Which Apollo astronaut guest-starred in season 6?

The Apollo astronaut who guest-starred in season 6 of The Big Bang Theory was Buzz Aldrin.

Aldrin appeared as himself in “The Holographic Excitation,” the seventh episode of season 6, which aired in November 2012. The story brings him into Howard Wolowitz’s orbit through a neighborhood Halloween encounter, where Howard recognizes the famous moonwalker and excitedly introduces him to the others.

Buzz Aldrin was the lunar-module pilot on Apollo 11 and became the second person to walk on the Moon, following Neil Armstrong. That makes him an especially fitting guest for a sitcom centered on scientists, engineers, space travel, and popular science.

A common mix-up is Michael J. Massimino, another astronaut who appears repeatedly in the series as Howard’s fellow astronaut. Neil Armstrong and Michael Collins were also Apollo astronauts, but neither is the season 6 cameo identified by this question.