(AP)—NASA does not have enough money to get its new, $12 billion rocket system off the ground by the end of 2017 as planned, federal auditors say.

The Government Accountability Office issued a report Wednesday saying NASA's Space Launch System is at "high risk of missing" its planned December 2017 initial test flight. The post-space shuttle program would build the biggest rockets ever—larger than the Saturn V rockets which sent men to the moon—to send astronauts to asteroids and Mars.

"They can't meet the date with the money they have," report author Cristina Chaplain said. She said it wasn't because the space agency had technical problems with the congressionally-required program, but that NASA didn't get enough money to carry out the massive undertaking.

The GAO report put the current shortfall at $400 million, but did say NASA was "making solid progress" on the rocket program design.

NASA's launch system officials told the GAO that there was a 90 percent chance of not hitting the launch date at this time.

This usually means NASA has to delay its test launch date, get more money or be less ambitious about what it plans to do, said former NASA associate administrator Scott Pace, space policy director at George Washington University.

NASA is working on the problems GAO highlighted, but delaying launch or diverting money from other programs would harm taxpayers, NASA Associate Administrator William Gerstenmaier wrote in the agency's response.

"Welcome to aerospace," Pace said, pointing out that large space projects often end up as much as 50 percent over budget. He said that "is why you shouldn't believe initial cost estimates."

The space agency has been reluctant to put an overall price tag on the Space Launch System. The GAO report says it will cost $12 billion to get to the first test launch and "potentially billions more to develop increasingly capable vehicles" that could be used for launches to asteroids and Mars.

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More information: GAO audit: GAO audit: www.gao.gov/assets/670/664969.pdf

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