U.S. President BarackObama’s proposed $18.7 billion budget for NASA in 2010 would provide a much-needed shot in the arm for a space agency that has been asked to do too much with too little in recent years. But the administration’s projected funding profile over the four subsequent years is not nearly so generous, which makes it very difficult to see how NASA will sustain the momentum.

In unveiling the first budget of his presidency Feb. 26, Mr. Obama sounded the right chords as far as NASA is concerned, committing to retiring the space shuttle by the end of 2010 and endorsing the previous administration’s goal of returning U.S. astronauts to the Moon by 2020. He also pledged to beef up NASA’s climate monitoring program, an increasingly important activity that was not a priority of his predecessor; launch new robotic probes for solar system exploration and astronomy; and to renew NASA’s commitment to aeronautical research.

For 2010, the president put his money where his mouth is. The NASA request is $900 million more than the agency stands to receive this year, and $700 million above what former President George W. Bush envisioned for 2010 when he submitted his 2009 budget, the last of his administration. Coming on top of the $787 billion economic stimulus package that includes some $1 billion for NASA, Mr. Obama’s proposed budget represents a real windfall for space, especially given the current economic circumstances.

The bad news is the following four years. According to the administration’s out-year projections, funding for NASA would decline slightly, to $18.6 billion, in 2011, and remain at that level until 2014, when it would rise a paltry 1.6 percent to $18.9 billion. That means that, assuming even a modest rate of inflation, the space agency’s spending power after 2010 would decline progressively over the next several years. This will substantially offset – if not neutralize – the impact of the one-time cash infusion provided by the stimulus bill.

The budget proposal lays out the administration’s NASA policy goals and priorities only in very broad terms; the detailed spending plan is expected to be released in April. But it is safe to say Earth science will consume a larger share of the budget than it has in recent years, and it seems quite possible that aeronautics will as well. If the current out-year projections for NASA come to pass, these activities are bound to put more pressure on human spaceflight and exploration efforts than envisioned by the Bush administration, which assumed steady growth in the agency’s budget over the next four years. One way to relieve some of that pressure is for NASA to collaborate closely with the U.S. National Oceanic and Atmospheric Administration (NOAA) on Earth science and environmental monitoring. NOAA satellite efforts received substantial funding in the stimulus package, and the agency is a partner with the U.S. Defense Department on a series of polar-orbiting weather satellites that have room to accommodate climate monitoring sensors.

Perhaps the biggest source of budgetary uncertainty for NASA is the space shuttle, which costs some $3 billion per year to operate. Retiring the fleet is a critical milestone that will free up the cash NASA needs to develop not only the replacement system but also the hardware specifically needed to return to the Moon, including a shuttle-derived heavy-lift rocket that promises to be an expensive undertaking. The Obama administration is holding firm to the end-of-2010 retirement date; documents released with the budget request said another mission could be added to the shuttle’s manifest – presumably to deploy the long-stranded Alpha Magnetic Spectrometer (AMS) astronomy instrument to the international space station – only if it can be safely carried out by then. But 2011 is just around the corner, and the latest series of delays to the launch of the Space Shuttle Discovery serves as a reminder that the orbiter fleet does not respond well to external schedule pressures. That makes it very plausible that the final space station assembly flight – or flights – will not have been completed by the time 2011 rolls around, which would make it all but impossible to retire the fleet by then.

Moreover, a decision on whether to fly AMS will have to be made in the next few months so that NASA can begin the necessary preparations, which will require a substantial funding commitment. Should the White House commit those funds, only to discover sometime over the next 18 months that the mission cannot make a 2010 launch, would it then cancel the flight and write off the investment? That seems doubtful.

Any delay to the shuttle’s retirement, even if it is only a matter of months, will be costly, and that money comes right out of the hide of the development budget for NASA’s next astronaut launching system. The former president’s budgets left no margin for error in terms the schedule for a return to the Moon; in fact, given the realities of the space business, 2020 seemed overly optimistic even under the best of circumstances. With Mr. Obama’s projected budgets, which in the out-years dip below those of his predecessor, the 2020 target date seems even less in tune with reality.