The US space agency awarded $50 million in grants to five private firms in a first step to implement President Barack Obama's vision of turning over space transportation to the commercial sector.
NASA Administrator Charles Bolden also deflected criticism by some US lawmakers that Obama's proposal would topple the United States from its preeminent role in space exploration.
"We are not abandoning human space flight by any stretch of the imagination, Bolden said at a news conference. "We are probably on a new course, but human space flight is in our DNA."
Obama's budget plan, unveiled on Monday, scuttles the Constellation programme, begun under former President George W Bush to return humans to the moon and spends $6 billion over five years to develop commercial space transportation.
Obama's NASA proposal hands over more space operations to the commercial sector, saying it will create thousands of new jobs and hold costs down. Some members of Congress promised a fight to save the symbolic but costly lunar programme. Senator Richard Shelby, the senior Republican on the appropriations subcommittee handling NASA funding, called the Obama plan a "death march" for human space flight.