With less than a minute to go, a combination of glitches Friday forced NASA to postpone the shuttle Atlantis' launch until Monday at the earliest, delaying the crew's plans to fire a science probe to Venus.

"It just wasn't our day," said launch director Robert Sieck, who added that getting Atlantis ready in time for a second launch try Monday "would be optimistic."The countdown for the year's second shuttle launch, running five minutes late because of trouble with a ground computer, was ticking smoothly toward zero when a problem with main engine No. 1 triggered an interruption a scant 31 seconds before the planned 12:29 p.m. MDT blastoff.

With countdown clocks at the Kennedy Space Center frozen on "-0:00:00:31," a technician radioed launch director Robert Sieck: "We're out of `engine ready."' A few minutes later, NASA spokesman Hugh Harris said launch was delayed for the day.

It was a frustrating disappointment for Atlantis commander David Walker, 44, co-pilot Ronald Grabe, 43, Mary Cleave, 42, Mark Lee, 36, and Norman Thagard, 45, who climbed out of the shuttle about an hour after the launch scrub, smiling and chatting with technicians.

If Atlantis is cleared for a Monday launch try, liftoff would be scheduled for 12:07 p.m. MDT. The launch opportunity would end 43 minutes later at 12:50 p.m. The weather outlook for Monday calls for increasing cloudiness, "but right now, it certainly wouldn't be any cause for concern," Sieck said.

The goal of the flight is the launch of the 9,337-pound Magellan Venus radar mapping probe.