What happens at the end of Project Hail Mary?

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Final Outcomes

1. Ryland Grace succeeds in creating Taumoeba, a microbe that preys on Astrophage, the organism draining stars—this solution ends up being sent back toward Earth via automated probes that travel while Grace turns his attention elsewhere.

2. Just before Ryland can return home, a fatal leak appears: Taumoeba organisms are eating through xenonite, the material Rocky’s ship (the Blip-A) is built from. Left unchecked, the contamination would kill Rocky and doom his home system. Ryland faces a hellish choice: return to Earth and abandon Rocky, or save Rocky but likely never make it back himself. He chooses to save Rocky.

3. Ryland rescues Rocky and goes to Erid (Rocky’s planet), where he settles in. He lives in a specially-built, climate-controlled habitat to survive the local environment. He becomes an educator, teaching Eridian youth—he finds a sense of home and purpose there.

4. Earth is ultimately saved. The Taumoeba-based cure works: Sol returns to full brightness, meaning Astrophage has been eradicated or suppressed. Humanity is given a second chance.

5. Will Ryland ever return to Earth? At the end, Rocky offers to help build a ship so he could go back. But Ryland pauses; he acknowledges that Erid has become home and doesn’t commit. The ending is hopeful, open for possibility, but he stays for now.

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If you want any differences between the book vs the movie, I can break those down too.