Rocket Comics was a comic book starring the fictional character Captain Rocket.
Rory Williams was a fan of the comic growing up: he called the titular character his hero.
The series was popular with NASA scientists, who put copies of the first issue in all of their explorer probes. The Eleventh Doctor owned a copy of the first issue, which he had the character's creator, Hamilton Wilson, sign for him at San Diego Comic-Con. (COMIC: Convention Special)