President Barack Hussein Obama on Thursday ordered a review of a rocket NASA hopes will replace its shuttle fleet, but the agency insisted the future of manned U.S .space flights was safe.

"NASA will review U.S. post-shuttle human space flight activities this summer," Obama's new budget said, sparking questions over the future of the Constellation project and its ambition to take astronauts to the moon and Mars. Chris Scolese, acting administrator of the National Aeronautics and Space Administration, said the agency's review would examine "possible alternatives" to Constellation, which consists of an Ares rocket and Orion crew capsule. But while acknowledging that Constellation has burned through six billion dollars in the past two years, Scolese told reporters: "If you're asking me if I think we're on the wrong path, no, I don't. We're making a commitment to this nation for human space flight for many years to come."

Obama's 2010 budget gave no further indication on the future of Constellation, which NASA intends to use as a replacement for the space shuttle in the years after the aging fleet is retired in September 2010. In an interview with U.S. newspapers in March, the president said NASA was guilty of a "sense of drift" and that it needed a "mission that is appropriate for the 21st century." The first Ares launch is tentatively scheduled for March 2015, as NASA returns to its 1960s model of a reusable crew capsule sitting atop a rocket, instead of a shuttle capable of returning to Earth on its own. In the five-year interval after the orbiters are withdrawn next year, NASA intends to rely on Russian spacecraft to take its crews to the International Space Station. Ares would then take over, with the aim of returning U.S. astronauts to the moon by 2020 and then taking humans to Mars for the first time.

The new rocket has suffered major problems in development, notably a violent shaking that is feared could drive the vehicle into its launch tower. Constellation's initial budget was 28 billion dollars and that could now range upwards to at least 44 billion. "The Obama administration wants to understand if it's the right way to spend the money, on human space exploration," said John Logsdon, ex-director of the Space Policy Institute at George Washington University. Constellation was approved by Obama's predecessor George W. Bush, and Scolese said it was natural for a new administration to examine NASA's priorities. He denied Obama was cool towards NASA — he emphasized he had met with the president three times in the past month and the new budget raises the agency's funding by roughly two billion dollars overall to 18.7 billion. Accentuating NASA's readiness to shift with new political priorities, Scolese said the extra money would help to beef up NASA's work on climate change and research into greener aviation.

Despite the announced review of Constellation, "we're going to continue ahead," the acting administrator added. "We're not stopping anything. We're progressing with the plans we have in place." The review panel is to be headed by Norman Augustine, a former chief executive of Lockheed Martin, and is expected to issue its recommendations in August. "I am a real believer in the value of this nation's human spaceflight activities," Augustine said in a NASA statement. "And will do everything I can to provide the information needed to help the country maintain the spectacular arc of progress NASA has fueled for five decades."