Part One At an unspecified point in the near future, five astronauts, commander Tom Kirby, engineer Yvan Grigorev, medic John Pearson, geologist Zoë Lessard, and exobiologist Nina Sulman depart Earth orbit on a mile-long nuclear-powered spaceship,
Pegasus. Six weeks later,
Pegasus arrives in orbit around their first destination, Venus, where Lessard and Grigorev pilot the lander
Orpheus to the surface, where Grigorev conducts a short walk in a specially designed reinforced pressure suit while Lessard remains inside the lander. During the walk, Grigorev visits the nearby derelict Soviet lander
Venera 14, which had arrived on Venus in 1982. He falls behind schedule, and, suffering from exhaustion and overheating, only narrowly makes it back to
Orpheus. After successfully launching and docking with
Pegasus, the crew depart Venus for Mars. After arriving in Mars orbit, the crew rendezvous with a pre-placed supply ship to refuel
Pegasus. Kirby, Pearson, and Sulman descend to the surface of Mars in the lander
Ares, landing near
Melas Chasma, a part of the
Valles Marineris canyon system, where they intend to search for liquid water buried beneath the surface at the bottom of the canyon using a robotic rover (named
Charlie) carried by a balloon. Upon landing, Kirby is hit by a
dust devil, but he is not injured. Their first attempt at finding water is thwarted by a solar flare, which forces the astronauts on the surface to take shelter in
Ares for a few days, while Lessard and Grigorev in orbit onboard
Pegasus take shelter too. Once the danger has passed, the crew of
Ares return to the edge of Melas Chasma, where Sulman successfully guides
Charlie to the bottom of the canyon, successfully finding a small amount of liquid water buried under the ground. However, Lessard reports that a large dust storm is developing in the canyon, which threatens to ruin their mission. After retrieving the water sample, the astronauts once again take shelter in Ares. Once the dust storm has passed,
Ares lifts off and rejoins
Pegasus. Departing Mars, the next destination of
Pegasus is
Jupiter, which they will reach by swinging near by the
Sun to pick up speed. On the way towards the Sun,
Pegasus passes by
Mercury, however mission constraints prevent landing there. To protect them from the intense radiation of the Sun, an artificial magnetic field is generated around
Pegasus, but the power requirements of the generator force them to turn off non-essential systems, such as their laboratory and the centrifuge that provides artificial gravity for their sleep compartments. The flyby is a success, and the crew head to Jupiter. On their way through the asteroid belt, the crew perform an unplanned close flyby of a binary asteroid system, which passes much closer than predicted owing to discrepancies in the data provided by mission control.
Part Two Pegasus performs an intense aerobraking manoeuvre in Jupiter's upper atmosphere to put them on course for
Io. En route to Io, Pearson receives a diagnosis of lymphoma, which he received from a high radiation dose during the solar flyby. Upon arriving in orbit over Io, the primary landing site is considered unsafe due to an ongoing volcanic eruption, as is the secondary landing site due to lethal levels of radiation. After a heated discussion at mission control over whether to land at the still potentially risky tertiary landing site, or to land at the safe but uninteresting quaternary landing site, the decision is made to land at the more interesting site, but the mission duration is cut to reduce radiation exposure. Lessard then descends alone to Io in the lander
Hermes. On the surface, she quickly becomes exhausted in her heavy radiation-shielded spacesuit. With the radiation levels climbing, Lessard is ordered by the crew to abort her walk. Returning to
Hermes, she leaves all the samples she had collected behind.
Pegasus then encounters
Europa, where they send a robotic probe to the surface to collect sub-surface ice samples. Their mission around Jupiter complete,
Pegasus embarks on the long journey to
Saturn. However, Pearson's condition continues to worsen, weakening him severely. Upon arriving at Saturn,
Pegasus enters orbit around its largest moon,
Titan, where they release a robotic probe to collect samples, however the probe malfunctions and is lost. Leaving Titan,
Pegasus is placed in an orbit in the
Cassini division, where Sulman performs a spacewalk to collect a fragment of Saturn's rings. However, during her spacewalk, Pearson dies. Subsequently, the remaining crew cut off communications with Earth for a whole day while they mourn Pearson and decide what to do next. During this period, Kirby sets adrift the body of Pearson into the rings of Saturn. Upon restoring contact, the crew reveals that they have decided to continue the mission (in the US version, they decide to return home), and they leave for Pluto shortly afterwards. Arriving at
Pluto, Kirby and Grigorev land on the surface in the lander
Clyde, where they set up a telescope array to detect
exoplanets, which they had modified on their journey from Saturn to Pluto, and the telescope is pointed at Earth for a calibration test. Before leaving Pluto, Kirby and Grigorev perform a short memorial service for astronauts who have died in the pursuit of space exploration, including Pearson, who was intended to land on Pluto with them.
Clyde later ascends and docks with
Pegasus. On the way back to Earth, they rendezvous with a fictional long-period
comet, Yano-Moore. Lessard and Sulman pilot the lander
Messier to collect samples, with Kirby, Grigorev and mission control observing increasing seismic readings from within the comet. Sulman and Lessard continue their excursion, but the nucleus of the comet suddenly breaks apart. Sulman and Lessard liftoff aboard
Messier to return to
Pegasus, which itself is struck by fragments of debris of the destructing comet and severely damages the spacecraft. Kirby tends to an oxygen leak while Grigorev is injured by comet debris. Unable to contact the damaged
Pegasus, Lessard and Sulman abandon
Messier and spacewalk to the airlock. After performing emergency surgery on Grigorev to remove the comet fragment from his lung, the three able astronauts repair
Pegasus and commence on the final journey home. Finally, after six years away,
Pegasus and her crew arrive home to Earth. == Cast ==