Elon Musk's SpaceX successfully launched a Falcon 9 rocket — carrying a top-secret spy satellite — from NASA's Kennedy Space Center Monday, before turning around and heading back to Earth.

Why this matters: This marks the fourth time SpaceX has brought one of its Falcon 9 rockets back to land, and is the company's first launch for the U.S. National Reconnaissance Office's (NRO's) mission.