Roberts was a member of the human colonial expedition of six sent to Deva Loka to assess its suitability for colonisation.
He had left the expedition's base in the Total Survival Suit (TSS) to explore the area and, like two others before him, failed to return. The TSS machine was empty when it was discovered in the jungle by the Fifth Doctor and Adric. (TV: Kinda).
Behind the scenes
The two unnamed members of the expedition who went missing before Roberts are named as Stone and Carter in Terrance Dicks's novelisation of the story. This was not derived from any information given in the televised version.