The U.S. Space Launch System (SLS), successor to the mighty Saturn V rocket that landed 12 Americans on the Moon, won't have the money to lift-off by its target date of December 2017.

NASA said it doesn't have enough money to fully fund its US$12 billion rocket system to ensure the SLS' initial test flight. The Government Accountability Office (GAO) said the SLS was US$400 million short of the funds it needs and is at "high risk of missing" its first launch in 2017.

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GAO noted, however, that NASA was making solid progress on the rocket program design and that technical problems weren't causing the delay.

NASA has admitted a 90 percent chance of not hitting the launch date at this time.

Aerospace experts said NASA is now left with the choice of either delaying its test launch date as it scrounges for more money or be less ambitious about what it wants to do with the money it does have.

NASA claims delaying the 2017 launch or diverting money from other programs to fund the SLS would harm taxpayers.

SLS will loft astronauts and hardware to near-Earth destinations such as asteroids, the Moon, Mars and most of the Earth's Lagrangian points. SLS will also make trips to the International Space Station.

The SLS program is integrated with NASA's Orion Crew and Service Module.

SLS succeeded the cancelled Constellation Program. It is being designed to replace the retired Space Shuttle.

The SLS launch vehicle, the most powerful heavy lift vehicle ever developed by NASA, is to be upgraded with more powerful versions. Its initial Block I version without an upper stage can lift a payload of 70 metric tons to Low Earth Orbit (LEO) while the Block IA/B can loft 105 metric tons.

The final Block II SLS will have a payload lift capability of at least 130 metric tons to LEO, or 12 metric tons greater than that of Saturn V.