The cuts come to $3 billion over the next five years, even as NASA's overall spending grows by 3.2 percent this year, to $16.8 billion.
Among the casualties in the budget, released last month, are efforts to look for habitable planets and perhaps life elsewhere in the galaxy, an investigation of the dark energy that seems to be ripping the universe apart, bringing a sample of Mars back to Earth and exploring for life under the ice of Jupiter's moon Europa — as well as numerous smaller programs and individual research projects that astronomers say are the wellsprings of new science and new scientists.
The agency's administrator, Michael D. Griffin, says NASA needs the money to keep the space shuttle fleet aloft, complete the International Space Station and build a new crew exploration vehicle to replace the shuttle.
That transition has produced an unexpected shortfall of money, but, Mr. Griffin told the House Science Committee last month, to postpone it would be more damaging than to put off some space science projects.
"We're delaying some missions," he told the committee, "but we're not abandoning them."
Yesterday, Mary Cleave, NASA's associate administrator for science, said she took Dr. Griffin at his word that the cuts were a one-time event. "There was no money available anyplace else," Dr. Cleave said. "We took a hit."
The programs could still be saved if Congress voted to increase the NASA budget. The agency has powerful allies in both parties, and some have expressed alarm at the proposed cuts, which will be discussed today at a hearing of the House Science Committee.
But at a time when fiscal conservatives are placing intense pressure on the Republican Congressional leadership to rein in government spending, programs that were previously considered sacrosanct are now vulnerable.
The cuts have alarmed and outraged many scientists, who have long feared that NASA will have to cannibalize its science program to carry out the president's vision of human spaceflight.
The new cuts, they say, will drive young people from the field, ending American domination of space science and perhaps ceding future discoveries to Europe.
"The bottom line: science at NASA is disappearing — fast," said Donald Lamb, an astrophysicist at the University of Chicago and chairman of a committee on space science for the Association of American Universities.
Representative Sherwood Boehlert, the New York Republican who is chairman of the Science Committee, called the new budget "bad for space science, worse for earth science," adding, "It basically cuts or de-emphasizes every forward-looking, truly futuristic program of the agency to fund operational and development programs to enable us to do what we are already doing or have done before."
As a result of the new cuts, NASA's expenditures for space and earth science will grow about 1 percent a year from now to 2011, far less than inflation, even as the Bush administration promotes its effort to bolster American competitiveness by doubling the research budgets at the National Science Foundation, the Department of Energy and the National Institute of Standards and Technology over the next 10 years.
Senator Pete V. Domenici, Republican of New Mexico, and 56 other senators have introduced a bill that would mandate a 10 percent increase per year in NASA's science budget from now through 2013, among other things.
Astronomers and planetary researchers say space science has provided NASA's brightest and most inspirational moments in recent years: the landing on Saturn's moon Titan, the exploits of the Mars rovers and the stream of cosmic postcards from the Hubble Space Telescope.
Despite Dr. Griffin's assurances, they say that delaying space missions can be a death sentence if there is not money to continue developing technology and to keep teams together until the mission is ready to fly again.
That is the case, said Charles Beichman of NASA's Jet Propulsion Laboratory, with the Terrestrial Planet Finder missions, which are intended to produce images of Earth-like planets around other stars. They are the culmination of a line of missions devoted to hunting for planets around other stars and investigating if they are habitable or already harbor life, a goal, planetary scientists point out, that is explicitly endorsed in Mr. Bush's space vision.
"We're getting ready to fire all the people we've built up," said Dr. Beichman, who is the project scientist for the second of the two spacecraft missions, once scheduled for about 2020. Once those scientists have found other jobs, he said, they are not likely to come back.
"What I feel bad about is turning away a generation," Dr. Beichman said, explaining that planet-finding has been one of the hottest fields in science lately, attracting, in particular, young scientists into astronomy.
"We were the new kid on the block," he said.
Much of the concern among scientists is for the fate of smaller projects like the low-budget spacecraft called Explorers. Designed to provide relatively cheap and fast access to space, they are usually developed and managed by university groups. Dr. Lamb referred to them as "the crown jewels in NASA's science program."
In recent years, one such mission, the Wilkinson Microwave Anisotropy Probe, produced exquisite baby pictures of the Big Bang, while another, the Swift satellite, has helped solve a 30-year-old mystery, linking distant explosions called gamma-ray bursts to the formation of black holes.
Explorers, Dr. Lamb said, are where graduate students and young professors get their first taste of space science. Until recently, about one mission was launched a year, but under the new plan, there will be none from 2009 to 2012. In a letter to Dr. Cleave last fall, 16 present and former Explorer scientists said, "Such a lengthy suspension would be a devastating blow to the program and the science community."
One author of the letter, Fiona Harrison, a physicist at the California Institute of Technology, said she first learned from a news conference that her own Explorer project, an X-ray satellite observatory called NuStar, was being cancelled after several years of development. Dr. Harrison said that she had been invited to reapply in 2008, but that in the meantime she had to tell her graduate student to find another thesis project.
Dr. Harrison said she was thinking of leaving the country or perhaps even the field of astrophysics.
In another move last month, NASA reduced this year's budget for individual research projects by 15 percent, retroactive to last fall, taking money from researchers and their institutions that had already begun work.
Dr. Cleave, the NASA associate administrator for science, acknowledged that she had been deluged with faxes and e-mail messages from scientists alarmed about these developments. She said the agency would be willing to adjust "the mix" in favor of more research and analysis, but added that something would have to give.
"I have my budget. I don't expect relief," she said. "There's no free lunch here."
Many scientists said the roots of their plight lay in the Bush administration's refusal to ask Congress for enough money to carry out the Moon-Mars program, announced with fanfare two years ago. But others said they were partly to blame as well for pursuing an overly ambitious agenda in the face of cold realities like the Columbia shuttle disaster and concern about the mounting federal deficit.
For example, the James Webb Space Telescope — the designated successor to the Hubble telescope, designed to see back in time and space within a whisper of the Big Bang — was ranked first on the astronomers' wish list in an influential National Academy of Sciences survey in 2000. But delays and technical problems have almost doubled its cost, to more than $4.5 billion, and postponed its launching by two years, to 2013. Meanwhile, planned repairs to the Hubble telescope will cost some $300 million.
Once the Webb and Hubble telescopes, the two highest priorities, are included, said John Mather, an astronomer at the Goddard Space Flight Center and project scientist for the Webb telescope, "almost everything else that isn't started had to be stopped."
"People assume that when Congress votes for something they send extra money," Mr. Mather said. "They don't."
Dr. Griffin and his colleagues, the scientists agree, have tough choices to make, but so far, the space scientists say, the choices have been made in a vacuum, without input from the community most affected, namely them.
Last year NASA dismantled a longstanding network of scientific advisory committees, and while a new network is in the works, it is not yet in place.
As a result, "scientists feel very much left out of this process," Dr. Beichman said. "You could have involved the community and said, 'Here's what we have to do.' "
He added, "In the end, even scientists can be responsible."