NASA managers Tuesday asked shuttle engineers to assess retargeting the final two space shuttle missions, moving launch of a mid-September flight with Discovery to Oct. 29 and a late November flight by Endeavour to Feb. 28. The changes would give engineers more time to optimize payloads bound for the International Space Station and avoid launch conflicts with other flights to the lab complex.

Discovery's crew. Credit: NASA

The status of a proposed final fight of the shuttle Atlantis next June to deliver a final load of supplies and equipment remains unresolved. Bill Gerstenmaier, director of space operations at NASA headquarters, said earlier this year he hoped to have a decision by the end of this month, but officials said Tuesday the discussion had been deferred to the August timeframe.

A decision on delaying the next two missions, however, is expected July 1, after a two-week review.

Assuming an Oct. 29 target date, Discovery would blast off at 5:44 p.m. and dock with the station the afternoon of Oct. 31. Two spacewalks would be carried out Nov. 2 and 4. Discovery would undock the morning of Nov. 7 and land back at the Kennedy Space Center the afternoon of Nov. 9.

Shuttle mission STS-133 will be commanded by former chief astronaut Steven Lindsey. His crewmates are pilot Eric Boe, station veterans Nicole Stott and Michael Barratt and spacewalkers Alvin Drew and Timothy Kopra, another station veteran.

Endeavour, commanded by Mark Kelly, would blast off around 5:31 p.m. on Feb. 28 to deliver critical supplies and a $1.5 billion physics experiment to the space station. Joining Kelly for mission STS-134 will be pilot Gregory Johnson, Hubble veteran Andrew Feustel, European Space Agency astronaut Roberto Vittori and station veterans Gregory Chamitoff and Mike Fincke.

NASA originally planned to end shuttle operations by the end of fiscal 2010, launching Endeavour in July and Discovery in mid September. But problems with the Alpha Magnetic Spectrometer scheduled for launch aboard Endeavour forced program managers to delay the July flight to the end of November.

Congress earlier promised an additional $600 million to cover shuttle costs through the end of the calendar year to avoid the sort of schedule pressure blamed in part for the Challenger and Columbia mishaps.

NASA managers then came up with additional savings, permitting operations through February or March without additional appropriations. Delaying Discovery to late October and Endeavour to late February will give engineers more time to optimize the equipment and spare parts being launched to the station.

Atlantis is being processed to serve as an emergency rescue vehicle for Endeavour's crew. But if a rescue flight is not needed, NASA managers believe the standby shuttle could be launched with a crew of four, relying on Russian Soyuz capsules to ferry the crew members home if a major problem blocked a safe re-entry.

Concerned about the near-term lack of a large rocket to deliver heavy payloads after the shuttle fleet is retired, NASA managers want to take advantage of the boosters and external tank being prepared for Atlantis' rescue mission that otherwise would go to waste.

But additional funding would be required and it's not yet clear whether NASA has the necessary political support.