David Bowman is the 35-year-old captain of the Discovery spacecraft during the initial phase of its deep-space mission. An experienced astronaut who has spent time in tropical lagoons on Earth, he approaches his work with the careful routine of a generalist. He shares ship management responsibilities and spends his free time reading classical literature and histories of great explorers. Unmarried and detached from his past, he focuses entirely on the present voyage.

Hal is the sixth active member of the Discovery crew, serving as the artificially intelligent brain and nervous system of the ship. Developed during the third computer breakthrough, he communicates through natural spoken language rather than a keyboard. He possesses an independent intelligence capable of passing the Turing test and operates the ship's vital systems. He harbors a classified directive regarding the true nature of the mission that he cannot share with the conscious human crewmembers.

Frank Poole is an astronaut aboard the Discovery, sharing ship duties with Bowman on an alternating schedule. He possesses a careful, conscientious personality suited for performing delicate external ship repairs using the spacecraft's pods. Like Bowman, he is unmarried and lives a strictly regimented life aboard the vessel, occasionally receiving delayed recorded messages from his family back home.

Dr. Heywood Floyd is an experienced astronaut and the chairman of the National Council of Astronautics. He has traveled to Mars and visited the Moon three times before this current assignment. A widower whose wife died ten years earlier, he leaves his three children behind to investigate the Tycho Magnetic Anomaly. He relies on social pleasantries and small talk to evade reporters and manage the international tensions surrounding his classified assignment.

Moon-Watcher is the leader of a tribe of early hominids struggling to survive against starvation and leopard attacks. As a child, he earned his name by reaching up toward the Moon in an attempt to grasp it. His species lacks long-term memory or deep emotional attachments, living strictly in the present moment. After encountering a mysterious crystal monolith, his cognitive abilities begin to change, sparking the earliest glimmers of advanced intelligence.

One-Ear is the leader of the 'Others,' a rival tribe sharing the savannah with Moon-Watcher's group. He leads his tribe in daily, five-minute territorial disputes that traditionally involve aggressive posturing without physical harm. He acts as the primary obstacle to Moon-Watcher's group accessing resources across the river.

Dr. Dimitri Moisewitch is a 55-year-old astronomer working for the USSR Academy of Science. He spends his time building a radio observatory on the far side of the Moon. Despite the ongoing geopolitical tensions between their respective nations, he maintains a close personal friendship with Dr. Floyd.

Ralph Halvorsen is the administrator of the Southern Province on the Moon, living and working within the underground Clavius Base. He manages the day-to-day operations and morale of the scientists stationed there. He prefers the comfort of the base's artificial environments to wearing a spacesuit out on the lunar surface.

Dr. Roy Michaels is the Chief Scientist and a geophysicist stationed on the Moon. He leads the briefing regarding the Tycho Magnetic Anomaly and dates the excavated monolith to three million years in the past. He openly disapproves of the strict secrecy surrounding the discovery.

Nick Miller works for Station Security on Space Station 1. He serves as an escort for high-profile visitors moving through the station's administrative sectors. He becomes easily flustered when sensitive topics are brought up in public areas.

Diana is Ralph Halvorsen's four-year-old daughter, though her rapid growth makes her appear closer to eight years old. Born and raised entirely on the Moon, she represents the first generation of humans adapted to a low-gravity environment. She possesses an unusually delicate bone structure and graceful carriage resulting from her lunar upbringing.

Whitehead is a geophysicist and one of the three crew members kept in hibernation for the ten-month voyage to Saturn. Like the other sleeping astronauts, he underwent specialized training regarding the mission's true purpose. He remains dormant until his specific expertise is required by the active crew.

Dr. Simonson is the Chief Programmer at Mission Control on Earth. He contacts the Discovery using a bandwidth-heavy visual link rather than standard audio to discuss a serious technical issue. He provides remote analysis of computer malfunctions and issues directives regarding ship system management.

Dr. Chandra is the computer scientist responsible for Hal's early instruction. He programmed the artificial intelligence's foundational knowledge. This included teaching the machine to sing simple songs during its initial boot sequences.