WASHINGTON — Less than 24 hours after Zero Gravity Corp. (Zero-G) flew wheelchair-bound cosmologist Stephen Hawking on a high-profile flight that gave him the opportunity to experience brief periods of weightlessness, executives at the Ft. Lauderdale, Fla.-based company were poring over a NASA solicitation that would open the door to companies that want to provide similar airplane services to the U.S. government.

Whether NASA entrusts Zero-G to fly its astronauts and scientists or continues to operate its own dedicated airplane is up in the air. These airplanes use a series of parabolic maneuvers to create brief periods of weightlessness.

A request for proposals NASA’s Cleveland-based Glenn Research Center issued April 27 says the agency is interested in buying between one and 20 weeks of flights per year from a qualified bidder offering safe and reliable services.

Zero-G, which has been certified by the Federal Aviation Administration since 2004 to carry paying passengers aboard specially outfitted Boeing 727 airplanes for brief bouts of weightlessness, thinks it fits the bill.

But the company’s co-founder and chief technical officer, Byron Lichtenberg, said he was disappointed that NASA did not structure the solicitation as a more straight-forward commercial services purchase agreement, noting that Zero-G is recognized by the U.S. General Services Administration — the government’s procurement agency — as a qualified vendor.

“We are a little disappointed that it still doesn’t take more of the form of a commercial services purchase but they did change some things that make it more palatable,” Lichtenberg said May 3 by telephone. He was referring to differences between the April 27 request for proposal and NASA’s initial draft request for proposals. “We are still looking at the issue of a one-week minimum but we think we’ve got some solutions to try make that a little bit easier to work with.”

Lichtenberg said NASA is asking companies to commit to a single price for flight services whether the agency ends up buying just one week of flights or 20 weeks. He would prefer to bid the contract on a sliding scale.

“The airplane costs go down the more you fly it,” he said, noting that if Zero-G wins, it would have to make some modifications to its aircraft, such as adding overboard venting equipment for combustion products generated by experiments, in order to meet NASA’s requirements.

NASA officials would not comment on the solicitation now that the final version is out for bids. When the draft request for proposals was issued in February, NASA spokesman David Steitz said the agency’s intent is to “procure commercial services provided it is in the taxpayer’s interest to do so.”

While Zero-G officials made clear they intend to bid, some long time commercial space advocates are concerned about what the microgravity solicitation says about NASA’s ability to buy services on a commercial basis.

“For years NASA has told entrepreneurs: ‘don’t ask for an up-front contract, just build it and pursue commercial markets … and then we will buy’,” said Jim Muncy, principal consultant for Alexandria, Va.-based PoliSpace. “Well, a private company has built it, is selling it to commercial customers every day, and has even put it on the GSA [U.S. General Services Administration] schedule. But NASA isn’t willing to just buy this off-the-shelf service, or even use a more commercial-terms procurement.”

Muncy said the shape NASA’s microgravity procurement has taken bodes ill for company’s hoping to sell NASA space station re supply flight under the Commercial Orbital Transportation Services, or COTS, program.

“The investors in COTS should be terrified by this,” Muncy said.