The agency doesn't yet have a rocket powerful enough to blast it into space that is also deemed safe enough to transport people. "It's kind of like sitting in a beautiful Cadillac, without an engine or tires," said Marco Caceres, director of space studies for Teal Group, a Fairfax, Virginia-based consultant.

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."