NASA said it needs more information before it decides how to do its signature space mission of the 2020s — bringing a space rock into orbit around the moon for study — but it's leaning toward having a robot pull a boulder off the surface of a large asteroid. The big question is whether taking that option is worth the extra expense and mission risk, NASA Associate Administrator Robert Lightfoot told reporters during a teleconference on Wednesday.

Lightfoot presided over meetings this week to decide between the boulder-grabbing scenario, known as Option B, and an Option A plan that involves intercepting a small near-Earth asteroid and bringing it back whole. After the meetings, Lightfoot said it would take another two or three weeks to get the data he needed for the decision. A mission concept review is scheduled for next February.

Option B would test more of the technologies that would be used during future missions to Mars, Lightfoot said — but the mission would be more complex and about $100 million costlier. The target price tag is roughly $1.25 billion, excluding launch costs. That figure also excludes the cost of sending astronauts to rendezvous with the space rock in the mid-2020s.

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One reason I prefer option B for asteroid return: testing ability to work on asteroid surfaces so we can learn to deflect larger asteroids. — Dr. Phil Metzger (@DrPhiltill) December 17, 2014

Asteroid Redirect Mission options (asteroid vs. boulder) are so awesome NASA can't yet make a decision. If only Congress thought the same. — Parabolicarc.com (@spacecom) December 17, 2014

— Alan Boyle