WASHINGTON — NASA watchdogs voiced caution Wednesday about changing the agency's course toward greater research rather than returning to the moon, as proposed by President Obama.

Obama's proposed budget, released Monday, would end the $9 billion Constellation program aimed at returning astronauts to the moon aboard an Ares rocket by instead ferrying people to the International Space Station aboard commercial rockets.

But Joseph Dyer, a retired Navy vice admiral who heads NASA's Aerospace Safety Advisory Board, told the House Science subcommittee on space that the Ares rocket represents the safest bet to return people to space after the shuttle retires later this year.

Dyer also warned against piecemeal extensions of the aging shuttle fleet, which is retiring after five more flights, to fill the gap in transportation between shuttles and the next rocket.

"If the goal is to minimize the gap between the shuttle and the follow-on vehicle, then Ares I offers the safest, quickest opportunity and probably the most cost-effective," Dyer said. "Extension of shuttle use significantly beyond what is planned through the current manifest is not recommended."

Dyer; Cristina Chaplain, director of acquisition management for the Government Accountability Office; and Paul Martin, NASA's inspector general, were officially called to describe challenges facing the space agency. But lawmakers focused on Obama's budget, which would provide $6 billion over five years to encourage commercial flight and boost scientific research about the climate and space travel.

The Kennedy Space Center already anticipates losing 7,000 workers with the shuttle retirement, so eliminating the Constellation program sparks more concerns.

"I'm extremely concerned about the lack of direction that we might have in the policy that is put forth," said Rep. Suzanne Kosmas, D-Fla. "I see a loss of workforce and workforce skills."

The witnesses declined to comment on the budget directly. Martin joked that inspectors general criticize policy only after it is adopted.

But the witnesses described safety concerns with shifting from the NASA-owned vehicles to private partners.

Chaplain said NASA still needs to adopt safety standards for commercial rockets that are more rigorous than for cargo.

"I think they should have a much stronger role than they have now," Chaplain said.

Dyer warned that problems loom if the Federal Aviation Administration or NASA alone were to handle the regulations for commercial spaceflight. The FAA lacks experience with space, and NASA has no experience in certifying commercial vehicles.

"I guess you could call this an interagency opportunity," Dyer said.

The open question is whether private companies will avoid the cost overruns and delays that NASA faced with the Constellation program, particularly if NASA is setting the standards the rockets must meet in order carry people. Company officials have voiced confidence that safety regulations could be met and that NASA wouldn't dawdle because of its goal of returning rockets to the space station.

But the Aerospace Safety Advisory Panel issued a report in January that said abandoning the Constellation program "without a demonstrated capability or proven superiority is unwise and probably not cost-effective."

Despite concerns about shifting to commercial rockets, the consensus remained that the shuttle is done.

Martin warned that delaying the final flights into 2011 would cost $200 million a month to maintain the program.

Dyer noted that after 129 shuttle flights and two catastrophic failures that killed 14 people, the shuttle's history, age and eroding supply and support chain all point to higher risks.

"We recognize that the shuttle is risky by inherent design, and it is becoming more so because of aging and wear," Dyer said.