The cost of the SLS and Orion could grow to more than $30 billion, said Marco Caceres, a senior space analyst with the Teal Group consulting firm. There is a fair amount of opposition to the asteroid mission in Congress, and a Mars landing, some 20 years away at best, remains a dream.

"To make matters worse," Caceres wrote in a recent blog, "is the painful fact that the rocket doesn't actually have a mission."

The Mars program is subject to funding constraints and a balky political system.

"This is the problem with anything that takes 20 years — people have very short attention spans," Caceres said. "And you have numerous presidential administrations and Congresses, and inevitably someone gets around to canceling it, or reducing it."

"Boeing's cost structure is higher than SpaceX — that's just a fact," said Marco Caceres, senior space analyst at the Teal Group.

Because the aerospace giant has been around longer, it most likely has an older, more experienced workforce with higher wages and labor costs, he added.

But the biggest cost differences likely come from manufacturing processes. Boeing builds its Delta rocket and Atlas V launchers in a vertical position, while SpaceX's Falcon 9 rockets are assembled horizontally, which is easier and saves on time.

Meantime, SpaceX is a vertically integrated company, building all of its parts in-house, with no subcontractors. Boeing employs numerous subcontractors to build components, which adds to costs. "When you become a more established company, you can't be a specialist in everything," Caceres said.

"SpaceX is the new kid on the block, but it's proven its capabilities very quickly," said Marco Caceres, director of space studies with Teal Group, a Fairfax, Virginia-based consultant. "SpaceX would love to be the first commercial company to land its own private astronauts on the moon and eventually go on to Mars."

"It's entirely possible that we wake up one day and the Russians say, 'We're not taking your astronauts up anymore,'" said Marco Caceres, director of space studies at Fairfax, Virginia-based consultant Teal Group. "NASA's anticipating this possibility. That's why they want to move as quickly as possible with this program."

Funding two ventures may raise development costs while also fostering competition and giving NASA an alternative if one vehicle encounters technical difficulties, said Marco Caceres, director of space studies with Teal Group, a Fairfax, Virginia-based consultant.

Congressional opposition to a similar arrangement for the crew contract has waned as the fraying U.S.-Russia relationship focuses attention on NASA's dependence on Soyuz rockets to put astronauts into orbit, Caceres said in a telephone interview.

"The Russians have done NASA a favor in terms of funding," Caceres said.