By Jeffrey Kluger
May 21, 2015

Nothing, absolutely nothing, is easy in space, and that includes leaving it, as the crew of the International Space Station (ISS) will be reminded on Thursday morning, May 21, when the Dragon cargo vessel undocks and heads home—a maneuver TIME will live-stream via NASA beginning at 6:45 a.m. ET. Dragon, the 24-ft. (7.3 m) cargo vehicle built by SpaceX, arrived at the ISS on April 17 carrying 5,200 lbs (2,360 kg) of cargo. It is returning after a five-week stay, bringing home 3,100 (1,400 kg) different lbs. of stuff—some of it trash, but a lot of it scientific samples that are part of the extensive biomedical studies being conducted on astronaut Scott Kelly and cosmonaut Misha Kornienko as they spend a marathon year in space.

No spacecraft leaving the ISS can simply cast off and go. Ever since the long-ago joint mission of Gemini VI and Gemini VII, 50 years ago this December, when the two manned spacecraft maneuvered to within inches of each other, NASA has appreciated the delicate dance involved when any two orbiting objects come anywhere near each other. Moving along at a matching 17,500 mph (28,160 k/g), the ships are effectively motionless relative to each other. If one adds even the tiniest bit of speed that the other one doesn’t match—going to, say 17,505 mph—the result can be a fender bender.

For that reason, the Dragon departure will be a process that will consume a whole morning’s work. Before 7 a.m., the station’s 58 ft. (17.6 m) robotic arm will grapple the Dragon, which will decouple from its berth on the station’s Harmony module. The arm will carry the Dragon as far from the station as it can, and Kelly, who will be controlling the operation from aboard, will give the signal for its release at 7:04 a.m. Over the course of the next four hours and 45 minutes, the Dragon will fire its thrusters three separate times, edging further away from the station until, at 11:49 a.m., it reaches the precise spot in its orbit to begin a reentry burn that will carry it to a Pacific splashdown at 12:42 p.m.

The station has been serviced by milk runs like these many times in the 15 years it has been continually occupied and there will be a lot more to come in the decade or so of service it has left to it. It’s a measure of the complexity of the up and down trips that they take so much planning and such deft execution; it’s a measure of the people doing the executing that the maneuvers can actually, after a time, seem routine.

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