NASA is unlikely to complete the last of its four remaining shuttle missions before January 2011, according to a March 25 report by the U.S. space agency’s internal watchdog.

NASA aims to fly its last shuttle mission before Oct. 1, the start of the U.S. government’s new budget year. As a hedge against a schedule slip, NASA included an additional $600 million in its 2011 budget request to cover shuttle operations through December 2010.

In its newly released report, the NASA Office of Inspector General says the agency’s chance of completing the remaining flights by the end of 2010 is improving, but the odds remain that the last flight will launch in January.

Space shuttle program managers estimate spending up to $54 million in personnel overtime costs to keep the shuttle on pace to complete its last mission by the end of September, but acknowledge that shuttle operations could still spill over into fiscal 2011.

“Nevertheless, the managers stated that setting September 2010 as a goal can reduce the amount of time that they will need to sustain Shuttle operations in [fiscal year] 2011 and potentially provide the Agency the opportunity to redirect millions of dollars toward other priority programs by limiting the need to continue Shuttle operations at $200 million a month,” says the inspector general report.