How realistic is Project Hail Mary?

How realistic is Project Hail Mary? 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That’s us: molecular biology reporter Tina Hesman Saey and Carolyn Gramling, who writes about climate and earth science.  We braved the aftermath of a big snowstorm in Washington, D.C., to get a sneak peek of Project Hail Mary, a film adaptation of a book by science fiction author Andy Weir. The film follows middle school science teacher Ryland Grace, played by Ryan Gosling, who wakes up on a spaceship light-years from Earth. He’s alone — two other astronauts died in transit — and can’t remember who he is or why he’s there. As his memory returns, he learns that he is on a one-way mission to save his home planet. Sign up for our newsletter We summarize the week's scientific breakthroughs every Thursday. Spacefaring microbes called astrophages appear to be eating the sun and many other nearby stars. Except one. Tau Ceti is at the center of the infected stellar cluster, but it is not dimming. Grace must figure out why this star is still healthy and send a solution back to Earth. Luckily, he doesn’t have to do it alone. Grace meets Rocky, an alien whose planet is threatened by the same menace. The movie is action-packed, funny and touching. We walked out full of questions for each other about the movie’s science chops. The conversation has been edited for clarity and length. Tina: The first thing we have to talk about is the whole reason for this mission. Astrophages are these fictional microorganisms that are basically eating the sun. At least that is what their name means. Carolyn: I was puzzled about what exactly the astrophages were doing to the sun. From what I understand, they’re reducing its luminosity. Tina: Yes. According to scientists in the movie, the sun would dim enough over 30 years to drop Earth’s temperature by 10 to 15 degrees. They say that would be enough to put us into an ice age. In the book, the sun is dimming by 10 percent in that 30-year period. Is that what caused past ice ages? Carolyn: So… yes and no. How much radiation the sun emits — its luminosity — has changed over time. But past ice ages are more related to other natural forces, including variations in Earth’s tilt and the shape of its orbit. Sometimes Earth is farther from the sun, or not tilted toward it as much in summertime, and so the planet’s average temperature is colder. During the last glacial maximum 20,000 years ago, Earth was maybe 10 degrees Celsius colder than now. The sun was still putting out the same amount of radiation, but less sunlight reached Earth’s surface. Tina: OK, so what would a 10 percent reduction in luminosity do to Earth’s climate? Carolyn: It would be colder, for sure. Billions of years ago, the sun was maybe 25 percent as dim as it is now. But there’s evidence that Earth wasn’t as cold as people thought it would be under those conditions. There was liquid water on the surface. And that could be because there was a high concentration of greenhouse gases in the atmosphere. So it’s hard to know how much colder Earth would have been just from a dimmer sun. Because we don’t know the other possible mitigating conditions. Tina: Although the atmosphere