The flight program intended to get the United States back to the moon, to Mars and beyond might not be able to get off the ground itself without more federal funding.

A new report issued by the U.S. Government Accountability Office warns the National Aeronautics and Space Administration and its commercial aerospace partners will likely not get the Space Launch System, a giant launch vehicle, into space by its December 2017 target period unless an added $400 million is found to bolster the program, according to a report by United Press International.

Officials with the federal watchdog agency said NASA's previous spending figures aren't realistic, and, as a result, the agency may well have to delay the maiden launch of the craft.

"While the technical challenges associated with those efforts appear manageable, the compressed development schedule in conjunction with the agency's relatively flat funding profile for SLS through 2017 place the program at high risk of missing the planned December 2017 launch date for the EM-1 initial test flight," the report noted.

The evaluation raises new doubts about the promise made earlier this month by Virginia Barnes, vice president and program manager of Boeing's SLS initiative, that the company's "teams have dedicated themselves to ensuring that the SLS -- the largest ever -- will be built safely, affordably and on time."

The dire report comes days after engineers began preparing to test the SLS rocket's RS-25 engines at the agency's Stennis Space Center near Bay St. Louis, Mississippi, a space agency news release said.

The Stennis center team is on track to perform developmental and flight certification testing of the engine, a modified version of the main engine that powered space shuttle missions into space from 1981 to 2011.

"This test series is a major milestone because it will be our first opportunity to operate the engine with a new controller and to test propellant inlet conditions for SLS that are different than the space shuttle," said Steve Wofford, SLS Liquid Engines Element manager. "This testing will confirm the RS-25 will be successful at powering SLS."

Early tests on the engine will collect data on the performance of its new advanced engine controller and other modifications. The controller regulates valves that direct the flow of propellant to the engine, which determines the amount of thrust generated during an engine test, known as a hotfire test. In flight, propellant flow and engine thrust determine the speed and trajectory of a spacecraft. The controller also regulates the engine startup sequence, which is especially important on an engine as sophisticated as the RS-25. Likewise, the controller determines the engine shutdown sequence, ensuring it will proceed properly under both normal and emergency conditions.

Said Gary Benton, RS-25 rocket engine test project manager at Stennis: "Installation of RS-25 engine No. 0525 signals the launch of another major rocket engine test project for human space exploration."