Ryland Grace, the protagonist of the movie "Project Hail Mary," wakes up on a spacecraft with no memory and belatedly learns that he has been tasked with traveling to the star system outside the solar system, Tau Ceti, to stop a mysterious space microbe that threatens the sun.
Project Hail Mary lays out several scientific concepts at once, including microbes that dim the sun, spacecraft propulsion using neutrinos, long-distance spaceflight, and artificial gravity in a rotating spacecraft. Among these, the setup in which the sun's brightness plunges over a short period and a microbe that feeds on a star's energy are far from reality, but the idea of generating artificial gravity through a rotating spacecraft structure and setting the story in the real star Tau Ceti can be explained by physics and astronomy.
◇ How much of the sun-eating microbe is fiction
The crisis in the story begins with the premise that the sun's luminosity drops 10% over about 30 years. In the movie, a space microbe called "Astrophage" that absorbs solar energy infects stars and triggers an ice age for planets, including Earth.
However, NASA's solar radiation data show that the real sun does not change so abruptly. The total solar irradiance (TSI), the total amount of solar energy reaching Earth, varies by only about 0.1% over the sun's 11-year activity cycle. Past ice ages on Earth are understood to have arisen not because the sun's luminosity suddenly weakened, but because Earth's axial tilt or orbit changed. The movie compresses the sun's long-term changes into a disaster within a human generation.
Astrophage, the key device in the story, absorbs solar energy to dim stars and stores that energy to use for propulsion. Jacqueline McCleary, a professor at Northeastern University in the United States, said, "That is similar to how real microbes absorb sunlight and use it for energy," but noted, "There is an orders-of-magnitude gap between the energy a microbe can store and the energy the sun actually emits."
She added, "The sun emits on the order of 10 to the 26th joules of energy per second, and the solar atmospheric environment can rise to about 2.78 million degrees," and, "There are extremophiles on Earth that withstand high temperatures, high pressures, and strong radiation, but a life-form that absorbs enough energy to affect a star's energy flow itself is a completely different order of problem."
The premise that Astrophage converts stored energy into neutrinos for propulsion is also far from reality. Neutrinos do exist, but they interact so little with other matter that they are called "ghost particles." Enormous numbers of neutrinos pass through Earth and the human body every second with virtually nothing happening. With today's physics, the technology to capture and store large quantities of neutrinos and emit them precisely in a desired direction to use like a spacecraft engine is closer to imagination.
◇ Not just imagination: elements that touch real science
By contrast, the artificial gravity of the rotating spacecraft where Grace stays is grounded in real physics. NASA explains that within a rotating structure, one can create an environment similar to gravity by using the effect of acceleration. If part of a spacecraft is rotated, crew members feel a force pressing them against the outer wall, and that effect can act similarly to gravity.
However, if the rotation speed is limited to about 4 rpm (revolutions per minute), a radius of about 56 meters is needed to generate the magnitude of gravity we feel on Earth's surface. In other words, while a rotating spacecraft is physically feasible as a concept, actual implementation would require a very large structure and precise control technology.
Tau Ceti, presented as Grace's destination, is a real sun-like star. And the planet "Adrian" orbiting Tau Ceti is modeled on Tau Ceti e, which was identified in 2017 as a candidate for a super-Earth exoplanet. But according to additional research over the past two years, many of the signals for Tau Ceti e likely arose from errors in data analysis or from surface activity on Tau Ceti.
Meanwhile, some see Project Hail Mary as revealing the realistic limits of astrobiology. Current searches for extraterrestrial life effectively use only life on Earth as a standard. So they first look for familiar traits such as cellular structures, carbon-based substances, deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and liquid water. If there are beings that use a star's energy like Astrophage or live in ways we have never seen, our current search methods could miss them.
That is why NASA is conducting research to find "agnostic biosignatures" such as complex molecular arrangements, hard-to-explain chemical imbalances, and unusual energy flows without assuming specific biochemistry. It is also searching for "technosignatures," such as radio signals or artificial pollutants, and plans to send a probe to Titan, a moon of Saturn, in 2028 to see whether prebiotic chemistry can form where methane and ethane flow instead of liquid water.