"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.

The problem is that the Space Launch System program lacks a clear mission, according to Marco Caceres, director of space studies at the Teal Group Corp., a defense industry think tank. “They’ve toyed with lassoing an asteroid and hauling it to the moon, or using it as a vehicle to take people back to the moon, but it’s not clear why” we’re funding SLS, Caceres said.

The political landscape may have changed, however, now that Russia has blocked sales of its Russian RD-180 rockets, which are used to launch U.S. defense satellites. Russia also has announced that it plans to end its operations on the ISS in 2020, effectively disabling the station. “There’s a good chance that SLS will be developed and built, mainly because politically it just looks bad to not be developing your own national launch vehicle and we have to rely on Russia to launch our astronauts,” Caceres said.