WASHINGTON — NASA will not rob its science budget to pay for human spaceflight, according to the U.S. space agency’s top official, though in the future NASA scientists should expect international cooperation to play a greater role in the projects the agency undertakes.

“The cost and complexity of space programs require both the achievement and the costs be shared among many nations, for no one nation can carry this burden alone,” NASA Administrator Charles Bolden told members of the American Astronomical Society during remarks at an annual conference here Jan. 5.

Bolden reassured the audience of mostly astronomers and astrophysicists that regardless of an anticipated realignment in NASA’s manned spaceflight activities and investments, the agency’s $4.5 billion annual science budget would not be used to pay for it.

“I can make this commitment to you as NASA administrator: The future of human spaceflight will not be paid for out of the hide of the science budget,” Bolden said to an enthusiastic round of applause.

Bolden and senior officials from the administration of U.S. President Barack Obama have spent the past several months mulling the report of a blue-ribbon panel that found the agency’s Constellation program, a 5-year-old effort to replace the space shuttle with rockets and spacecraft optimized for the Moon, is incompatible with NASA’s budget.

Bolden blamed a lack of adequate funding for a robust human spaceflight program over the last “10 or 15 years” for the program’s current setback, adding that in the past few years the agency “had to steal money away from everybody just to try to allow the human spaceflight program to survive,” he said. “That’s not a way to run a program.”

In addition to funding woes, Bolden said, NASA’s fear of failure since the 1986 Space Shuttle Challenger accident has hindered efforts to return humans to the Moon.

“We became intimidated when we lost Challenger,” he said. “And we became risk averse. And we have not recovered from that as a nation yet.”

Bolden said NASA must become more willing to take chances.

“Every once in awhile, things aren’t going to go right,” he said, citing the launch mishap that destroyed NASA’s Orbiting Carbon Observatory in February 2009.

“We should have been better than that, but we weren’t, so we lost an incredibly capable and incredibly important scientific satellite. But that’s OK,” he said, noting that while human error will always be a factor in the agency’s endeavors, NASA can learn from its mistakes.

In the meantime, Bolden said, NASA must make tough choices when prioritizing missions and funding them.

“What does that mean for us in terms of fiscal measures?” he asked. “It means we’ve got to find cheaper ways to do some of the things we want to do.”

Although NASA lacks the resources to fly more frequent flagship-class missions, Bolden said, smaller efforts to develop enabling technologies could gradually lead to large-scale projects.

“We cannot do big things very much anymore,” he said.

International partners will be key to helping fund space missions both large and small, Bolden said. Whether it involves human voyages beyond low Earth orbit, a robotic Mars sample-return mission or the building of large space telescopes, a new era of international cooperation is at hand.

“We need to just suck it up and buy into it — the world has changed,” Bolden said. “We have incredible partners when it comes to technical capability,” he added, citing the Japanese H-2 Transfer Vehicle that made its first supply run to the international space station last year.

Bolden said he met earlier in the day with U.S. Secretary of State Hillary Clinton to discuss NASA’s potential to partner with space agencies in developing nations. Citing a meeting he held with the head of Nigeria’s space agency during a visit to South Korea in October, Bolden said he and Clinton discussed ways to involve small, nontraditional partners in cooperative space projects.

“There are other nations that can benefit so much [from] being able to collaborate in the things that we do,” he said.

During a subsequent NASA town hall meeting during the conference Jan. 6, NASA Astrophysics Division Director Jon Morse said the agency must find new ways to fund science programs.

“We can’t afford to keep doing everything the same way we have been in the past,” Morse said. “We’ll be looking for ways to keep things going, but there might be some changes in the way things are supported.”

Morse said that while lawmakers made only minor “tweaks” to the agency’s astrophysics budget, they had provided “more content than money” in the Science Mission Directorate’s $4.5 billion appropriation for 2010, which Obama signed into law Dec. 16. As a result, the Astrophysics Division can expect to make up some $15 million in funding within its $1.1 billion allocation that lawmakers expect the directorate to find from within.

“It’s not a huge amount of money,” Morse said. “But it’ll come from somewhere.”

Referring to Bolden’s comments on Nigeria, Morse said astronomy projects are a natural fit for international collaboration with nontraditional partners.

“Keep your eye out for opportunities to form partnerships with developing countries,” Morse told the audience during a question-and-answer session following his remarks. “Astronomy is accessible to everybody, and we’re one of the best fields for outreach in that.”