Peterson, along with Heldorf, were scientists hired by Charles Carrington to examine after his captive aliens. He was shot and killed by one of Reegan's thugs when he tried to attack him. (TV: The Ambassadors of Death)
Behind the scenes
Peterson is never identified in the episode, only in the paperwork for episode three. The actor who portrayed Peterson, Roy Scammell, was a stunt performer on the series and played Peterson, without credit.