NASA is finally done assessing its two options for the Asteroid Redirect Mission (ARM). Its final choice? Option B: it plans to grab a chunk of its target asteroid and take the boulder-sized piece to the moon's orbit. While Option A sounded more exciting -- it entailed capturing an entire asteroid -- NASA has decided to go with this one, because it will allow the agency to test new technologies for future manned trips to Mars. One of those technologies is called Solar Electric Propulsion (SEP), which uses sunlight instead of traditional fuel. More specifically, it uses solar arrays to convert sunlight into power, which propels charged atoms that move the spacecraft -- it's not as fast as rocket propulsion, but it could save manned missions a lot of money.