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."
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Neil Armstrong, James Lovell, Eugene Cernan criticize manned space flight plans
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President Obama set to officially announce his space plans Thursday
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Obama's plans would shift  funding away from  human space flight program
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Three astronauts applaud plan for overall increase in NASA funding
Armstrong, Lovell and Cernan praised @placeholder's increase in total funding for space exploration, which includes money for research, the international space station and a heavy-lift rocket.