Apollo 11 commander Neil Armstrong’s open letter was also signed by Apollo 17 commander Eugene Cernan, the last man to walk on the moon; and Apollo 13 commander Jim Lovell, who is marking the 40th anniversary of his famous lunar non-landing this week.
The letter was released to NBC News just two days in advance of Obama’s trip to NASA’s Kennedy Space Center for a space policy summit.
The most controversial part of the president's policy is the cancellation of the Constellation program, which was aimed at developing a new generation of Ares rockets and Orion spacecraft to send astronauts into Earth orbit and beyond.
The idea was that such spacecraft would replace NASA's space shuttle fleet, which is due to be retired by the end of this year. But acting on the advice of an independent panel, the Obama administration determined that the Constellation program could not fulfill NASA's goals on the retired timetable. NASA's budget proposal, released in February, puts the return to the moon on indefinite hold and instead focuses of developing technologies for future exploration.
Canceling Constellation is likely to lead to thousands of layoffs at some of America's biggest aerospace contractors, including Lockheed Martin, the Boeing Co. and ATK. Such job losses are among the factors behind congressional opposition to the cancellation. Armstrong and his fellow astronauts take a different tack, however, saying that the decision would put the nation on a "long downhill slide to mediocrity."
The letter notes that the U.S. space effort will be dependent for years to come on the Russians for transport to the International Space Station, at a cost of more than $50 million per seat.
NASA is budgeting billions of dollars to support the development of U.S. commercial spaceships that could help fill the gap. The beneficiaries of those billions would include smaller aerospace ventures, such as California-based SpaceX and Virginia-based Orbital Sciences. In their letter, the astronauts say that the availability of such craft "cannot be predicted with any certainty, but is likely to take substantially longer and be more expensive than we would hope."
Armstrong and his colleagues complained that the cancellation would amount to wasting the roughly $10 billion that has been allocated to Constellation over the past five years. "Equally importantly, we will have lost the many years required to re-create the equivalent of what we will have discarded," they wrote.
"For the United States, the leading spacefaring nation for nearly half a century, to be without carriage to low Earth orbit and with no human exploration capability to go beyond Earth orbit for an indeterminate time into the future, destines our nation to become one of second- or even third-rate stature," they said in the letter.
"America must decide if it wishes to remain a leader in space," the astronauts said. "If it does, we should institute a program which will give us the very best chance of achieving that goal."
The letter did not make specific policy recommendations, but the astronauts spoke approvingly of NASA's step-by-step Ares rocket development program. "Enthusiasm within NASA and throughout the country was very high," they said.
However, an independent panel headed by retired aerospace executive Norman Augustine concluded last year that the Ares I rocket would not be ready in time to serve its planned role for space station transport. The panel said that NASA's plan to return to the moon by 2020 was unrealistic, due to underfunding, and that Constellation's first moon landing couldn't occur until 2027 or later.
The Obama administration decided to pursue a different option, known as the "flexible path." This option involves developing the technologies for trips beyond Earth orbit, but holding off on a schedule for landings on the moon or Mars until such technologies are further along. Operations on the space station would be extended to 2020, and interim steps might include trips around the moon, or to low-gravity destinations such as near-Earth asteroids or a Martian moon. Such steps would set the stage for eventual landings on Mars and the moon.
"Safely sending humans into deep space for years at a time and exploring destinations across the solar system is not something to be done overnight or to be taken lightly," NASA Administrator Charles Bolden said Tuesday at the National Space Symposium in Colorado Springs, Colo.
Bolden told reporters last week that NASA's total employment might actually go up, based on the fact that the White House wanted to increase the space agency's budget from $18.7 billion to $19 billion in fiscal 2011. He acknowledged, however, that the increases might come in climate research and aeronautics rather than human spaceflight.
The flexible-path strategy embraced by the Obama administration just might get astronauts into deep space and on to the moon and Mars sooner than the Constellation program would have, said James Oberg, a former NASA mission controller who is now NBC News' space analyst.
"It was widely felt outside NASA, and also within it, that Constellation was like an unguided missile, ignoring changing circumstances and blindly pushing ahead," Oberg said. "Bolden has suggested that there are cleverer and better strategies to achieve the same goals — and I'm inclined to agree with him."
Apollo-era astronauts have been divided on the wisdom of canceling Constellation. One of the most outspoken advocates of NASA's change in plans is Armstrong's crewmate for the Apollo 11 moon landing: Buzz Aldrin.
"Many said the president's decision was misguided, short-sighted and disappointing," Aldrin wrote in an op-ed piece for The Wall Street Journal. "Having the experience of walking on the moon's surface on the Apollo 11 mission, I think he made the right call. If we follow the president's plan, our next destination in space, Mars, will be within our reach."
This report includes information from NBC News' Jay Barbree and Tom Costello, and msnbc.com's Alan Boyle.