WASHINGTON —As the Senate Commerce Committee begins work on a 2010 NASA authorization bill, science and space subcommittee chairman Sen. Bill Nelson (D-Fla.) is questioning whether $6 billion the U.S. space agency is seeking for developing a commercial crew taxis might be better spent on a heavy-lift rocket that could take humans beyond low Earth orbit.

Nelson told a NASA Kennedy Space Center-area audience March 19 that he expects U.S. President Barack Obama to “revamp his budget” and set specific goals for the nation’s human spaceflight program when he visits Florida April 15 to talk space.

“Now, if he doesn’t do that, it makes our job a little rougher,” said Nelson, speaking to a forum at Brevard Community College in Cocoa, Fla. “But I can tell you that we have a consensus that what we will do in the Senate at least is put money into aggressive [research and development] of a heavy lift [rocket],” Nelson said. “And to try to take parts that we already have — and obviously what we already have are parts of the Ares rocket — and utilize that as we start to develop this heavy lift capability.”

Nelson also said he hopes Congress will fund continued development of the Orion Crew Exploration Vehicle, or something like it. The Obama administration has marked Orion and its Ares 1 launcher for cancellation along with the rest of NASA’s Constellation program.

“It’s also my hope that we would get the funding in the budget that would continue the development of a spacecraft, call it Orion or Orion light or something completely different, but we’re going to do that,” Nelson said.

During a March 18 subcommittee hearing on U.S. commercial space capabilities, Nelson asked repeatedly about the $6 billion intended to fund commercial crew taxis proposed in Obama’s 2011 budget request for NASA.

“What would happen if Congress decided —  since the Congress controls the purse strings —  that we wanted to take the $6 billion projected by the president over the next five years and use that not for human certification of the commercial vehicles but instead to accelerate the [research and development] for a heavy-lift vehicle for the Mars program?” Nelson asked United Launch Alliance President and Chief Executive Officer Michael Gass , one of seven witnesses Nelson quizzed about NASA’s plan to outsource astronaut transportation to the private sector.

NASA’s 2011 budget request includes about a $500 million down payment on a five-year, $6 billion initiative to foster development of commercial crewed vehicles for the international space station. NASA also proposes to spend $3.1 billion through 2015 on a heavy-lift and propulsion research and development program. The White House has proposed paying for these and other initiatives by scrapping plans to go to the Moon and increasing NASA’s budget by $6 billion over the president’s previous five-year plan.

Earlier in the week NASA Administrator Charles Bolden said the agency’s 2011  budget plan balances a number of priorities neglected under previous administrations, including new heavy-lift propulsion research and development, commercial crew initiatives, technology demonstrations, Earth science missions and modernization of the nation’s air traffic control system.

“We should make no mistake that these are the drivers for NASA’s budget increase of $6 billion over the next five years,” Bolden said during a March 16 Washington Space Business Roundtable lunch in Oxon Hill , Md.

Nelson  asked former NASA comptroller Malcolm Peterson how Congress can verify  whether NASA needs the entire proposed $6 billion to foster  a commercial crew transportation system over the next five years.

“I don’t think you can do so at this stage ,” he said. “Our history is not exactly one of unparalleled excellence in cost estimating … we manage to miss it usually by an order of magnitude or so.”

Nelson later asked Gwynne Shotwell, president of  Space Exploration Technologies (SpaceX), whether  fielding a crewed version of the Falcon 9-launched Dragon capsule the Hawthorne, Calif.-based company is getting ready to test for space station-bound cargo runs would require  the entire $6 billion NASA is seeking for the commercial crew initiative.

Shotwell said $6 billion would be enough for NASA to fund between five and 10 efforts of the magnitude SpaceX intends to propose.

Frank Culbertson, a former astronaut who is now a senior vice president  at Orbital Sciences Corp.,  said his Dulles, Va.-based employer is less optimistic than SpaceX about the number of crewed systems NASA could foster for $6 billion.

“We’ve looked at the previous systems that have been flown and we looked at how we develop spacecraft and what it takes to ensure they accomplish missions and to be safe and reliable,” said Culbertson, whose company stands to receive $1.9 billion from NASA to make eight cargo runs to the space station using the Taurus 2 rocket and Cygnus spacecraft now in development. SpaceX has promised NASA 12 cargo flights for $1.6 billion.

Culbertson said Orbital used cost models based in part on the space shuttle’s development to come up with a roughly $3 billion estimate for developing a 9,000-kilogram spacecraft capable of delivering three or four people to low Earth orbit. “Given today’s productivity and efficiencies that we have, we think the number could be significantly less than that, but at least that gives you an upper-bound on a conservative approach of what one company might need to do that level of development,” he said.

During the hearing, Nelson urged Obama to consider adding a fifth shuttle flight to the orbiters’ current manifest, comprised of four remaining logistics missions to the station by September this year .

“I would argue to the president to have a fifth shuttle flight,” he said.