When NASA sends a rocket out to tow an asteroid into Earth orbit, it'll be ion propelled.

Ion propulsion engines harness magnetic fields to create thrust, instead of depending on chemical explosions the way chemically fueled engines do. NASA engineers are developing such an engine for the agency's plans to bring an asteroid into Earth's orbit and then send astronauts there to study it. NASA Administrator Charles Bolden went to see a prototype engine yesterday at the Jet Propulsion Laboratory in Southern California, the Associated Press reported.

Bolden also praised the bring-it-to-us asteroid study plan. Originally, the Obama Administration wanted to send people to the asteroid belt, but bringing an asteroid here instead will be cheaper. The technologies the Asteroid Retrieval Initiative develops will help NASA shuttle people to Mars in the future, Bolden told the Pasadena Sun.