Jackson Roykirk was an early-21st century Earth scientist who created the Nomad space probe – an artificially-intelligent machine sent out with the mission to seek out new life in 2002. Roykirk's ambition was to build a perfect thinking machine which was capable of independent logic. Spock described him as one of the most brilliant but erratic scientists of his time.
In 2267, after merging with Tan Ru and becoming damaged, Nomad mistakenly believed Captain James T. Kirk was its creator, given the similarities between their names. As a result, it ceased its hostilities against the crew of the USS Enterprise.
Kirk later used this mistake, Nomad not realizing it later, and failing to self-destruct as of that moment, being imperfect as it was, as reasons why it should destroy itself according to its own prime directive. (TOS: "The Changeling")
The photographic image of Roykirk seen in "The Changeling" was "portrayed" by director Marc Daniels. Although he was a 21st century scientist, he wore a standard 2260s Starfleet dress uniform in the photo (albeit it seems to be either a reversed image or configured slightly differently based on the opposite positioning of the "decorations").
In an homage to Star Trek, a character named "Jackson Roykirk" appeared in an episode of Team Knight Rider entitled "Apocalypse Maybe," played by William Christopher of M*A*S*H fame. While this may seem to be a crossover, constant references to "Star Trek" on the show make it unlikely. 
Jackson Roykirk is mentioned in The Rise and Fall of Khan Noonien Singh, Volume Two as being part of the team which designed and created the DY-100 class ship which carried Khan and his remaining followers to the stars.
Jackson was incorrectly referred to as Jason Roykirk in the All Our Yesterdays sourcebook.