Endeavour will have to wait until Tuesday to launch. Photo: NASA

Shuttle Endeavour won't blastoff on its voyage to the International Space Station until Tuesday so cosmonauts can perform a contingency spacewalk outside the station to assist in getting a wobbly cargo ship firmly docked to the outpost before the shuttle arrives.

NASA managers made the decision to delay the launch until next week during a meeting Friday morning. Earlier, during a Thursday evening teleconference, space agency officials concluded that tentative plans to launch Endeavour on Friday evening would be scrapped, instead pushing back the liftoff for at least another 24 hours to Saturday.

Endeavour was originally slated to launch on its 11-day mission to exchange resident crews aboard the station on Thursday evening from Kennedy Space Center, but the failed docking of a Russian Progress freighter on Wednesday scuttled NASA's plans.

A piece of debris, suspected to be a cable or wire or maybe a separated seal, was on the docking mechanism as the Progress linked up to the aft port on the Zvezda service module. The freighter's probe penetrated the port and was captured, however, the debris prevented the probe from retracting as designed to bring the craft and station together in a tight fit. The end result was the inability of eight latches to engage that would firmly hold the Progress in place for its 80-day stay at orbiting complex.

So the Progress was left in a "soft docked" state. A "hard dock" configuration with the latches locked would mean the supply ship is snugly mated to the Zvezda module with an air-tight seal, allowing the station crew to open hatchways to enter the freighter to retrieve more than one-ton of food, clothing and other items packed inside.

Russian flight controllers say they saw the cable resting on the docking port in video captured by a camera mounted to the front of the last Progress ship when it undocked from the station a week ago. That same cable was seen again Wednesday when the new Progress approached for docking.

The identity of the cable and how it got there isn't known, though it could have been brought to the station by the last Progress. The docking port is only open during the short periods between the departure of one ship and the arrival of the next.

Plans are being developed to stage a spacewalk on Monday, starting at about 8:30 a.m. EST, by cosmonauts Vladimir Dezhurov and Mikhail Tyurin, to remove the debris from the docking mechanism. Expedition Three commander Frank Culbertson will monitor the spacewalk from inside the station.

A file close-up image shows the probe on the front of a previous Progress spacecraft. Photo: NASA

The Progress' docking probe will be re-extended, physically pushing the craft away from the docking port 8 to 12 inches while still keeping it connected to the station. The one-foot gap created should allow the spacewalkers the chance to snare the debris. The probe would then retract fully and permit the latches to close.

Sources said Thursday night that the NASA engineering community unanimously recommended delaying Endeavour's launch until after the spacewalk. Not all senior NASA managers were able to attend Thursday evening's teleconference, so until everyone, including the Russians, were briefed on the latest developments, the official selection of Endeavour's new launch date was put off for the night.

Although a Friday launch attempt was ruled out, the Kennedy Space Center launch team began preparing for a Saturday liftoff just to keep options open.

The NASA Mission Management Team met at about 9:15 a.m. EST Friday to make a formal decision on how to proceed. And it was then made official: Endeavour will wait until Tuesday to fly. The launch time will be 5:45 p.m. EST.

"We agreed that the best use of our resources, the best way to minimize the risk to the (Endeavour) flight, to be sure we can get it off before the end of the year and accomplish all the critical mission objectives was to target our first launch attempt on Tuesday evening," James Van Laak, space station operations manager at the Johnson Space Center in Houston, told a news conference Friday.

"The principle benefits of doing that is that it allows us to have a clear understanding of the consequences, the outcome of the EVA on Monday. We feel good it can be done and will be successful, but it uses a good part of the day Monday and we want time to assess the outcome and make certain we're in a good safe, configuration.

"Clearly, if we were to try and launch on Monday it would require that we tank and have the crew get aboard the vehicle while we were still assessing things. And that puts a great deal of pressure on all involved and was not the way to make the best decision possible."

But what if the spacewalk isn't successful in clearing the obstruction?

Van Laak said in such a situation plans would call for the Progress to be locked down as best as possible, like it is now, allowing Endeavour to launch Tuesday and dock two days later since structural analysis shows the shuttle can link up with the station with the Progress in its current state. Further work on the Progress to be delayed until after the shuttle departs the station.

In a worst-case scenario in which the cosmonauts can't get the debris removed and the Progress isn't re-secured as it is currently, then the craft would undock and move to a parking orbit until after the shuttle visit.

Earlier on Thursday NASA had considered an option of launching Endeavour on Friday and have the spacewalk occur while the shuttle was docked to the station. But that would have most likely forced a spacewalk scheduled for two shuttle astronauts -- to install thermal blankets on the drive motors of the U.S. solar arrays -- to be deferred to the Expedition Four crew, as well as forcing mission planners to rework parts of the game plan for Endeavour's docked time at the station.

Although trained for the job of attaching the blankets, the spacewalk would have disrupted the Expedition crew's planned timeline that is already full with stowing supplies brought by the Progress and shuttle and tending to the station's science experiments. The crew -- commander Yuri Onufrienko and flight engineers Carl Walz and Dan Bursch -- are scheduled to spend five months on Alpha.

The official reason for Thursday's launch attempt to be scrubbed was the need to give engineers more time to ensure it was safe for the shuttle to link up to the station with a Progress not "hard docked." The shuttle imparts structural loads when it attaches to the station and NASA needed to ensure the wobbly Progress wouldn't rock back and forth and damage the complex.