How Long Was Rocky In The Tau Ceti System In Project Hail Mary?

When Ryland Grace (Ryan Gosling) reaches the Tau Ceti system in "Project Hail Mary," we learn that he's been on a multi-year journey. According to the "Project Hail Mary" timeline, Grace has been en route for three years, while 13 years have passed back on Earth due to time dilation. One thing we don't find out, though, is how long Rocky has been in the area. The Eridian alien just shows up, and no one says a word — at least in the movie.

In the book, though, author Andy Weir reveals that Rocky has been hanging around the Tau Ceti star system for a whopping 46 years trying to solve the Astrophage problem. As Grace says in the book, "He's been stuck in this system for longer than I've been alive."

Even worse? Rocky's been alone all that time. This is another one of the more confusing parts of the "Project Hail Mary" movie, but again, the book comes to the rescue. Weir explains through a detailed conversation between the two interplanetary astronauts how Rocky was one of 23 Eridians sent on their ship, which Grace calls the Blip-A. However, the entire crew is unknowingly exposed to radiation, and they all get sick and die along the way. 

The only exception? Rocky, who is safe from the radiation because he is the ship's engineer and is working in an insulated space in the back of the xenonite spacecraft, where he's protected by the Astrophage tanks. That's good news for the survival of both Erid and Earth, but Rocky finds himself living, eating, and even resting alone (an Eridian's worst nightmare, thanks to the unique way they sleep) as he spends decades searching the Tau Ceti system for answers before Grace arrives.

If Rocky's been hanging around the Tau Ceti system for nearly half a decade before Grace shows up, it begs the question: how old is the mineral-based sentient life form in the first place? For the answer, we once again turn to Andy Weir's text, where Rocky says he's been alive for 291 Earth years. Grace notes, "Rocky is older than the United States. He was born around the same time as George Washington."

Again, the natural follow-up here: Is Rocky old for his race? When Grace asks how long Eridians live, the laconic answer comes back: "Average is six hundred eighty-nine years." As Grace reflects after the conversation, "[Rocky is] not even that old for his species. There are old Eridians out there who were alive when Columbus discovered (a bunch of people already living in) North America." We get even more context when Rocky explains that he's been with his mate, Adrian (or at least, that's Grace's nickname for his mate after Rocky sings their musical name to him), for nearly two centuries.

The sheer difference in life spans from those of short-lived humans is staggering. Eridians simply don't have the same pressure to squeeze every drop of life out of each passing day. They have as many centuries as most humans have decades to live.

Rocky doesn't just have 10 times the lifespan of Grace. His planet has a lot more time to figure out the Astrophage problem compared to Earth. Andy Weir breaks down the numbers in the book: Rocky and Erid have roughly 72 years before they'll be in race-exterminating trouble as the Astrophage consumes their star. By that time, the book explains, "Earth will be a frozen wasteland and 99 percent of the human population will be dead."

In fact, Earth's time crunch is much shorter than Erid's. In less than a fifth of those 72 years, it's going to go belly up. The reason for the difference? Heat. Rocky's home planet (which happens to be in the same region as that of an iconic Star Trek alien) is big and hot. As Grace explains in the published text, "Erid is much hotter than Earth ... and Erid is much larger with a much thicker atmosphere. So Erid has a whole lot more heat stored in its air. Earth is getting cold fast. Very fast. In fourteen more years, most humans will be dead."

The time differences aren't highlighted in the film adaptation, but you don't need to know those details to understand that Rocky and Grace need to work as fast as possible to find a solution. Yet when you step back and look at the two races working side by side, one is actually on a much stricter timeline than the other. The real tragedy, though, is Rocky's isolated half-century floating around Tau Ceti. We're just glad he finally gets a friend again who can watch him while he sleeps.