Alex is an optimistic eleven-year-old boy living in Rockview, Colorado. He possesses a deep passion for aerospace science and greatly admires the astronomer Carl Sagan. Taking on adult responsibilities at home, Alex cooks meals for himself and his mother while preparing for a rocket festival. He travels with his dog and uses an iPod to document his thoughts and experiences as audio recordings for an alien audience.

Karen is Alex and Ronnie's mother, who lives with Alex in Colorado. She spends long hours resting on the couch, watching television, and going for long walks. She leaves Alex to handle household chores and prepare meals, appearing detached from his daily activities and his ambitious plans to attend a rocket festival.

Ronnie is Alex's older brother, who works in Los Angeles as an agent for professional athletes. He moved away from Rockview to establish his career and has not visited home in almost a year. He occasionally checks in on Alex over the phone regarding finances but remains physically distant from the household's daily reality.

Zed is a calm, bald traveler whom Alex initially mistakes for a martial arts master because of his robes. He is taking a vow of silence and communicates using a small chalkboard, or "chalkpad." He takes a genuine interest in Alex's journey and offers steady support.

Terra is a young woman living in Las Vegas who works at the Domino Grill. She opted out of college, feeling it was too expensive, and lives independently. She crosses paths with Alex during his travels and takes an active, caring role in looking out for him as they spend time together.

Carl Sagan is a small stray dog that Alex adopts and names after his hero, the famous astronomer. He accompanies Alex on his train trip to the Southwest High-Altitude Rocket Festival and provides constant companionship.

Steve is Zed's roommate, an entrepreneurial young man who seeks quick ways to make money, such as selling energy drinks at the rocket festival. He worries about keeping his car clean and can be impatient, offering a more cynical outlook compared to Alex's persistent optimism.

Joseph is Alex and Ronnie's deceased father. Alex knows very little about him but looks him up on Ancestry.com for a school project, prompting curiosity about his life, whereabouts, and history.

Ken is an event organizer for the rocket festival and owns a rocket supply store in Taos, New Mexico. He encourages Alex's interest in rocketry and offers support to the young competitor.

Diane is Ken's wife, an active and understanding woman who provides a welcoming and stable environment for visitors at her home in Taos.

Hannah is Ken and Diane's baby daughter, residing with them in Taos.

Justin is an old friend of Ronnie's. He provides Alex with the used iPod that Alex uses to record his messages for extraterrestrial life.

Donna is Terra's mother, living in Las Vegas. She has a complicated relationship with her daughter.

Nathan is the third roommate living with Zed and Steve in Los Angeles. He participates in social gatherings at the apartment.

Lander is the famous owner of CivSpace, a major aerospace corporation that sponsors the rocket festival. Alex admires him greatly as a visionary in space exploration.

Benji is a peer of Alex's from Colorado. Alex talks about him to Zed while sharing details about his ordinary life.

Noah is a young boy competing at the Southwest High-Altitude Rocket Festival. He launches his rocket successfully with the hands-on help of his father.