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Atlantis will join Discovery and Endeavour in retirement after this 13-day journey, the 135th for the shuttle program. All three will become museum displays.

As it turns out, Tuesday marked the 36th anniversary of the undocking of the Apollo spacecraft from a Soviet Soyuz in the first-of-its-kind joint flight. Nearly six years passed between the end of that 1975 mission and the start of NASA’s next: the space shuttle.

Mission Control said that gap — five years and nine months — is the mark to beat this time around, and it said it was starting the clock.