Jackson, who goes by Jack, serves as Orbiting Jupiter’s narrator and protagonist. Throughout much of the novel, Jack is a passive protagonist. The son of Maine farmers, Jack is a hard worker who is well-liked by the adults in his life. Jack is 12 years old at the beginning of the novel, and he is more naïve than Joseph. Jack is childlike in many ways, which explains why he is shy about sex. While Jack doesn’t speak often about his own religious views, he and his parents are practicing Christians. They attend church services on Christmas Eve and their Christmas tree ornaments evoke Christian imagery.

Jack is haunted by the memory of seeing a dog drown in the Alliance River when he was six years old. This memory informs his reaction to Joseph’s misadventure on the river and foreshadows the loss of his foster brother. Saving Joseph counters the guilt of not helping the drowning dog, and it spurs Jack into his first significant action of the novel, and as the novel progresses, Jack becomes more active. He intervenes in Joseph’s fight with Jay, and he decides to split from his parents while searching for Joseph in Brunswick.