NASA tried a rocket scientist at the helm for four years under President George W. Bush. Now the world's biggest space agency may need a salesman as it aims to raise as much as $120 billion to get back to the moon.

The National Aeronautics and Space Administration, preparing for the May 11 space shuttle launch of a risky repair mission to the Hubble Space Telescope, faces critical decisions about returning to the moon by 2020. And 50 years after its founding, the agency is struggling to prove its relevance.

Bush's last administrator, Michael Griffin, an aerospace engineer who wrote a textbook on spacecraft design, came up short as a pitchman, said Representative Ralph Hall of Texas, the top Republican on the U.S. House Science and Technology Committee.

"Mike wasn't able to sell" the space program, Hall said. "Maybe we need a guy with a shine on his shoes and a smile on his face."

Whoever takes the job will need to persuade lawmakers to commit billions to developing a new spacecraft for a return to the lunar surface 51 years after Neil Armstrong set foot there. The moon is a step toward the eventual goal of humans landing on Mars after a journey of perhaps six months.

"The main job of the NASA administrator" is to go to lawmakers "and beg for money," said Jonathan McDowell, a scientist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

Bolden, Lampson

Speculation in the media and Congress about Griffin's successor has focused on retired Marine Major General Charles Bolden, a former shuttle commander, and former Texas Representative Nick Lampson, a Democrat who represented NASA workers in a Houston suburb.

Pioneering astronaut and former Senator John Glenn said NASA is underestimating the amount needed for the new missions by billions of dollars.

Glenn, an Ohio Democrat who was the first American to orbit Earth, said it is "obviously impossible" for NASA to pursue its moon and Mars quest on its existing budget.

Bush requested incremental increases in the spending plan, which Congress cut in two of the past three years. The 2010 budget Obama requested May 7 would boost by 13 percent to $4 billion the NASA unit in charge of the moon and Mars missions.

Space Station

The president also would increase by 7 percent to $6.2 billion the unit that operates the shuttle and the International Space Station.

The space agency "this summer" will review human space- flight plans following the shuttle, according to the budget documents.

Obama told reporters in March that he wanted to "restore that sense of excitement and interest that existed around the space program." He also chatted with astronauts on the space station, asking them if they cut their hair short and still drank Tang.

The delay in naming a space chief raises questions about the agency's direction and Obama's commitment, Senator David Vitter said.

The new administration is a "bit of an unknown quantity with regard to space policy," said Vitter, a Louisiana lawmaker who is the senior Republican on the Science and Space Subcommittee. "Without a new administrator that period of uncertainty has been extended."

Recent NASA leaders haven't been known for their skills as agency boosters. Sean O'Keefe, the administrator between 2001 and 2004, was a former deputy director of the Office of Management and Budget brought in to bring discipline to NASA's finances. O'Keefe ended up consumed by the investigation into the destruction of the shuttle Columbia -- and the deaths of its crew of seven -- during the craft's return from a 2003 mission.

Columbia Disaster

Griffin took over in 2005 in the aftermath of the Columbia disaster with a mandate to retire the shuttle and get NASA back into deep space.

He said the president should take the lead on pushing for bigger budgets for missions and leave management and technical details to the agency administrator.

"NASA needs a pitchman like a fish needs a bicycle," he said. "Technical excellence is crucial."

Among the nine missions left, including one proposed by Obama, is Atlantis's trip to replace a camera, batteries and gyroscopes on Hubble, an orbiting observatory. The mission carries extra risk because the shuttle can't get to the space station if something goes wrong. The shuttle Endeavour is on a launch pad in case a rescue is needed.

Cost Concerns

While polls show Americans generally support NASA's missions, many are reluctant to pay for them.

Fifty-seven percent of those asked in an April 2008 survey by Gallup for the Coalition for Space Exploration said they wouldn't back higher taxes to help close a five-year gap between the shuttle's retirement and the completion of the next- generation Constellation lunar spacecraft.

"The public right now is a little cynical," McDowell said. "Many don't see the immediate benefits to going back to the moon."

NASA's popularity peaked with the Apollo program, which ended in 1972 after six trips to the lunar surface, about 250,000 miles from Earth. Since then, astronauts haven't traveled more than 500 miles (805 kilometers) from our planet.

Senator Barbara Mikulski, chairwoman of the subcommittee that determines NASA's budget, said the agency needs to find a person with a rare mix of qualities to engage Americans.

Wanted: Charisma

"We want somebody who brings intellectual rigor, technical competency and a charismatic approach," she said. A likeable, driven person is needed to "really promote the agenda."

Bolden and Lampson, who lost re-election in 2008, have the ability to re-energize the American public about the space program, their supporters said.

White House spokesman Nicholas Shapiro declined to comment on the administrator search.

When asked if Bolden would be the pick, Senator Bill Nelson, a Florida Democrat, put his hands together in prayer. The lawmaker and Bolden were both members of a 1986 space shuttle crew.

The next administrator will have to figure how to keep the human spaceflight program functioning during the five-year gap without NASA spacecraft, lawmakers and analysts said. U.S. astronauts will fly on Russian spacecraft to the station.

Glenn called the situation "ludicrous."

"Here we are, the greatest space-faring nation ever and we can't even get up to our own space station," he said.