The Science Behind Project Hail Mary
When an alien microbe begins feasting on starlight, a frantic, galactic scramble to save humanity ensues in Project Hail Mary (2021), a science-fiction novel by Andy Weir. If you’ve read the book or watched the 2026 film adaptation, starring Ryan Gosling, you’ll know that there’s a lot of real science mixed in with massive flights of fancy—the lifeblood of science fiction. In case you’re scratching your head about some of those big ideas—mass-energy conversions, space travel, and artificial gravity—Encyclopædia Britannica has you covered. This article goes into everything from consequences of a dimming Sun to the counterintuitive science of near-lightspeed travel.
Editor’s note: The following discussion contains spoilers.
What is Project Hail Mary about?
In the story, sunlight reaching Earth begins to weaken as Astrophage, a type of mysterious microorganism that travels through space and lives on or near the surface of stars, begins to blot out a significant amount of light from the Sun. As the dip in solar energy threatens to wipe out all life on Earth, the world’s governments vest sweeping powers in Eva Stratt (played by Sandra Hüller in the movie), tasked with understanding Astrophage and reversing its spread. The survival of life on Earth hinges on sending a space probe named the Hail Mary on a mission to Tau Ceti, the only nearby star seemingly free of Astrophage, in the hope of learning what makes it immune. On board the spaceship are astronauts Olesya Ilyukhina (played by Milana Vayntrub) and Yáo Li-Jie (played by Ken Leung) and the story’s unlikely protagonist, schoolteacher Ryland Grace (played by Gosling). However, Ilyukhina and Yáo do not survive the interstellar journey, and Grace is left to find a solution on his own—until he makes contact with an intelligent spiderlike alien, whom he calls Rocky (voiced by puppeteer James Ortiz). The fate of both their planets hangs in the balance.
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Why is the Sun dimming in Project Hail Mary? And why is it reason to panic?
The dimming of the Sun’s light, caused by the rapid spread of Astrophage, is the major crisis that triggers the events of Project Hail Mary. Astrophage is incredibly efficient at absorbing electromagnetic radiation, which includes visible light. As the Astrophage multiplies across the Sun’s surface, they intercept an increasing fraction of its light, dimming the Sun.
All life on Earth ultimately depends on energy from the Sun in the form of heat and light. It provides plants with light for photosynthesis, maintains Earth’s temperature, and drives weather and climate. Even a modest reduction in the solar energy reaching Earth could lead to significantly lower surface temperatures and spell doom for most species. While it is true that the Sun’s output regularly fluctuates by small amounts—only about 0.1 percent over an 11-year cycle—the novel imagines that Astrophage could block up to 10 percent of the Sun’s total output. That would translate to a 10–15 °C (18–27 °F) drop in Earth’s temperature, according to the book, triggering a brutal ice age. For context, during Earth’s last major ice age (2.6 million to 11,700 years ago), global temperatures were only about 6 °C (10.8 °F) cooler than today, yet glaciers covered large portions of the continents. During the Little Ice Age (roughly 1500–1850), average global temperatures across the Northern Hemisphere dropped only 0.6 °C (1.1 °F). This was enough to cause widespread crop failures and famines.
What is Astrophage? How does it “feed” on light and move around in space?
Astrophage (from ancient Greek, meaning “star eater”) is a microscopic alien life-form that originated on the planet Adrian, as Grace names it. Adrian orbits the star Tau Ceti. Astrophage is similar to mold and algae, in that it is DNA-based, contains a broadly comparable cell structure to life-forms on Earth, and reproduces and spreads to nearby stars in a sporelike manner. Primarily water-based, Astrophage maintains its internal temperature at 96.415 °C (205.547 °F) no matter how hot or cold its surroundings are. It is able to maintain this temperature even though it sits close to the surface of the Sun and travels through space thanks to its special ability (explained by a complex fictional mechanism involving neutrinos) to absorb excess energy from stars and store it as mass. It then converts it back to energy to propel itself through space.
To understand how this might work, it’s important to understand the implications of physicist Albert Einstein’s famous equation E = mc2. Essentially, the equation establishes that mass (m) and energy (E) are two forms of the same physical entity and that they can be converted from one to the other. The fact that the speed of light in a vacuum (c) appears squared in the equation means that even a small mass has a huge amount of energy stored in it, given that c = 3 × 108 meters per second.
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(Read more about Einstein’s theory of special relativity.)
In the real world, even nuclear reactions can convert only a tiny fraction of mass into energy. (Over the past 4.5 billion years, the Sun has only converted 0.03 percent of its mass into energy.) However, Astrophage is extraordinarily efficient at storing nearly all energy as mass and then converting that mass back into energy in the form of infrared radiation. The infrared radiation emitted from the Astrophage carries a certain amount of momentum, and, in keeping with the conservation of momentum, the Astrophage gains momentum in the opposite direction—similar to how a rocket experiences thrust by expelling mass through its exhaust. It may seem weird that light, which has no mass, can have momentum, but this is a direct consequence of Einstein’s theory of special relativity.
