CAPE CANAVERAL, FLA — CAPE CANAVERAL, Fla. -- The space shuttle Columbia's problems went from bad to worse yesterday when it sprang a third fuel leak, forcing NASA to cancel today's planned liftoff and raising more doubts about the reliability of the U.S. shuttle fleet.
Columbia's mission -- to deploy the $150 million Astro-1 observatory -- will now be pushed back to November or December, officials said, because NASA workers must crawl back inside the engine area to find the new leak.
In what has become a recurring nightmare for space agency officials, Columbia yesterday began leaking unusually high concentrations of explosive hydrogen fuel shortly after technicians began pumping a half-million gallons of cold, liquid propellant into the ship's external tank.
The leak was in the same area where Columbia sprang a leak Sept. 6 -- the shuttle's main engine compartment. Kennedy Space Center launch Director Bob Sieck canceled the flight at 6:35 p.m.
Columbia's countdown has now been delayed four times over 111 days since it was first fueled for blastoff the evening of May 29. That is the longest delay in the history of the shuttle program.
"We still have a leak in the orbiter's aft compartment and we're 'no go' for a launch tonight," NASA spokesman Lisa Malone said.
The delay in Columbia's launch puts NASA's highest-priority mission for the year -- the shuttle Discovery's deployment of the Ulysses solar probe -- up next. Discovery's launch is scheduled for Oct. 8.
Shuttle managers were at a loss yesterday to explain the leak, especially since workers had replaced a crushed seal and three engine pumps that were thought to have caused the Sept. 6 leak.
"The mood is pretty depressing after all summer trying to run down these leaks," said Keith Hudkins, director of the shuttle orbiter division, in Washington. "All the experts, and I mean experts, have been working very long hours."
The fueling operation began after officials changed a launch rule to allow a small increase in the level of hydrogen gas that could creep into the shuttle's engine compartment and yet be within safety limits.
The previous limit was 600 parts per million. The new limit they decided on was 1,000 parts per million. Hydrogen gas levels have to reach 40,000 parts per million before they become flammable.
William Lenoir, chief of the National Aeronautics and Space Administration's space flight office in Washington, said the change was made after a "tremendous amount of analysis" had been done on leaks that have kept the shuttle fleet grounded all summer.
In the end it did not matter. Sensor readings showed Columbia's leak reached 3,500 parts per million.
Mr. Hudkins said the leak might have been caused by "bad seals in smaller lines" that feed fuel to the shuttle's engines. He also said Columbia, which ushered in the shuttle era in 1981, could be getting old.
"If it turned out to be a cracked weld, that would be indicative of an aging problem," he said. "But I don't think that's going to be the case."
To hunt down the leak, NASA will probably do something it has not done since the shuttle was under development in the late 1970s: install television cameras in the engine area to try to pinpoint the problem.
When hydrogen gas escapes, it appears as white smoke and thus could be spotted.
"My guess is that we'll put cameras in there and do more tanking tests," Mr. Hudkins said. "We've done that before in the development process. We've got the equipment."
The shuttle fleet's fuel leaks began May 29 when Columbia sprang its first leak while it was being fueled for launch. Its sister ship Atlantis sprang a leak one month later, which left NASA no choice but to halt flights until the trouble was solved.
Both leaks were blamed on a faulty seal in a 17-inch valve that controls the flow of fuel to the spaceship's main engines. The seals were apparently damaged by tiny glass beads that somehow got into the fuel lines of the two orbiters.
The Astro-1 mission is supposed to make use of the Hopkins Ultraviolet Telescope, a project begun a decade ago.
Johns Hopkins University astrophysicist Samuel T. Durrance and Ronald Parise of Computer Sciences Corp. in Silver Spring are members of the mission's seven-man crew. They began training for the flight in 1984.