2061: Odyssey Three

2061: Odyssey Three is Arthur C. Clarke's third novel in a four part Space Odyssey series. It was preceded by 2010: Odyssey Two and succeeded by 3001: The Final Odyssey. The novel follows the adventures of Heywood Floyd returning from Halley's Comet, and investigating Jupiter's moon Europa.

Plot
Returning from Halley's Comet, Floyd is directed to Europa on a rescue mission using the Universe spacecraft that travels to the aid of Galaxy, trapped on Europa. During the rescue, Mount Zeus is discovered, a diamond mountain on Europa.

Synopsis
2061 is set 60 years after the events of 2001: A Space Odyssey and 51 years after those of 2010: Odyssey Two. At the end of the second novel, the enigmatic aliens who built the Monolith had transformed the planet Jupiter into a mini-sun in order to aid the evolution of life on Jupiter's moon Europa. A message was sent to Earth referring to Jupiter's moons:

ALL THESE WORLDS ARE YOURS—EXCEPT EUROPA. ATTEMPT NO LANDING THERE.

The opening chapters of 2061 gradually explain the events that have taken place in the interim years. The new sun, dubbed 'Lucifer', has transformed the moons of the former Jupiter: Io has become a volcanic hellhole, Europa an ocean world shrouded by clouds, and Ganymede a temperate world that the human race is colonizing. Large-scale interplanetary travel is now commercially viable with muon-catalyzed fusion-powered spacecraft. But humanity is wary of sending spacecraft close to Europa despite its fascinating mysteries, including the appearance of a large mountain, dubbed "Mount Zeus", on its surface. On Earth, a period of relative peace has evolved between the United States, Soviet Union, and China, although a non-violent revolution has taken place in South Africa (now the United States of Southern Africa or USSA); the white population has fled to Europe, taking most of the country's wealth with them and leaving the black population to rebuild the economy, which they do in a matter of weeks thanks to the country's diamond mines.

Dr. Heywood Floyd, the protagonist of 2010, has had an accident and become a permanent resident of an orbital space hospital. His estranged grandson, Chris II, works aboard the spacecraft Galaxy. Now, at the age of 103, Floyd is chosen as one of six "celebrity guests" to travel aboard the privately owned spaceliner Universe for the first-ever human landing on the surface of Halley's Comet as it again nears Earth. The Universe lands on Halley, and the crew explore its surface and its caves.

On Ganymede, Rolf van der Berg, a second-generation Afrikaner refugee, studies data on satellite surveys of Mount Zeus and forms an astonishing thesis about its nature. He communicates his discovery to his uncle Paul Krueger, and van der Berg is also invited to join the crew of the spacecraft Galaxy for its flyby of Europa. As Galaxy nears Europa, Rosie, a stewardess, attempts to hijack the craft, forcing it to crash into Europa's ocean. Failing in her (unexplained) plan, she commits suicide, and the crew of the Galaxy crew are stranded. Observing the burgeoning aquatic life forms of Europa, the crew pilots Galaxy to an island that they name Haven.

Universe abandons its exploration of Halley to rescue Galaxy. After siphoning water from Halley's vents to refuel, an idea conceived by Universe&apos;s navigation officer and advanced by Floyd, Universe heads directly for Europa. During the flight, the celebrity passengers discuss the mystery surrounding Dave Bowman and the monoliths, and whether they would be allowed to land on Europa to rescue the crew of the Galaxy crew. Floyd follows a suggestion that he simply try to call Bowman on the radio, and later has a strange dream in which he sees a small monolith floating at the foot of his bed.

On Europa, Van der Berg and Chris Floyd take the shuttle William Tsung (nicknamed Bill Tee) to study Mount Zeus. Near Mount Zeus, van der Berg relays the message "LUCY IS HERE" to his uncle Paul. It is revealed that Van der Berg's hypothesis, now proven true, was that Mount Zeus is one huge diamond, a remnant from the core of the exploded Jupiter. The revelation about Mount Zeus explains how Van der Berg got onto Galaxy and why Rosie tried to hijack it; both the rulers of the USSA and the exiled Afrikaners are concerned about what might happen to the world diamond market if a vast mountain of diamond was discovered.

Travelling further, the two men find the wreck of the Chinese spacecraft Tsien (which had crashed on Europa in 2010), which has been completely stripped of its metals, and then find the enormous, 3 kilometer long monolith lying on its side at the border between the dayside and nightside, dubbed the "Great Wall". Beneath it is a town of igloo-like dwellings, but the inhabitants are not in sight. There, Chris sees an image of his grandfather, who appears to Chris in the same way that Bowman appeared to Floyd in 2010, and tells him that the Universe is coming.

Universe rescues the crew of the Galaxy; they are brought to Ganymede, where they watch as Mount Zeus, which has been steadily sinking, finally disappears beneath the Europan surface. Krueger writes a follow-up article for Nature, stating that Mount Zeus was a mere fragment of Jupiter's diamond material, and that it is almost certain that many more such large pieces of diamond are currently in orbit around Lucifer. Krueger therefore proposes a scouting project to seek diamond in the Lucifer system, for eventual mining.

Floyd, Chris and van der Berg become close. In a later chapter, another Heywood Floyd, now a disembodied creature of pure consciousness, talks with David Bowman. It is revealed that the small monolith duplicated Floyd's consciousness; there are now two Heywood Floyds, one an immortal being who resides with Bowman and HAL inside the Great Wall, another who will live and die without knowing this. Bowman shows Floyd images of his experience of studying the life forms of Jupiter before they were killed in the creation of Lucifer, explaining that the monolith weighed the Jovians against the Europans and decided the latter held more promise. Bowman states that during the diamond meteor impact, the immensely powerful monolith tipped over on its side, and may be damaged. He and HAL believe that when Lucifer begins to fail, the monolith will weigh the Europans against humanity and they have only about a thousand years to prepare for that moment.

In an epilogue, set in 3001, the original monolith discovered on the Moon in 1999 has been placed in the plaza of the ancient United Nations Building. Humans have found more quantities of diamond from the former Jupiter and have used it to create space elevators and an orbital ring connecting them, as suggested by Krueger. (This idea will later be a central concept in 3001: The Final Odyssey.) Suddenly, Lucifer's light begins to fade and the Monolith reawakens for the first time in four million years.

Outside references to scientific literature
In a late chapter (56, Perturbation Theory), two real scientific articles are explicitly cited by Paul Krueger, van der Berg's uncle, in 2061's otherwise fictional narrative. One article lends plausibility to 2061's basic premise that other Solar System bodies may contain massive quantities of diamond material, and serves as a factual basis for 2061's fictional premise of a large diamond being discovered on Europa. The second article examines the feasibility of various materials for the theoretical construction of a space elevator, identifying diamond as the best available known material, and likewise serves as a factual basis for the space elevators, or towers, which extend from Earth's surface into outer space. The elevators are major pieces of technology in the sequel novel, 3001, and settings of much of the latter novel's action. The intervening implication is that over the next millennium, humanity finds other large sources of diamond in the Lucifer/Jupiter system, precipitate which resulted from the transformation of the latter into the former, and mines this diamond material, using it for previously impossible construction projects.