Why does Astrophage need to do all this work? Because it uses the energy from a star to travel to the nearest source of carbon dioxide—the planet Venus in the case of our solar system—which it needs to reproduce. It then travels back to the nearest star with its offspring to repeat the process.
How does space travel work in Project Hail Mary?
Within the context of the story, while Astrophage is the biggest threat to life on Earth, it also makes for an extraordinarily powerful and efficient fuel. Chemical fuels produce a limited amount of energy relative to their mass. Nuclear fuels generate more, but still far below the theoretical maximum described by E = mc2. In contrast, Astrophage can release almost all the energy stored in its mass, allowing much more energy to be produced from a relatively small amount of fuel. Conventional fuels would be too inefficient to supply the enormous amounts of energy that interstellar travel demands; the sheer mass of fuel required to make such travel possible would be impractical. So, in the narrative, technology is developed to harness Astrophage to power the Hail Mary’s journey to Tau Ceti. However, the rocket can carry only enough fuel and resources for a one-way trip to the star, effectively making it a suicide mission for Grace.
Here’s the potentially confusing bit: Tau Ceti is about 12 light-years from Earth. However, for Grace, on board the rocket ship, that journey takes only about four years to complete. How does that happen? The answer once again comes from special relativity, which says that the faster an object moves, the slower time passes for it (at least relative to an outside observer). This effect, called time dilation, would be especially pronounced for the Hail Mary, which is moving at more than 90 percent of the speed of light. The reason is that the speed of light remains constant for every observer in the universe, whether they’re on Earth or on the speeding spacecraft. Time and distance adjust to make sure of that; clocks inside the moving spacecraft run slower for an outside observer, and the distance to the destination contracts for those on board. This is one of the concepts that other films, such as Interstellar (2014), also use. In Project Hail Mary, Rocky’s species has not discovered relativity, leading them to overestimate the amount of fuel the trip to Tau Ceti would require.
(Read more about time dilation and length contraction.)
How does the spacecraft create artificial gravity?
Among the subtler physics embedded in Project Hail Mary is how acceleration mimics gravity on the spacecraft. As the spacecraft accelerates and then decelerates at 1.5 times Earth’s gravity (9.8 meters per second squared) toward Tau Ceti, Grace feels the floor of the ship pressing up against him, producing a slightly stronger effect of gravity than that on Earth. This points to another one of Einstein’s early insights on the path to general relativity: the equivalence principle, which states that it is impossible to tell the difference between gravity and uniform acceleration in a small, contained space. This insight helped guide Einstein toward his revolutionary reframing of gravity as a consequence of the curvature of space-time by massive objects rather than a force pulling us toward the center of Earth.
(Read more about Einstein’s theory of general relativity and how he developed the idea.)
The other way gravity is imitated on the ship—so that Grace can use lab instruments designed for Earth’s gravity—is by splitting the ship into two sections connected by cables and spinning them around their common center. This circular motion around the central axis requires centripetal force, provided by the floor of the ship, which pushes Grace toward the center. From Grace’s point of view, he experiences an equal and opposite force outward toward the floor, which feels exactly like gravity.
What is the panspermia hypothesis (did life come from outer space)?
When pondering how life on Earth and on Rocky’s home planet of Erid share some similarities in terms of biology and cellular makeup, one of the theories Grace and Rocky discuss is panspermia, the hypothesis that rudimentary life—or its most primitive ingredients—did not originate on Earth but arrived from space, carried by a meteor or some other celestial body. Some proponents of this idea argue that it may have come from other planets, possibly even Mars, and others suggest that aliens may have been involved. If Earth-based life-forms, Astrophage, and Eridians shared a common ancestor billions of years ago in the fictional world of Project Hail Mary, it would explain how their cells contain similar building blocks, such as the energy-carrying molecule adenosine triphosphate.
Certain discoveries have sparked an interest in this idea. Microorganisms have been shown to survive in Earth’s stratosphere, some of which researchers have claimed are new species. Compounds critical for life, such as water, carbon, and ammonia, are present in space and produced by stars. It should be noted, however, that the discovery of organic molecules is not direct evidence for biological life. While panspermia continues to generate debate among scientists, the evidence for the hypothesis is far from conclusive.
(Read more in Britannica’s article on extraterrestrial life, coauthored by Carl Sagan.)
Despite some simplifications, the physics in Project Hail Mary is perhaps its strongest suit. Its exaggeration of biology, linguistics, and evolution requires much greater suspension of disbelief. In a story about an astronaut schoolteacher trying to solve the puzzle of light-guzzling amoeba, that is perhaps inevitable.