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 . What do you think of the proposed space plan ? 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 `` can not 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 .
Neil Armstrong , James Lovell , Eugene Cernan criticize manned space flight plans . President Obama set to officially announce his space plans Thursday . Obama 's plans would shift funding away from human space flight program . Comment: