Project Hail Mary
by Andy Weir
The plot is delivered both through Ryland Grace's flashbacks to the events leading up to the launch of the Hail Mary spacecraft, and the present events on board the Hail Mary. The two perspectives are often intercut within the story, swapping between the current ongoing events and his previous memories as they gradually return.
Before launch
Ryland Grace is a devoted junior-high school science teacher in the United States, having left his former career as a molecular biologist. He is recruited by Eva Stratt, head of a UN task force, to find the cause of a solar dimming event that could end humanity.
A probe sent to Venus discovers an alien single-celled organism reproducing uncontrollably around the Sun, absorbing crucial amounts of solar energy, which will cause a new ice age within 30 years. Grace names the microbe "Astrophage," Greek for "star eater." Grace soon discovers it uses carbon dioxide to reproduce, and emits extraordinary amounts of light energy to migrate between Venus and the Sun. Other scientists quickly conceive of using it as a high-output fuel source.
Eva Stratt is given absolute authority to procure materials for a project to bring the Astrophage under control. Astronomers discover a nearby star, Tau Ceti, has resisted Astrophage infection, while the surrounding stars have not. Stratt spearheads the development of a starship, the Hail Mary, to travel to Tau Ceti in order to study and figure out why it has been unaffected in the hope its discoveries will help Earth find a solution to deal with Astrophage before humanity goes extinct. If a solution is found, four probes called "Beetles" will be sent back to Earth, but Hail Mary cannot return. Despite the massive amounts of energy produced by astrophage, the limited time Earth has to build the ship and fly to Tau Ceti before facing environmental collapse means there is only time to cultivate enough Astrophage to fuel the vessel for a one-way trip, making it a suicide mission.
To spare food and water, and preserve the crew's sanity, the crew will be placed in a 4-year-long coma aboard the ship. Genetic research finds a gene that improves the likelihood of surviving the coma. Grace finds out he has this genetic marker during routine testing. Two crews of three volunteers, each with the genetic marker, are chosen for the Hail Mary mission. The primary crew consists of Chinese Commander Yáo Li-Jie, Russian engineer Olesya Ilyukhina, and American scientist Martin DuBois. Grace is tasked to train DuBois and his backup for the Hail Mary mission.
However, nine days before launch, an explosion at the Baikonur Cosmodrome kills both DuBois and his backup, during an experiment on Astrophage. A replacement cannot be found and trained in time for the launch, forcing Stratt to ask Grace to become the third crew member. Grace refuses and threatens to sabotage the mission if he is forced to go, but Stratt, knowing he is their best remaining option by a long way, detains him until the departure, saying that she will have him sedated before the launch and placed directly into the coma for the trip. To avoid him revealing his unwilling presence on the ship to his crewmates upon waking up and causing distrust or tension, she plans to program the ship's caretaker computer to issue him an amnesia-inducing drug before reviving him at their destination. Stratt counts on the assertion that Grace is a "fundamentally good person" and that by the time he remembers what Stratt has done, he will have become involved with the mission and feel compelled to see it through.
Aboard Hail Mary
Ryland Grace wakes up with no memory of his name or where he is. He finds himself in a starship, and finds the two other crew members, Yáo and Ilyukhina, long since dead. His cognition slowly returns after a few days. After gaining access to the ship's control room and airlock, he gives the other crew members a space burial, and tries to remember where he is, and why he's here.
He finds out his ship, the Hail Mary, is in a stable orbit around Tau Ceti. Soon after this, the ship detects another starship close by which is of extraterrestrial origin. The other ship, which the radar and later Grace calls Blip-A, sends over a physical star chart model to communicate to Grace that it comes from 40 Eridani for the same reason as the Hail Mary: its star is infected by Astrophage. Blip-A connects to Hail Mary, and the single crew member aboard Blip-A comes to meet Grace. Since the alien came from the Eridani star system, Grace names the new species an "Eridian."
The alien resembles a dog-sized five-legged spider with a stone-like carapace. It lives in an atmosphere 29 times denser than Earth's, composed mostly of superheated ammonia. Eridians never evolved eyes, using echolocation as their primary sense. Grace names the alien "Rocky".
Rocky communicates in tones, and Grace sets up a computer program to translate Rocky's speech to English. Like Grace, Rocky is the sole survivor of his crew, and has been alone for the last 40 years. He is also able to learn about Eridian biology, including their nearly 700-year lifespan and unique processes by which they eat and sleep. He is a genius-level engineer, but lacks Grace's scientific knowledge to do the research needed. Rocky constructs enclosures with suitable Eridian living conditions aboard the Hail Mary so they can work together to solve the Astrophage problem.
Because of the nature of their environment and evolution, Eridian technology and science is slightly less advanced than human technology. Although the Eridians have built a space elevator, they have never before left their home planet's orbit, with their lack of vision meaning space lacks any sort of sensory input to them. With the Blip-A being their first venture outside their atmosphere, the strong magnetic field of their home planet has also shielded them from any radiation. They do not know about radiation sickness, which killed the rest of Blip-A’s crew, or time dilation due to Special relativity. Because of this, the Blip A had enough fuel for 30 [[light-year]]s of travel, but would have perceived less than 10 years of relative flight. Rocky assures Grace that he can give the Hail Mary some of his excess fuel, allowing Grace to return to Earth after the mission.
They find out Tau Ceti's third planet contains Astrophage, but the population is controlled by a natural predator, a microbe which Grace names "Taumoeba." After a dangerous low orbit maneuver, they recover a sample of Taumoeba, but find out it cannot survive any exposure to nitrogen. As both Venus and the Astrophage breeding planet in Rocky's system have nitrogen-rich atmospheres, they have to breed a nitrogen-resistant strain of Taumoeba. They succeed, but the initial Taumoeba sample escapes and infects the Hail Mary, consuming its remaining Astrophage fuel.
After disinfecting and refueling the Hail Mary with Astrophage from Blip-A, the two part ways, knowing that they won't meet or communicate ever again. However, Grace soon discovers that the nitrogen-resistant Taumoeba has inadvertently also developed the ability to pass through the material the Blip-A is made from. It will get into the fuel tanks, consume the Astrophage, and cripple the Blip-A, dooming Rocky's civilization. Grace sends out the Beetle probes with the mission's findings and live-samples of Taumoeba, then reverses course to catch up to and rescue Rocky, and take him back to his planet so he can save his people.
On their journey to 40 Eridani, Grace resigns to the fact that he will starve to death after they reach Rocky's homeworld, because Eridian food is toxic to humans and he would not have enough food left for a return journey to Earth. Rocky points out that Grace could eat Taumoeba, until his people find out a way to produce human-friendly food. They continue on their journey and the book skips ahead to years later, showing Grace on Rocky's planet, living in a dome with Earth Like conditions. The planet's higher gravity has meant he has aged and weakened at a faster rate, but he is nonetheless happy. He receives news from Rocky that Earth's Sun is no longer visibly infected with Astrophage, indicating that the Hail Mary’s mission was a success. With the repaired and refueled Hail Mary in orbit, Grace has the option to return to Earth, but delays the decision to another day. The book ends with Grace teaching science to a group of young Eridians, leaving it unclear whether he eventually decides to return to Earth or live out his days with the Eridians.