Project Hail Mary

Grace and Rocky trace the Astrophage threat to its source: a planet they nickname Adrian, which hosts a natural predator microbe they call Taumoeba that keeps Astrophage populations in check by consuming it. After a dangerous sample-collection mission using a ten-kilometer xenonite chain (during which a hull breach nearly kills them both, and each saves the other), they isolate the Taumoeba and begin selectively breeding a strain hardy enough to survive both Venus's environment and the Astrophage breeding grounds near 40 Eridani, so it can be deployed to save both Earth and Rocky's homeworld, Erid.

The plan nearly falls apart when stray Taumoeba get into the Hail Mary's fuel lines and devour most of its remaining Astrophage; Grace and Rocky have to jettison the contaminated tank and sterilize the ship with nitrogen to kill the rest. After repairing and refueling the ship together, Rocky prepares to head home. But on the return trip, Grace realizes that his breeding program has inadvertently produced a Taumoeba strain capable of eating through xenonite — the material Rocky's entire ship, Blip-A, is built from. If he simply proceeds home, the cure he's carrying to save Earth's sun will, if it ever reaches Rocky's people, consume their ship and doom Rocky.

Grace is faced with an impossible choice: return to Earth as the mission's celebrated hero, or divert to save Rocky and the Eridians, stranding himself in the Tau Ceti system for good, since Erid's food is toxic to humans. He chooses Rocky. He launches the unmanned "beetle" probes back toward Earth loaded with Taumoeba samples and full instructions for saving the sun, ensuring humanity's survival even though he himself will not return. He then finds and rescues Rocky's ship before its fuel runs out, and Rocky, far from being upset by the change of plans, is overjoyed to have his friend back and points out that Grace could theoretically survive on Erid by eating Taumoeba himself.

The novel closes sixteen years later. Grace is living permanently on Erid, sustained by synthetic food the Eridians engineered for him using the archive of human knowledge he brought along. Rocky confirms that back on Earth, the sun has returned to its normal brightness — proof that the beetles arrived safely and humanity was saved. Knowing Earth is safe, Grace briefly considers returning home, but instead we leave him settled into his new life, working contentedly as a science teacher for young Eridian students.