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WASHINGTON • Even as President Barack Obama signed off Monday on plans to steer the nation's space program toward Mars, Senate staffers on Capitol Hill reported growing speculation that the White House was preparing to replace NASA administrator Charles Bolden.

Bolden, a former astronaut, has the reputation of being a can-do leader known for sometimes ignoring bureaucratic constraints. However, top administration officials have eased him into the background as the midterm congressional elections approach.

Without fanfare and without a public ceremony, Obama quietly signed into law legislation adding a third and final shuttle flight next June, extending space station operations for five years to 2020, providing federal support for commercial spacecraft and hastening the development of deep-space exploratory craft to reach an asteroid by 2025 and orbit Mars a decade later.

White House Press Secretary Robert Gibbs merely issued a 71-word written statement announcing Obama's signature, a low-key bookend to a contentious nine-month struggle with Congress over the future of the U.S. space program.

A White House photo showed Obama seated alone at his Oval Office desk, without the customary backdrop of senior officials or lawmakers.

The legislation killing the back-to-the-moon program under President George W. Bush represented the most dramatic course change since President John F. Kennedy made the moon the nation's destination in 1961 in the midst of the U.S.-Soviet space race.

Sen. Bill Nelson, D-Fla., chairman of the Senate subcommittee with direct control over the space agency and a kingmaker in the electoral battleground of Florida, lavishly praised Sen. Kay Bailey Hutchison, R-Texas, the top Republican on the Senate panel with jurisdiction over NASA, for collaborating on the compromise bill that Obama signed.

"Kay Bailey and I were joined at the hip as we were trying to stop all the misinformation and trying to build a consensus among our colleagues," Nelson recalled.

The upbeat comments largely obscured the challenges that lie ahead. Senate insiders report growing talk that the administration will replace Bolden after the midterm congressional elections. Names of some potential successors were already circulating.

Bolden read a prepared statement but took no questions during Monday's telephone conference call with reporters organized by the White House to showcase Obama signing the measure into law.

"Our nation's leaders have come together to endorse a blueprint for NASA — one that requires us to think and act boldly as we move our agency into the future," said Bolden, a Marine aviator who flew four space shuttle missions and retired as a lieutenant general from the Marine Corps before being chosen to take over the beleaguered space agency in mid-2009.