NASA has decided to launch a robotic probe to a near-Earth asteroid in 2020, have it pull off a boulder from the space rock and bring it to a spot near the moon where astronauts can study it. The mission is expected to cost $1.25 billion and is a big milestone for a mission concept that was born in the early days of the Obama administration. Option B was chosen over Option A, which would have cost about $100 million less.