The Discovery journey had begun five years ago as a plan to send a man to Jupiter. With artificially induced human hibernation now shown to be safe, however, the extent of the journey grew—the astronauts were to be sent to Saturn. Though the expedition had begun only thirty days ago, David Bowman felt far, far away from earth and from his home. His own pre-flight experimentation with hibernation seemed but a distant memory. His only sentient traveling companion Frank Poole shared these sentiments, as the two were alone in an incredibly intelligent ship that also contained three hibernating astronauts.

Additionally, the ship contained a sixth crewmember. Hal was a product of the most advanced research in artificial intelligence. His brain had been grown through self-replicating neural networks, in a process extremely close to that with which the human brain developed. He was responsible for maintaining the trip throughout the journey. He could communicate with the crew by speaking with them. Further, only he knew the real purpose of the journey and could execute it alone were anything to happen to the humans on board.

Bowman and Poole's day-to-day activities had been immaculately planned. They were never both asleep at the same time. Food had been carefully prepared for their journey. Further, they received daily news updates and were able to spend time each day learning and relearning scientific material relevant to their journey. Many hours each day were spent checking and rechecking all of the controls and gauges on board to ensure that nothing had malfunctioned in the interim. On board, a rotating carrousel recreated the effect of gravity in one part of the ship. Here, the crew could shave or consume hot drinks without worry of stray hairs getting caught in the machinery or being burned by floating globules of hot coffee. Their days progressed rhythmically and methodically.

The ship passed through the asteroid belt in between Mars and Jupiter with relative ease. At one point they were to cross within 900 miles of an asteroid—the astronauts sent out a probe to collect data on the astronaut and proceeded, uneventfully, toward Jupiter.

Discovery began to approach Jupiter. First, it had to pass by many of Jupiter's planets, gathering information, particularly, as it passed by. During this phase of the journey, Bowman would often listen to a low frequency sound emitted by Jupiter that scientists had discovered nearly a half-century earlier; it amazed him to think that this sound, coming across the radio, had nothing to do with humans or Earth. As they got closer to Jupiter, it seemed that they were going to plunge into it; but the well-charted course they were on actually had them passing several hundred thousand miles away. The astronauts readied to release two probes to gather information from Jupiter. As Discovery passed to the other side of Jupiter, there was no direct line to earth and, as planned, radio contact was temporarily lost. The ship emitted its probed and positioned itself to use Jupiter's gravitational field to gain speed and head toward Saturn.

The first probe burned up almost immediately upon entering Jupiter's atmosphere. The second one made it a bit further along. Back on board, a television displayed the pictures from Jupiter as the probe descended into the atmosphere. It was able to provide only a brief picture though, as the probe eventually collapsed under the immense pressure of the Jovian atmosphere.