ASTRONAUT'S RETURN POSTPONED FROM MIR.

Byline: William J. Broad The New York Times

NASA on Friday delayed by six weeks the return of astronaut Shannon Lucid to Earth from a Russian outpost so as to replace rocket boosters on the shuttle that was to bring her home.

The Atlantis mission, which was to retrieve Dr. Lucid from the Russian space station Mir, was delayed from the end of July to mid-September so NASA could try to make sure that there was no trace of the sort of problem that led to the Challenger explosion a decade ago.

It was a leaky booster rocket that destroyed the space shuttle Challenger in 1986, killing all seven crew members, including a high school teacher.

Evidence of new danger with the reusable booster rockets emerged earlier this month from the last shuttle flight, and ever since then the National Aeronautics and Space Administration has been analyzing the evidence and wrestling with how to best deal with the risk.

NASA officials suspect that a new adhesive used in the rocket joints allowed hot gases and flames to get dangerously close to the joints' seals. In the Challenger disaster, hot gases leaked through the rocket joint, igniting the hydrogen fuel tank.

The delay of the Atlantis mission from July 21 to around Sept. 15 was announced late Friday at the Johnson Space Center in Houston.

Tommy Holloway, NASA's shuttle program manager, said at a news conference that the agency was not concerned about the design of the booster joint. The joint's sealing system was extensively redesigned after touching off calamity a decade ago. ``This event and decision in no way affects our confidence in the joint,'' he said.

``I cannot quantify the increased protection'' that will result from the booster replacement, Holloway added, ``but it was something we felt was the proper thing to do.''

Dr. Lucid, a 53-year-old biochemist and mother of three, has been aboard Mir since March 24 and was launched into space aboard a shuttle on March 22. She reacted mildly earlier this week to the possibility of a delay in her return to Earth, saying she had brought along plenty of books to read.

``Obviously, like anybody, I was looking forward to being with my family and my friends again,'' Dr. Lucid said in an interview with The Associated Press. ``But like I told them the other day, 10 years from now you won't ever remember that we had to wait an extra few weeks before we saw each other.''

Dr. Lucid is the second American astronaut to live on the Mir station. On Monday she will surpass the American space-flight endurance record set last July by Dr. Norman Thagard, 52, an astronaut and physician who spent 112 days on Mir.

On Friday as the delay was announced, Dr. Lucid orbited Earth with her two Russian colleagues, Yuri Onufrienko and Yuri Usachev. NASA said that she and her crew mates were asleep when the delay decision was made and that she would be informed of it first thing this morning.

Frank Culbertson, the American head of the shuttle-Mir flights, said at the news conference that NASA's medical specialists were closely monitoring Dr. Lucid's physical and psychological health and were satisfied that she was ready to stay in orbit longer.

``Everybody is in concurrence that Shannon is in good shape,'' Culbertson said. He added that extra supplies were being readied for shipment to her up in the Mir station.

NASA officials said Dr. Lucid would be involved in a news conference from space Monday and would discuss the delay.

The vexing boosters are like giant Roman candles - 15-story tubes packed with rubbery, highly explosive fuel that once ignited cannot be extinguished. A pair of boosters, mounted on either side of the shuttle, assists the spacecraft into orbit on each flight.

After falling away from the speeding shuttle and parachuting into the ocean, they are recovered and repacked with fuel for another flight. The rockets are shipped from Utah in pieces and assembled in the ``field'' in Florida, resulting in three field joints on each rocket.

NASA was first alerted to the new problem with the boosters after discovering scorching and soot in the ``field joints'' of the pair that helped push Columbia into space on June 20.