CAPE CANAVERAL, SEPT. 5 -- The mysterious hydrogen leakage that has kept the space shuttle fleet grounded all summer forced NASA to scrub the launch of Columbia again tonight.

Columbia had been scheduled to lift off at 1:20 a.m. Thursday on a nine-day astronomy mission, but officials said last night that the launch will now be sometime next week at the earliest, assuming baffled engineers can locate the leak, which somehow eluded intense earlier tests and inspections.

Initial indications were that this leak was from a second source not associated with one engineers previously had located and fixed, Robert Crippen, head of the shuttle program, said tonight.

"This hydrogen leak is a real bugger to try to find and isolate," he said, acknowledging the disappointment and frustration of the engineers, astronauts and scientists.

This is the third time the launch has been postponed and the second time a hydrogen leak has halted its countdown. In May, alarms went off when hydrogen flooded Columbia's engine compartment six hours before the scheduled launch. Engineers later pinpointed the leak, they thought, in a part of the fuel line known as the 17-inch disconnect.

The National Aeronautics and Space Administration has not launched a manned mission since April 24, when Discovery lifted off carrying the Hubble Space Telescope, which also suffered a major setback.

Engineers discovered that its primary mirror was manufactured in the wrong shape a decade ago.

Just days after that was announced in late June, NASA officials decided to ground all three of the manned shuttles when a leak similar to the one on Columbia was located on Atlantis shortly before it was to be launched on a Defense Department mission.

The troubled space agency, under mounting congressional pressure to make the space program work in a way that will justify the budget increases sought by its supporters, has launched three shuttle missions this year and 10 since resumption of manned flight 22 months ago. NASA has already scaled back to six its proposal to launch nine shuttles in 1990, and has launched only three.

NASA officials had hoped to launch Columbia by mid-month in order to start preparations for the next launch. The Ulysses mission to examine the sun's polar regions must lift off by Oct. 22 or be delayed 13 months.

In hopes of salvaging most of the year's launch schedule, NASA officials had decided against testing Columbia's fuel system for leaks by filling it with supercold hydrogen well in advance of its launch. They gambled that there would be no leak, but they also reviewed the system of sensors and inspections, they said, to make sure that any leak would be detected.

That part of the plan, at least, worked, and tonight's countdown became a test instead.

About 30 minutes after technicians at Kennedy Space Center began filling the external tanks with 385,000 gallons of highly volatile supercold liquid hydrogen, launch managers announced that sensors in the orbiter's aft engine compartment had detected a leak.

Another 30 minutes later, at about 5:45 p.m., launch director Robert Seick decided to scrub the mission but continue filling the fuel tanks to test and trouble-shoot the fuel system.

Tonight's leak in the aft engine compartment was 6,500 parts per million, or more than 10 times the limit NASA considers acceptable for launching. However, Crippen noted, that limit is set well below the level where the hydrogen becomes flammable.

Leak investigations this summer have focused on the 17-inch disconnect, a part of the fuel system that carries the hydrogen from the tank through the orbiter's belly to the three main engines during ascent.

But tests were never able to duplicate the entire amount of the leak and some engineers were "concerned" that too much attention was paid to the 17-inch disconnect and not enough to the possibility of a second leak in the aft compartment, a NASA engineering official said tonight. "We put our money on the wrong horse."

Ed Weiler, NASA project scientist for Columbia's astronomy mission, said he was frustrated at the delay but added that some of the crew members are fellow astronomers. "We want our friends to be safe. . . . Thank God NASA's being careful."

The coincidental bombshells of the Hubble and shuttle problems subjected NASA to a firestorm of criticism from some members of Congress, the news media and standup comedians and surfaced long-simmering doubts within the aerospace community -- and even within the space agency -- about NASA's institutional capabilities.

But NASA officials, led by Administrator Richard H. Truly, a former astronaut, have defended the agency.

He and others maintain that, by halting the shuttle launches, they have been giving the nation the "safety-first" approach it had demanded following the 1986 Challenger disaster.