The last space shuttle flight has smallest space shuttle crew because it's the last space shuttle flight. Bear with me, that's not a trick answer.
Since the loss of Columbia in 2003, NASA has strictly required a Launch On Need (LON) parallel mission for every shuttle flight. Put more simply, every time a shuttle flies, another shuttle is prepped on standby for an orbital rescue mission, should the primary shuttle suffer damage during launch that prevents it from safely returning to Earth.
STS-135 was originally STS-335, the Launch On Need standby flight for STS-134, the final flight of Endeavour that was intended to be the last shuttle mission. As NASA had already spent the money getting Atlantis flight-worthy, they convinced Congress to backdoor STS-335 into STS-135 -- provided Endeavour didn't need a rescue during its final flight. When Endeavour landed safely on June 1, 2011, STS-135 was finally, officially a go.
However, as both Discovery and Endeavour have been retired, there is no rescue shuttle on standby for STS-135. The Launch On Need plan for Atlantis involves not one, but a pair of three-man Russian Soyuz capsules, which can rescue two shuttle astronauts apiece with the help of each capsule's respective pilot.
Requiring two LON craft is a daunting enough prospect, but flying Atlantis with a standard 6-man crew would require a logistically staggering triple-Soyuz Launch On Need mission. That was a non-starter for greenlighting STS-135, so the Atlantis mission profile was developed for a modern minimum four-man crew. (Only the first four shuttle flights, all shakedown missions for Columbia, included two-man crews. No shuttle has ever flown with a three-man complement.)
That's not just a scientifically sagacious skeleton crew, it's a fundamentally unforgettable return flight for Geek Trivia.
The quibble of the week
If you uncover a questionable fact or debatable aspect of this week's Geek Trivia, just post it in the discussion area of the article. Every week, yours truly will choose the best quibble from our assembled masses and discuss it in a future edition of Geek Trivia.
Jay Garmon has a vast and terrifying knowledge of all things obscure, obtuse, and irrelevant. One day, he hopes to write science fiction, but for now he'll settle for something stranger -- amusing and abusing IT pros. Read his full profile. You can also follow him on his personal blog.