"Uncontrolled events" are likely to push the final Space Shuttle flight back by about four months and into 2011, according to internal NASA studies.
As things stand the Shuttle fleet is scheduled to retire with Discovery's completion of mission STS-133 in September 2010, but an analysis by the space agency's office of inspector general has concluded that a delay is likely.
Delay may cause no immediate programme budget crisis, because President Barack Obama's fiscal year 2011 budget request included $600 million for Shuttle operations beyond 1 October 2010, given expectations that the Shuttle would have to fly beyond its scheduled September finale.
The inspector general report says Shuttle managers estimate that completing scheduled flights by September will consume up to $54 million in personnel overtime costs, while delays beyond September would require "$200 million...for every month that they need to sustain Shuttle operations in FY2011".
However, a much longer extension may come regardless of any operational events that affect the timing of STS-133. Fierce Congressional opposition to Obama's vision for NASA - which retires the Shuttle and cancels the Moon-return Constellation programme and its Ares rockets, leaving the USA with no manned launch capability before private-sector rockets are expected in 2016 - has seen legislation proposed to keep the Shuttle flying.