembles the solid rocket boosters, the solid propellant in the rockets is not intended for use at temperatures below 40 degrees Fahrenheit and is not supposed to be stored at temperatures below 32 degrees.

These temperatures are for the interior of the rocket, but fears have been voiced that the outdoor chill may have been a factor.

According to the National Climatic Data Center at Asheville, N.C., outdoor temperatures at the space center here fell to a low of 24 at 6:55 a.m. and again at 7:30 a.m. on the day of the launch. They were in the 20s through 9:30 a.m. and did not rise above freezing until about 10 a.m. The thermometer stood at 38 degrees at the moment of liftoff.

"These motors are not made for anything under 40 degrees," said Morton Thiokol spokesman Thomas Russell. "This is one of the things our technical people are researching."

Dr. Edward T. Price of Georgia Tech, a leading U.S. rocketry expert and longtime NASA consultant, said "the burn rate of solid-fuel rockets is temperature dependent."

He said solid rockets can be fired in below-freezing weather, noting that "the military fires theirs at extreme temperatures to see how they operate." But he said he was quite sure NASA did not do much experimentation along this line.

Other possible explanations for what may have gone wrong have included faulty assembly of the SRBs, a failure in the inch-long steel pins or bonding between the four main sections, a defect in the metal casing, excessive vibrations, or internal pressure changes.

While recovery ships operated by Morton Thiokol concentrated on the two offshore areas, Coast Guard cutters and supporting aircraft kept up their search for surface debris in a 12,600-square-mile area today from St. Catherine's Island, Ga., to Cape Fear, N.C. They are looking for floating objects that may have been carried north by the Gulf Stream.

At the NASA budget briefing, Graham reaffirmed that "the shuttle is the nation's principal transportation into space and will remain so."

But he and other NASA officials repeatedly declined to answer questions concerning any plans to construct a new shuttle craft either from existing parts in inventory or by other means.

But shuttle program director Jesse W. Moore estimated that there were roughly $400 million worth of shuttle spare parts in inventory and while "you still have a substantial ways to go" to complete a shuttle "you've got a leg up" on the situation.

Meanwhile, the families of Challenger's crew thanked the nation today for its outpouring of support. "So that their lives were not lost in vain, we must rededicate ourselves to the exploration of space and to keep the dream alive," the families said in a statement.