The U.S. space agency on Tuesday 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 U.S. 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 program, 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 program. Senator Richard Shelby, the senior Republican on the appropriations subcommittee handling NASA funding, called the Obama plan a death march for human space flight.

Bolden said NASA teams were working on developing a new plan for exploration with the moon, Mars and asteroids as possible destinations, but no timetable has been set.

I think we're going to get there, perhaps, quicker than we would have done before, Bolden said, referring to the plan to partner with private industry.

We're departing from the model of the past, in which the government funded all human space activities, Bolden said. This represents the entrance of the entrepreneurial mind-set into a field that is poised for rapid growth and new jobs.

The firms selected by NASA to spur development of private sector manned spaceflight are:

* Sierra Nevada Corp, Louisville, Co. $20 million

* The Boeing Company, Houston $18 million

* United Launch Alliance, Centennial Co. $6.7 million

* Blue Origin, Kent, Wash. $3.7 million

* Paragon Space Development Corp, Tucson $1.4 million

NASA already has contracts with Space Exploration Technologies and Orbital Sciences Corp to deliver cargo to the International Space Station. SpaceX and other firms also are developing spaceships that can carry passengers to orbit and back.

(Reporting by JoAnne Allen, editing by Anthony Boadle)

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