Washington (CNN) -- The Obama administration's vision for the future of manned space flight will bump the United States to "second or even third-rate" status as a space-faring nation, the commanders of three U.S. moon missions warned Wednesday.
The letter was signed by the first and last men to walk on the moon -- Neil Armstrong from Apollo 11 and Eugene Cernan from Apollo 17 -- and James Lovell, who commanded the heroic Apollo 13 flight.
"Without the skill and experience that actual spacecraft operation provides, the USA is far too likely to be on a long downhill slide to mediocrity," the letter said. "America must decide if it wishes to remain a leader in space. If it does, we should institute a program which will give us the very best chance of achieving that goal."
President Obama is scheduled to announce his space plans Thursday during a visit to the Kennedy Space Center in Florida, the White House announced. The White House said the five-year strategy involves a $6 billion increase in NASA's budget and additional support for new space technologies.
Armstrong, Lovell and Cernan praised Obama's increase in total funding for space exploration, which includes money for research, the international space station and a heavy-lift rocket. But the astronauts said the decision to cancel the Constellation program for manned space flight "is devastating."
"America's only path to low Earth orbit and the international space station will now be subject to an agreement with Russia to purchase space on their Soyuz (at a price of over 50 million dollars per seat with significant increases expected in the near future) until we have the capacity to provide transportation for ourselves," they wrote.
NASA's space shuttle fleet will be retired at the end of this year, leaving the Russian Soyuz capsules as the only avenue into space until commercial ventures are ready to do the job, expected to be years away. Obama's proposal to use commercial transport to reach orbit "cannot be predicted with any certainty, but is likely to take substantially longer and be more expensive than we would hope," the astronauts said.
Cernan, Lovell and Armstrong said the more than $10 billion spent so far on Constellation -- including the Orion space capsule and the Ares rockets to boost it into space will be wasted by the cancellation "and, equally importantly, we will have lost the many years required to recreate the equivalent of what we will have discarded."
NASA's future, as outlined in the White House documents, would include a multibillion-dollar modernization of Kennedy Space Center, expansion of private-sector and commercial space industries, creation of thousands of jobs and eventually human travel to Mars.
But Allard Beutel, news chief at the Kennedy Space Center, told CNN that layoffs at the center will likely reach the 7,000 range with the end of the shuttle and the cancellation of the Constellation program.
The president's plans would shift some funding away from NASA's costly human space flight program to NASA's scientific programs, including robotic missions to other planets.
During a briefing in early April, NASA Administrator Charles Bolden praised the new future being charted for the agency.
"This budget provides an increase to NASA at a time when funding is scarce," Bolden said. "It will enable us to accomplish inspiring exploration, science and (research and development), the kinds of things the agency has been known for throughout its history."
CNN's Dan Lothian and Sarah Baker and CNN Radio's Dick Uliano contributed to this report.