WASHINGTON -- NASA sent Congress a revised spending plan for 2005 that would significantly cut the Project Prometheus nuclear power and propulsion program, cancel a host of international space station-based biological and physical research activities, and postpone some space science missions, including two advanced space telescopes and a Mars science lander slated to launch in 2009.

The cuts were necessary, according to NASA, to pay the remaining $287 million tab for preparing the space shuttle for its return to flight, to make a substantial down payment on a potential Hubble Space Telescope servicing mission, to accommodate $400 million worth of special projects that lawmakers added to NASA's budget last year, and to cover larger than predicted bills for a variety satellite projects being prepared for launch.

NASA informed Congress of these intended changes in an updated 2005 Operating Plan sent to Congress May 11. A copy of the operating plan, obtained by Space News, details changes both big and small that NASA says it needs to make to its $16.2 billion budget 2005 to get through the end of the fiscal year.

NASA's latest operating plan includes the full $291 million Congress directed it to spend this year preparing for a possible Hubble servicing mission. NASA's last spending plan, sent to Congress in December for review, allocated only $175 million of that amount to a Hubble mission. In February, NASA announced, to the chagrin of Hubble-supporters in Congress, that it would abandon any effort to save Hubble.

Since taking over as NASA administrator last month Mike Griffin has reversed that decision and ordered engineers at Goddard Space Flight Center to start preparing a Hubble servicing mission on the assumption that one will ultimately go forward. A formal decision is expected after the shuttle makes its return to flight.

Griffin explained his rationale in a May 10 letter accompanying the operating plan, saying that funding return to flight, Hubble servicing, programmatic overruns and releasing the $400 million in congressional earmarks "has created some difficult choices" for NASA.

"Given a choice, my preference as Administrator is to eliminate lower-priority programs rather than reducing all programs in the face of budget difficulties, to maintain efficient execution of the programs which remain," Griffin wrote lawmakers. "Delays and deferrals inevitably lead to increased life cycle costs and erode the overall performance of the Agency's programs. Thus, NASA must set clear priorities to remain within the budget which has been allocated."

Cut from NASA's latest operating plan are about $160 million worth of space station-based biological and physical research efforts that a recently completed, although unreleased, NASA review concluded were unnecessary in light of NASA's new focus using the space station for research that directly serves the needs of its space exploration goals.

While that is bad news for fundamental biological and physical research, some newly identified high priority areas of investigation will receive more money in the months and years ahead, according to Griffin.

"These high- priority areas include space radiation health and shielding, advanced environmental control and monitoring, advanced extra-vehicular activities, human health and countermeasures, advanced life support, exploration medical care, and space human  factors," Griffin wrote. "The highest priorities for research on ISS have been identified as medical research with human subjects and microgravity validation of environmental control and life support technologies. Lower-priority tasks, which are now subject to reduced  funding, include basic research using model organisms (such as cells or rodents), and fundamental research in physics, material science, or basic combustion - with no direct link to exploration requirements."

NASA also plans to refocus Project Prometheus on the development of "space-qualified nuclear systems to support human and robotic missions" especially those needed to support NASA's near term exploration goals. NASA started the program known as Prometheus in 2002 to develop nuclear power and propulsion systems for deep space probes like the Jupiter Icy Moons Orbiter, a flagship-class mission that NASA deferred indefinitely earlier this year once it became clear that the undertaking would cost tens of billions of dollars and not necessarily help NASA accomplish its goal of returning to the Moon and sending humans to Mars.

The operating plan sent to Congress would cut $171 million from the Prometheus budget, leaving the program with $260 million for the time being.

Money for the Crew Exploration Vehicle, meanwhile, would remain untouched at $421.9 million for the year, even though NASA has said it intends to accelerate the program in order to minimize any gap between retiring the shuttle in 2010 and fielding the new system. NASA is still evaluating its options for accelerating the program, but has already announced that it intends to pick the contractor it wants to build the system in early 2006 instead of late 2008.

In addition to the cuts and increases, the operating plan also indicates that NASA intends to take planning for a Hubble servicing mission away from its Exploration Systems Mission Directorate and give it back to the Science Mission Directorate. Exploration Systems, however, picks up full responsibility for NASA's nascent Lunar Robotic Exploration Program and the ISS Crew and Cargo Services effort to find alternatives to the space shuttle for delivering cargo and potentially people to the space station. That effort, initiated in 2004, had been under the management of the Space Operations Mission Directorate, which is in charge of the shuttle and station programs.

NASA would also delay the Space Interferometry Mission and Terrestrial Planet Finder, two advanced space telescope projects slated to launch some time after the James Webb Space Telescope. Griffin's letter also says that NASA is considering delaying the Mars Science Laboratory mission from 2009 to 2011. NASA's operating plan cuts nearly $72 million from the program.