NASA is considering the possibility of moving up space shuttle Endeavour's liftoff date by one day -- from April 19 to April 18 -- to provide three opportunities to get the shuttle airborne before a mandatory Russian launch to replace the space station's Soyuz lifeboat.

Space shuttle Endeavour's launch date is up in the air. Photo: NASA-KSC

Endeavour remains officially targeted for blastoff on April 19 with seven astronauts on an 11-day flight to deliver a Canadian-built robot arm to the international space station.

Endeavour is scheduled to undock from the complex on April 28, the same day a Russian Soyuz-U rocket launches from the Baikonur Cosmodrome in Kazakhstan carrying two cosmonauts and, presumably, millionaire tourist Dennis Tito.

The Soyuz capsule will dock with the station two days later, giving the station's full-time residents a fresh emergency evacuation vehicle.

Soyuz spacecraft are certified for 180 days in space. The Soyuz currently attached to the station has been in space since October 31 when it carried the station's first full-time crew into orbit.

The Russians have balked at delaying the Soyuz swap, forcing NASA officials to ponder how to squeeze in more launch opportunities for Endeavour.

The issue is complicated by the nature of the shuttle's rendezvous profile. A launch on April 19 would result in a docking on flight day three, but a launch on April 20 would require a flight-day four rendezvous, stretching out the mission and causing additional impact to the Soyuz flight.

There isn't enough room for the shuttle and two Soyuz vehicles to be docked to the station. So if Endeavour doesn't launch before the Soyuz replacement, the shuttle mission would have to postponed until May 6 so Endeavour's docking occurs after the old capsule and its taxi crew depart for the journey back to Earth.

One possible scenario under discussion, NASA sources say, calls for Endeavour to launch on April 18. The launch window that day extends from 2:58:16 to 3:08:16 p.m. EDT with a preferred liftoff time of 3:03:16 p.m. EDT.

The crew patch for Endeavour's STS-100 mission. Photo: NASA

If weather or a technical problem prevented launch on April 18, NASA would have backup shots on April 19 and 21st with a day off in between to rest the ground teams. All three launch opportunities feature flight-day three dockings.

Under this scenario, the flight-day four launch day would not be needed and the worst case delay to the Soyuz launch would be two days. And that would come only if Endeavour's first two tries were scrubbed and the shuttle didn't fly until April 21.

The launch window on April 19 runs from 2:35:39 to 2:45:39 p.m. EDT with a preferred liftoff time of 2:40:39 p.m. EDT. The window on April 21 is 1:47:21 to 1:57:21 p.m. EDT with the preferred time of 1:52:21 p.m. EDT.

NASA managers are holding discussions with the Russians this week in advance of Thursday's Flight Readiness Review. The FRR is the agency-wide meeting to establish the official launch date of Endeavour. As of this writing, the Russian position on any delay for the Soyuz flight is not known.

During Endeavour's seven days docked to the space station, two astronauts will make a pair of spacewalks to assemble and attach the 58-foot long Space Station Remote Manipulator System arm to the Destiny laboratory module. The arm will serve as a "construction crane" to aid in building the station and maintaining it in orbit.

The shuttle astronauts will also mount the Italian-made Raffaello "moving van" cargo module to the station using Endeavour's robot arm. Raffaello carries several tons of equipment and supplies, which will be unloaded before the module is returned to Endeavour's payload bay for the ride back to Earth.