Editor’s Note: Spoilers ahead for Project Hail Mary.
Two science journalists walked into a movie theater. 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.
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 in 30 years wouldn’t be that much different from now.
Carolyn: That’s true. The astrophages work fast! A 10 percent luminosity drop in just a few decades. For reference, the sun’s luminosity has been increasing at a rate of about 10 percent every billion years. But sci-fi likes to speed things up for dramatic effect.
Tina: Well, microbes grow fast. I can imagine that those astrophages were getting unlimited free buffets whenever they reached a new star.
Andy Weir told me that algae and mold were the inspiration for astrophages. He envisioned these microbes soaking up energy from the sun and then using that energy to propel themselves through space.
But they also need another thing: a planet or moon with a carbon dioxide–rich atmosphere so they can breed. For astrophages, that’s Venus — and presumably exoplanets around other stars they’re infecting.
Weir said they’d form moldlike spores to travel between stars.
Carolyn: Can microbes really live in space?
Tina: Maybe. Many Earth organisms can survive in space, usually in some inert state. Moss spores survived in the vacuum of space for nine months. And tardigrades can basically turn to glass and survive just about anything, including outer space.
The point is, those organisms aren’t really living in space. They’re just riding it out in a state of suspended animation. But astrophages are actively living and propelling themselves between the sun and Venus.
Carolyn: These things are really incredible. They can survive on the surface of the sun and on Venus where it is incredibly hot. But they can also live in space where there is no atmosphere and it’s barely above absolute zero. Can any Earth organisms do that?
Tina: There are some single celled organisms, especially archaea and bacteria, that can live in extreme hot or extreme cold, crushing pressure, high radiation, salt, acid — pretty much any nasty condition you can think of.
One type of bacteria can live in temperatures as low as –100° C. And there is an archaeon that can grow at 122° — above the boiling point of water. Of course, that is nowhere near as hot as the surface of the sun or even Venus, which has an atmospheric temperature of 467° C. That is hot enough to melt lead.
And I don’t know of any organisms that could live in both the extreme heat and cold, not to mention survive in a vacuum and Venus’ intense atmospheric pressure and getting blasted by solar radiation. But if any organism can do it, it’s a microbe.
When I spoke with Andy Weir, he said something I love: “Like 99.999 percent of the awesomeness that is life can be found in a single-celled organism. The rest of it is just cells cooperating.”
Carolyn: We should probably discuss xenonite. I saw a review that called it a technobabble material. Like unobtainium in Avatar.
Tina: Yeah, like Star Trek’s dilithium. Or vibranium or adamantium from the Marvel universe.
Carolyn: In this case, it’s a metal or rock made of xenon, which is a noble gas.
Tina: How do you make it solid?
Carolyn: To make a rock, atoms bond together in a 3-D structure. But noble gases don’t like to share electrons or get tied down into crystals. I don’t know how you’d keep it from escaping.
Tina: And how could Rocky’s people make it into something you can build with and change almost instantaneously?
Carolyn: No idea. Apparently scientists have actually crystallized xenon. It has to be cooled to below –111.79° C. Or, by subjecting it to extreme pressures, like 140 gigapascals, you can make it metallic. For reference, that’s similar to the pressure at the boundary of Earth’s mantle and core, or about 1.4 million times the pressure at Earth’s surface.
Tina: The cold is a problem, because Rocky’s planet is very hot. But it does have very high atmospheric pressure, 29 times Earth’s atmospheric pressure, according to Weir.
Carolyn: Maybe Rocky’s people have come up with a way to create these extreme conditions in a controlled portable diamond anvil cell.
Tina: They didn’t really get into this in the movie, which was probably the right choice. But Weir wanted his alien buddy to be better at something than humans. So Rocky and his species can do complex math in their heads, have perfect memory and are just incredibly good at materials science.
It was so fun going to the movie with you and nerding out about it. I really did enjoy the movie.
Carolyn: I did too. I loved it. I loved Ryan Gosling. And despite what it sounds like, it wasn’t like I was unable to suspend my disbelief! I enjoyed everything. It’s just that for me, it’s more fun to think about these things than annoying or aggravating.
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