NASA has cut the crew size for its new Orion spacecraft down from six seats to four in order to keep the space shuttle replacement on track for a March 2015 debut.

The space agency made the decision earlier this month in order to meet its commitment to begin operational manned flights on the Orion Crew Exploration Vehicle — NASA's successor for its retiring space shuttles — by 2015.

Before the crew reduction, the agency was preparing two parallel Orion designs: a six-person version to ferry crews to the International Space Station, and a four-seater to send astronauts back to the moon by 2020.

"We're not giving up on the six-crew capability of Orion," said Jeff Hanley, NASA's program manager for the Constellation project that includes Orion and its Ares rockets, on Wednesday. "We will need it someday. We don't need it early."

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NASA's Orion spacecraft are capsule-based vehicles designed to replace the space agency's three aging space shuttles: Discovery, Atlantis and Endeavour.

The orbiters are due to retire at the end of 2010, though Congressional lawmakers have drawn up a budget resolution that could delay mothballing the fleet until at least 2011 if approved.

Hanley said the decision to reduce Orion's crew size for the near-term stemmed from a recent report on ways NASA could accelerate its Constellation program efforts to build the new spaceship, retire the space shuttles and begin operational flights with the new craft in 2015.

Cutting the crew size eliminates the extra time and funding needed to design, build and certify two different Orion vehicles — the four-seater and six-seater — at the same time.

"We felt that that was a good program management option to go exercise to simplify the design a little bit," Hanley told reporters in a teleconference today.

Bridging the shuttle gap

NASA already expects a gap of up to five years between the shuttle fleet's retirement and the first operational flight of Orion.

During that time, NASA would have to rely on Russia's Soyuz spacecraft to launch astronauts to the station, as well as cargo ships built by Russia, Europe and Japan.

The agency is also hoping to use American-built commercial spacecraft, such as the Dragon vehicle and a rocket under development by Space Exploration Technologies, to carry U.S. cargo to the station. Those privately built spacecraft, however, have yet to fly.

Last week, a Congressional Budget Office report questioned NASA's ability to meet its March 2015 target for operational Orion flights without an infusion of billions of dollars in extra funding.

But Hanley disagreed, saying Wednesday that there is sufficient funding and congressional support for NASA's plans, but only if the agency draws up a streamlined schedule for testing.

Hanley said his team is considering canceling a second test flight of the Ares I rocket that will launch Orion capsules into space.

The first test , Ares I-X, is slated to launch no earlier than late August. Engineers are now studying whether the effort to launch a second planned flight — Ares I-Y — is worth the money and manpower required, or if it should be folded into a full-up Ares I launch or other test.

More space for station

A four-person Orion option will free up some mass and cargo space for more supplies for station-bound missions, Hanley said.

But it also means the space station will likely still require a second crew-carrying spacecraft, such as a Russian Soyuz, on hand to provide escape capability for the outpost's full six-person complement.

Soyuz spacecraft can ferry three spaceflyers to the space station and stay parked in orbit for about six months. NASA's space shuttles can carry seven astronauts on missions that average just over two weeks in duration.

Soyuz vehicles were already expected to be flying to and from the space station at the time as Orion, NASA officials said.

"The four-seat Orion will still meet all U.S. needs, both for [space station] crew exchange and rescue," NASA spokesperson Katherine Trinidad told SPACE.com. "No additional Soyuz seats are needed as a result of the change in crew size."

The space station is expected to begin full six-person operations in late May, when a second Soyuz vehicle will ferry three new astronauts to the outpost.

"I think it's safe to say that our Russian partners will always want to have their own means to arrive at space station," Hanley said. "Having a diversity in access to the International Space Station is actually a good thing."

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