This story was updated at 3:34 p.m. EST.

WASHINGTON - The White House's $17.3 billion request for NASA in 2008 demonstrates U.S. President George W. Bush's commitment to replacing the space shuttle by 2014, but might not be enough to offset the impact of a lower-than-expected budget for this year, agency officials said.

"This increase demonstrates the President's commitment to NASA and to maintaining our nation's leadership in space and aeronautics research," NASA chief Michael Griffin said in a Monday briefing.

The request, which would give the space agency a raise of roughly $1 billion over this year's likely budget, keeps science and aeronautics spending relatively flat, but does include funds to begin purchasing a new series of data-relay spacecraft.

The annual budget request tracks very closely with the spending plan the White House sent to Congress this time last year, which sought 3 percent increases for NASA in both 2007 and 2008. However, NASA is unlikely to get a raise this year due to a plan by the Democratic-led Congress to fund most U.S. domestic federal agencies at their 2006 levels.

"The key message we have with this budget is the agency is on track," NASA Comptroller David Schurr told reporters Feb. 2 during a budget briefing at agency headquarters here. "We are making progress toward the vision ... there are no real strategic changes in this request."

However, Schurr and NASA's director of strategic investments, Chris Shank, said progress on the shuttle replacement effort is threatened by NASA's likely budget for 2007.

Congress adjourned last year without finishing nine of 11 annual spending bills, among them the one that funds NASA. The new Congress has decided to fund most government agencies, including NASA, at their 2006 levels rather than spend time trying to pass the unfinished spending bills.

The House passed a broad spending measure Jan. 31 that would fund NASA at $16.2 billion this year, Fiscal Year 2007 (FY07) denying the agency any increase even as it seeks to ramp up spending on its space shuttle replacement. The Senate is expected to pass the measure by mid-February.

"The FY07 appropriations, if enacted as the House has resolved, will jeopardize our ability to transition safely and efficiently from the shuttle to the Orion Crew Exploration Vehicle and the Ares I Crew Launch Vehicle," Griffin said. "It will have serious effects on people, projects, and programs this year and for the longer term."

NASA officials said the spending measure endangers the agency's plans to field the Orion Crew Exploration Vehicle and Ares I rocket by 2014.

"If we do not quickly come to grips with this issue, we may have a prolonged gap between the end of the shuttle program and the beginning of the operational capability of our new systems," Griffin said.

NASA plans to retire its three aging space shuttles in September 2010 following the completion of the International Space Station.

"Naturally there are going to be challenges with the [2007] appropriation and that threatens our progress," Shank said last week. "But this [2008] budget, with a 3.1 percent increase, really does demonstrate that we've got the commitment and the resources to carry off a lot of these challenges."
 
Under the 2008 plan, spending on Earth science satellites, planetary probes and other unmanned science craft would remain flat at $5.5 billion through the end of the decade so that NASA can complete the International Space Station and build a new crew capsule and expendable rocket to replace the space shuttle after it retires in 2010.

Funding for aeronautics research also would remain flat, although a change in the way NASA accounts for overhead expenses makes it appear that aeronautics is in for a big budget cut.

The budget request includes $554 million for aeronautics for 2008, down $170 million from the agency's $724.4 million request for 2007. NASA officials, however, said that if the new agency-wide overhead rate was in use last year, the 2007 request would have been $529.3 million so the agency is actually seeking a modest increase for aeronautics.

NASA informed Congress last September that it would be simplifying the way it calculates overhead and administrative costs in part to ensure that its 10 regional field centers can compete for internal projects on a level playing field. The change was necessary, agency officials said, because the smaller centers tend to have higher per-capita overhead costs.

Because all parts of NASA's budget have been affected by these accounting changes, year-to-year comparisons are made that much more complicated.

Under the request, NASA's Exploration Systems budget would climb to $3.9 billion in 2008 and reach $4.7 billion in 2010 before making a big jump to $8.7 billion in 2011, the first year NASA expects to be unburdened by the space shuttle's hefty annual tab.

NASA's Science Mission Directorate also is projected to benefit from the shuttle's retirement, although not until 2012. That is when NASA and the White House foresee being able to afford 2.4 percent increases for the science mission portfolio.

For 2008, NASA science spending would grow about 1 percent to $5.5 billion, a sum that includes nearly $1.4 billion for planetary science, $1.6 billion for astrophysics, $1 billion for heliophysics, and $1.5 billion for Earth science.

NASA's Space Operations Mission Directorate, which runs the space shuttle and international space station programs, would receive $6.8 billion for 2008, a significant increase over the likely 2007 budget. Part of the increase is needed for new satellites NASA expects to order later this year to replace the Tracking and Data Relay Satellite System that NASA and the U.S. military use to communicate with Earth-orbiting spacecraft.

SPACE.com staff writer Tariq Malik contributed to this report from New York City.