Those who follow Elon Musk's "other" company, Space Exploration Technologies, already know that SpaceX's Falcon 9 rocket booster is on the verge of becoming re-usable, a milestone that will drop the cost of getting into orbit by several times.
On September 21, a SpaceX Dragon capsule was launched to resupply the International Space Station. During that flight, the first stage of the Falcon 9 went through a series of maneuvers that will eventually allow a fully controlled landing and recovery of the first stage. (Since the first stage contains nine of the ten rocket engines on the Falcon 9, its recovery and re-use represents recovery of most of the value of the rocket.)
Two NASA aircraft with infrared cameras captured this amazing video showing the first stage during its post-separation maneuvers and engine re-start. The data obtained by NASA will be analyzed for use in a possible Mars entry and landing mission. Before the Falcon 9 did it, no rocket had ever flown through its own exhaust plume for any significant amount of time, which would be required for some types of Mars entry missions.
h/t Innerspace
Let's walk through what we're seeing here. This is an infrared camera, which shows heat. Since it's cold up at those altitudes, the rocket will show up readily, even when the engine is turned off.
0:15. In the first images, we're looking at the Falcon 9 immediately after stage separation, when the first stage has separated and fallen behind the second stage. The second stage has ignited and we can see the large exhaust plume from the second stage and the bright dot of the hot-and-running second stage engine. Seconds later the dot seems to split in two, as we can see the spent first stage falling far enough behind the second stage to be separately resolved, trailing along behind the second stage in the exhaust plume.
0:22-0:53 The trailing first stage seems flash and brighten. These are the maneuvering thrusters of the first stage, orienting the stage properly for re-start of the main engines. The first task is to maneuver out of the exhaust plume.
0:55-1:50 The first stage reaches apogee (it's highest point) and re-starts one engine to control its final downrange landing point. The exhaust plume is ahead of the descending stage, forming a "bow wave".
1:54 Three engines re-ignite, which decelerate the stage from supersonic to the subsonic range. The stage and plume of hot exhaust gas both show up very brightly.
2:09 Engines shut down again and the stage free-falls at subsonic speeds.