What do you think they could utilize as energy on the asteroid? there will be water ice but what else could they use?

http://www.newsmax.com/TheWire/nasa-asteroid-mars/2012/12/26/id/469057

NASA scientists plan to capture a 500-ton asteroid, move it through space, and then keep it in a predestined position to act as a refueling space station for astronauts making their way to Mars.

The proposal will be on the docket of the White House Office of Science and Technology in the coming weeks, as the space agency plans its budget for space exploration in the coming years. The asteroid project costs an estimated $2.5 billion, according to the Daily Mail.

This would be the first time a celestial object would be captured and moved through space. Here's how it would work:

When the space rock passes between Earth and the moon, it would be captured by an "asteroid capture capsule" attached to an old Atlas V rocket. 

When the spacecraft approaches the asteroid, the capsule will release a bag engulfing the spinning asteroid. The bag then will cinch up with drawstrings. Once the asteroid is roped in, the spacecraft will deploy its thrusters and tow the asteroid to a gravitationally neutral spot.

A feasibility report of the project was prepared by NASA and the California Institute of Technology.

"The feasibility is enabled by three key developments," the report said. "The ability to discover and characterize an adequate number of sufficiently small near-Earth asteroids for capture and return; the ability to implement sufficiently powerful solar electric propulsion systems to enable transportation of the captured asteroid; and the proposed human presence in cislunar space in the 2020s enabling exploration and exploitation of the returned asteroid."

NASA declined comment on the project, saying discussion could compromise negotiations with the White House. But the space exploration agency did say the technology could be available within 10 to 12 years. If the project succeeds, the technology could be used later to mine asteroids for resources. 

On Dec. 11, a newfound asteroid about 120-feet-wide zipped between Earth and the moon. The near-Earth asteroid 2012 XE54 came within 140,000 miles of the planet. For comparison, the moon orbits Earth at an average distance of 240,000 miles or so. The next day, the near-Earth asteroid 4179 Toutatis, which is about 3 miles wide, zoomed within 4.3 million miles of Earth.