He had a reputation as an eccentric, stemming from his experience of integrating himself into a primitive humanoid culture which had not yet discovered fire for over a year. He had participated in more than twenty-four concealed observation missions in his career prior to 2265.
In that year, Roberts was part of a Starfleet observation mission to NGC 667, homeworld of the Grennai. In trying to determine the Grennai's natural resistance to the radiation of their atmosphere, Roberts decided to start taking tissue samples from recently deceased Grennai corpses, and later, covertly testing living beings.
After a mutated strain of Ametan rubeola killed the village Robert's team was observing, they were recalled by Starfleet. Starfleet classified his actions and the aftermath, claiming publicly that Roberts had retired due to health issues and withdrew from public life. In fact, Roberts was convicted of violating the Prime Directive, and sent to a penal colony. From there, he continued to occasionally publish in the Starfleet Medical Journal. (TOS short story: "First, Do No Harm")