NASA is preparing for the maiden launch this week of its new Orion spacecraft, which could help jump-start America's return to human exploration of space, including a journey to Mars.

This unmanned mission is relatively simple, less than five hours long and headed to no place particularly interesting.

Yet the flight's success and what NASA can learn from it are critical to the agency's dreams to send astronauts deep into space.

NASA plans to launch Orion from Cape Canaveral Air Force Station on Thursday atop a Delta IV Heavy rocket. The plan is for Orion to orbit Earth twice, swinging out to a point 3,600 miles high, then splash down in the Pacific Ocean off the coast of Baja California, Mexico.

It plans a second test launch in 2018 that would send up another unoccupied Orion, this time around the moon and back.

The first manned mission, expected in 2021, probably would also go around the moon and back. Later in the 2020s, NASA intends to send Orion and astronauts to an asteroid. By the late 2030s, it wants to send them to Mars.

NASA hopes to develop annual missions for Orion in between, but is leaving those plans undetermined, for "space destinations we cannot yet imagine," said Orion Flight Director Mike Sarafin.

That uncertainty puts the future of the program up in the air. If Orion struggles with delays, cost overruns and a lack of clear goals, as did its predecessor project, called Constellation, it could derail. In 2010, President Obama canceled Constellation, after NASA had spent $13 billion and five years on it.

The U.S. General Accounting Office estimated NASA would spend $19 billion to $22 billion on the Orion program through the first manned mission in 2021. The GAO said there was no way to estimate what would be needed beyond that.

Supporters in Congress express confidence in the space agency's agenda. "NASA knows exactly what it wants to do with this program," said Sen. Bill Nelson (D-Fla.), who chairs the Senate subcommittee on science and space. "This is the beginning of the trip to Mars."

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NASA set to launch Orion spacecraft, paving way for human Mars visit