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.