how did grace have enough astrophage to get to erid after rescuing rocky

Grace had enough astrophage because Rocky didn’t just hand over a tiny emergency top-up—he effectively gave him enough fuel for the long trip, and Grace’s ship was already built for interstellar travel. In the story’s logic, the Hail Mary had the range to get to Erid once it was refueled, especially since the route to Erid was still within the mission’s capabilities.

The key detail is that the fuel question is easy to mix up with the return trip to Earth. Rocky’s refuel was meant to help Grace get back, but the ship’s remaining fuel and the way the travel legs were staged meant Grace could still make the detour to Erid when needed.

Why it works

• The Hail Mary was designed for very high-speed interstellar travel.
• Rocky’s species loaded plenty of astrophage for their own travel needs.
• The story treats the Erid/Tau Ceti/Earth legs as separate legs with different fuel demands, which is why it can feel confusing at first.

In plain English

Grace didn’t “magically” have extra fuel; the ship still had enough astrophage in reserve after Rocky’s help, and the mission’s fuel margins were larger than they first seemed. The confusion mostly comes from how the book and film explain the timing and destination changes.

TL;DR: Grace could get to Erid because the ship still had enough astrophage after Rocky’s refuel, and the interstellar travel setup gave him the margin he needed.