Work to replace a powerful magnet in a $1.5 billion physics experiment bound for the International Space Station has forced NASA to move a summer flight by the shuttle Endeavour to mid November, agency officials said Monday, delaying the planned end of the shuttle program.

The STS-134 crew portrait. Credit: NASA

NASA will press ahead with plans to launch the shuttle Atlantis May 14 on a space station assembly mission, but Endeavour's planned July 29 flight will slip to mid November, leap frogging a Sept. 16 launch by the shuttle Discovery that was to have been the shuttle program's final flight.

NASA did not specify a firm target launch date for Endeavour and mission STS-134 pending completion of work to modify its primary payload, the Alpha Magnetic Spectrometer.

The AMS payload is now expected to be delivered to the Kennedy Space Center in late August, but as of this writing it is not clear whether November is a realistic target for Endeavour's launch on the final planned shuttle mission.

As it now stands, temperature constraints related to the station's orbit would preclude a shuttle launch between Nov. 8 and 25. Complicating the picture, three of the station's six crew members are scheduled to return to Earth Nov. 26. Their replacements are scheduled for launch Dec. 10.

To avoid a conflict with the December Soyuz flight, Endeavour would need to take off between Nov. 26 and the end of the month, it would appear, to complete its mission and undock before the new crew members arrive.

Other options appear limited. Looking further downstream, shuttle software issues would preclude launching Endeavour after Dec. 15 to avoid having the shuttle in orbit during an end-of-year rollover. Another so-called beta-angle cutout - the period when the angle between the sun and the station's orbit precludes a shuttle visit - begins Jan. 4 and extends through Jan. 20.

A NASA spokesman said the schedule will be re-evaluated later this summer, depending on how the AMS magnet swap-out proceeds.

The Alpha Magnetic Spectrometer is designed to study high-energy cosmic rays, helping determine the balance of normal matter to antimatter. AMS also may be capable of detecting evidence for the so-called dark matter that is believed to exist throughout the universe. Dark matter has not yet been directly detected, but it is believed to make up about a quarter of the mass-energy density of the universe. Normal matter accounts for about 5 percent and repulsive dark energy, believed to be accelerating the expansion of the cosmos, makes up the balance.

The AMS experiment was designed to use a liquid helium-cooled superconducting magnet to bend the paths of electrically charged subatomic particles, allowing scientists to study their properties. Payload managers, however, recently decided to use a less powerful, non-superconducting magnet that will enable AMS to operate through the planned life of the space station instead of a relatively short three years. While it will not be as sensitive, the longer run time is expected to make up for the shortfall.

"Given that the ISS lifetime has been extended (to at least 2020), the permanent magnet gives them a much longer science mission than the cryo magnet," Mike Moses, director of shuttle integration at the Kennedy Space Center, said last week. "From a thermal challenge standpoint, I think they'll get a lot better science out of this other magnet."

The AMS experiment was removed from NASA's shuttle manifest in the wake of a 2004 directive by the Bush administration to complete the space station and retire the space shuttle by the end of fiscal 2010. Completing the station was the priority, NASA officials said at the time, and AMS could not be accommodated.

Congress later provided funding for an additional shuttle fight to get the international experiment to the space station. The payload was assigned to Endeavour and launch was targeted for July 29.

The delay to mid November, or later, means NASA will miss the Sept. 30 deadline for completing the shuttle program. But the agency earlier secured an additional $600 million in funding to pay for flights through the end of the calendar year if necessary.

Under the revised manifest, Discovery will serve as the emergency rescue vehicle for the Atlantis astronauts. If all goes well, Atlantis would serve as the rescue shuttle for Endeavour.

Late last year, NASA managers privately discussed the possibility of adding a fourth and final shuttle flight to the remaining manifest using the boosters and tank set aside for the final emergency mission. By launching the shuttle with a crew of four, a rescue flight would not be needed; the astronauts could rely on the space station and downstream Soyuz flights to make it back to Earth if a major problem crippled the shuttle.

Talk about an extra flight has lingered, but given the end-of-year launch constraints and the delay for Endeavour, an additional flight does not appear to be possible before the end of the year and there is no funding currently available for a flight in 2011.