he overdue second flight of the space shuttle Columbia was postponed today for at least a week because two filters feeding lubricating oil to the spacecraft's hydraulic lines jammed just before liftoff.

The oil problem apparently was caused by dirty oil clogging the filters, due to the failure of the space agency's technicians to change the oil--five quarts of it--after the spacecraft's maiden flight last April. The jamming, which could have been disastrous if it had happened in flight, occurred on the ground after the launch was delayed for two hours because of a computer programming error, which apparently had no connection with the oil failure.

"We're as sorry as they are for what happened, but the prudent thing to do was to try another day," flight director Neil Hutchinson said an hour and a half after astronauts Joe Henry Engle and Richard H. Truly were told their five-day flight had been scrubbed.

Two apparently unrelated mishaps culminated in the last-minute clogging of the oil filters for the hydraulic lines, which direct the winged spaceship on its flight away from earth and control its landing gear, rudder, elevons, body flaps and speed brakes when it returns to earth.

So unexpected was the mishap that flight directors were unable to estimate how long it would take to get Columbia ready again for spaceflight. The flight already has been postponed twice, from Sept. 30 to Oct. 9 and then to today.

"We may have to drain the oil, remove the filters, clean them or replace them," L. Michael Weeks, acting associate administrator of the space agency, said late today. "We may even have to replace the entire system."

The chance of getting Columbia into space in a week was in doubt today, partly because of the complexity of the problem that scrubbed the launch, partly because Florida's east coast was being drenched with rain less than three hours after the launch was called off, and partly because of a storm that is lying off the coast of Cuba.

"We know we have that tropical depression called Katrina sitting just north of Cuba right now," Hutchinson said. "The potential for lots of cloud cover and rain is certainly with us for the next few days."

More worrisome than the weather were the jammed oil filters, which flight directors conceded had been used on Columbia's maiden voyage last April and had not been cleaned or changed since.

"We did not service the lubrication systems between flights," Johnson Space Center Flight Engineer Dwayne Weary said. "They were checked in July, when we failed to find any reason to change out the oil. Perhaps we underestimated the possibility of trouble."

The filters service two of the three pieces of machinery in the shuttle's auxiliary power units, which power the three hydraulic lines that swivel the spacecraft's engines in flight so it will change directions in space, feed fuel to its pumps and activate all its maneuvering systems when it lands.

If Columbia had taken off before flight directors noticed the oil filters had jammed, Engle and Truly might have had trouble landing their spacecraft next Monday at Edwards Air Force Base in California. Once the shuttle is in orbit, the auxiliary power units are turned off. They are not turned back on until the astronauts are ready to land and need the hydraulic lines to maneuver the spacecraft and lower the landing gear.

"If we had not stopped the countdown," Hutchinson admitted, "the shuttle could have been launched with two troublesome power units."

Flight directors had no idea the oil filters would give them trouble until the onboard flight computers stopped the countdown 31 seconds before Columbia was to take off. At this point in the countdown, ground computers automatically hand control of the spacecraft over to the on-board computers.

By late today, flight directors still did not know why the flight computers had stopped the countdown with 31 seconds to go. One theory was that the flight computers had not been programmed to know that the ground computers had decided an apparent drop in pressure in a fuel tank oxygen chamber was just a malfunction of monitoring instruments. When the on-board computers assumed control, according to this theory, they overrode the ground computers and stopped the countdown.

Whatever stopped the countdown, it delayed the launch almost two hours, and during that time the oil filters jammed up.

Less than 15 seconds after the countdown resumed, the oil pressure in two of the three pumps that feed the lubricating oil through the filters jumped from 60 pounds per square inch to 100 pounds per square inch. This told flight directors that the lube oil was struggling through a bypass line and that the filters were clogged. The bypass line is built into the unit so that oil will continue to lubricate the machinery even if it is contaminated and has clogged up the filters.

"It's just like your car," Hutchinson said. "It's better to get contaminated oil into the machinery than no oil at all."