CAPE CANAVERAL, SEPT. 17 -- Like a recurring bad dream, another hydrogen leak tonight foiled NASA's fourth attempt to launch the space shuttle Columbia on a 10-day astronomy mission. The launch was then postponed indefinitely.

Shuttle chief Robert L. Crippen acknowledged that the delays raise questions about the program's quality control and engineering approach but asked for understanding of the technical difficulty of tracking down a hydrogen leak.

There is no reason to believe a similar problem exists in the shuttle Discovery, he said, which is now next in line to lift off, Oct. 5 at the earliest. It is scheduled to deploy the robot probe Ulysses on a high-priority mission to study the sun, but must be launched by Oct. 23 or wait 13 months for the right planetary alignment.

Columbia's astronomy mission could be rescheduled for November if the leak is located and fixed quickly, officials said.

The countdown was halted shortly after the leak was discovered at about 6:30 p.m., as ground crews began filling the shuttle's giant external tank with a half million gallons of supercooled liquid hydrogen and liquid oxygen.

After a plague of elusive hydrogen leaks grounded the shuttle fleet for the summer, shuttle managers were braced for at least a small leak this time. They relaxed their criteria for approving the launch in order to accommodate more escaping hydrogen than was previously allowed.

The new limit was far below the level where the hydrogen would pose a significant risk of exploding, officials said, and far below the level of previous launch-stopping leaks this year. But tonight's leak was well above the new limit of acceptability.

The seven-man crew of the astronomy mission, known as Astro-1, includes the first non-astronauts to fly since Challenger exploded in January 1986. They are Samuel T. Durrance, of the physics and astronomy department of Johns Hopkins University in Baltimore, and Ronald A. Parise, a senior scientist at Computer Science Corp. in Silver Spring.

Astro-1's planned May launch was delayed by a hydrogen leak which later delayed the launch of a sister ship, Atlantis, and grounded the shuttle fleet for the summer while frustrated engineers struggled to locate and analyze the leaks.

The investigators focused their attention on a part of the shuttle plumbing known as the 17-inch disconnect, which carries fuel from the external tank into the aft engine compartment. That plumbing was replaced on Columbia and no leaks sprang from that hardware during the last countdown.

But sources said engineers, preoccupied with the 17-inch disconnect unit, failed to search exhaustively for other possible leak points.

The result was yet another hydrogen leak from a different location that forced NASA to scrub its third attempt to launch Astro-1 early this month. (A second attempt had been postponed by a communications problem in one of the mission's telescopes.)

At first engineers thought the new leak was coming from a recirculation pump in the engine compartment and installed new pumps.

But during a check of fuel lines last week they discovered "almost by chance," as one engineering source put it, that a small Teflon seal had been crushed when it was improperly installed after Columbia's last flight, creating a leak path.

Tests showed there was still more hydrogen leaking than could be accounted for by the crushed seal, sources said. The limit of allowable leakage was raised to avoid delay over a leak said not to be serious.

The previous limit called for a launch to be scrubbed if more than 600 parts per million (ppm) of hydrogen accumulated in the engine compartment. The new limit is 1,000 ppm. The leak that halted the last countdown was 6,500 ppm, and tonight's leak peaked at about 3,800 ppm.

Hydrogen becomes dangerously flammable at a concentration of about 40,000 ppm.

Engineers plan to install lights and cameras inside the aft compartment and fill the system with supercold hydrogen in order to flush out the leak, Crippen said.

This leak and the last one may have been caused when the fuel system was taken apart, cleaned and replaced after Columbia's last flight in January, he said.