CAPE CANAVERAL, MAY 29 -- Wednesday's early morning launch of the space shuttle Columbia, with its cargo of four telescopes, was delayed tonight because of a possible fuel leak.

The shuttle's handlers began loading Columbia with liquid hydrogen and oxygen fuel late this afternoon but stopped after traces of hydrogen were detected outside the shuttle's huge silo-shaped external tank.

"Right now, indications are that there was a leak from the main propulsion system," said NASA spokeswoman Pat Phillips. "The vehicle was never in an unsafe condition."

The launch was scrubbed after technicians detected "higher than allowable" levels of hydrogen in the shuttle orbiter's aft fuselage. Ambient levels of hydrogen had risen to several thousand parts per million, a level that Phillips described as "not dangerous."

National Aeronautics and Space Administration spokesmen said the cause of the fuel leak was unknown and that NASA technicians would work through the night if necessary to uncover its source.

No new launch time or date was given.

The shuttle's external tank carries the fuel that propels the orbiter's three main engines. The leak was detected soon after workers began to "fast fill" the tank with liquid hydrogen. Only about 15,000 gallons of the 385,000 gallons of hydrogen, or about 4 percent, had been loaded.

Columbia's mission, which had been delayed almost two weeks because of problems with the shuttle's cooling system, is the first in five years dedicated entirely to science.

Four telescopes, including one from Johns Hopkins University, are nestled in the shuttle's cargo bay where they will be pointed at more than 200 celestial objects during the nine-day mission. The telescopes will use special sensors and lenses to "see" the invisible, such as X-rays and ultraviolet radiation.

Scheduled for viewing are the dense, hot embers of white dwarf stars and the swirling debris that is sucked like suds down a drain into the maw of black holes. The astronomers also plan to point their telescopes at star clusters, supernovas, pulsars and distant galaxies.

In addition to telescopes, Columbia will carry two astronomers from Maryland: Ronald A. Parise, 38, a senior scientist at Computer Sciences Corp. in Silver Spring, and Samuel T. Durrance, 46, a research scientist at Johns Hopkins.