On the scientific accuracy of the book “Project Hail Mary”

Since its publication in 2021, Andy Weir’s “Project Hail Mary” has become a defining example of modern science fiction. What makes this book so unique is the central role that scientific reasoning plays in the story as Weir takes us through the thought process of the main protagonist, Dr. Ryland Grace, as he uncovers his mission, identity and ultimately works out how to save humanity from an existential threat. From simple tabletop experiments to particle physics, scientific ideas drive almost every major development in the novel. 

The film adaptation released this year has brought Weir’s story back into the spotlight, and the question remains: How believable is the narrative? Several of its plot elements are grounded in established laws of nature, while some are a leap, pushing on currently-open questions in physics. 

Artificial Gravity on the Hail Mary

One of the many obstacles to sending humans on a multiyear interstellar mission is the microgravity environment, in which astronauts and objects appear nearly weightless. Gravity influences humans’ musculoskeletal, cardiovascular and nervous systems – real life astronauts on board the International Space Station (ISS)  exercise rigorously to combat the effects of microgravity, and missions are generally restricted to only six months at a time. 

On the Hail Mary, Grace’s team stayed in an induced coma for three years during their trip to the Tau Ceti system. While unconscious — and during any attempt to save humanity — prolonged exposure to microgravity would pose serious physiological challenges to the team, leading Weir to equip the spacecraft with artificial gravity. In lieu of the unexplained artificial gravity generators seen in popular science fiction shows like Star Wars, Weir instead equips the Hail Mary with two plausible methods of producing artificial gravity: continuous acceleration and a rotating centrifuge. 

Dr. Grace first awakens during the journey towards Tau Ceti. Having suffered from amnesia, he attempts to determine his surroundings by performing a series of experiments including timing a test tube falling to the floor, and measuring the time period of a simple pendulum. He is shocked to find that the acceleration due to gravity is actually 14.7 ms-2, greater than Earth’s 9.8 ms-2, leading to his conclusion that he is aboard a spacecraft that has artificial gravity. At this stage of the journey, artificial gravity is produced by the Hail Mary’s continuous acceleration: as the spacecraft accelerates, the floor exerts a force on Grace that accelerates him together with the ship, producing the same sensation as weight. According to Einstein’s equivalence principle, a uniformly accelerating frame of reference is indistinguishable from a gravitational field, explaining why Grace’s experiments measure an apparent gravitational acceleration despite the absence of a true gravitational field. 

Later in the novel, when the Hail Mary is no longer under continuous acceleration, Weir instead has it produce artificial gravity by rotating like a large centrifuge. The rotational motion causes an apparent centrifugal force that mimics gravity, pressing Grace against the floor. For a comfortable artificial-gravity environment, the ship should rotate to produce a large enough force, but at a low enough rate so as to not cause dizziness or motion sickness. Rather than constructing an enormous, rigid spacecraft that would have been extremely difficult to build, especially while humanity was in a rush to save Earth, Weir has the Hail Mary separated into two sections connected by approximately 100m of Zylon cable, with the entire structure rotating about its centre of mass. This reduces the rotation radius, but means that the spacecraft needs to rotate at around 4 rpm to generate an artificial gravity comparable to Earth’s. While recommended rotation rates are below 3 rpm, studies have shown that humans can adapt to environments rotating at speeds up to 10 rpm. Weir’s design for artificial gravity on the Hail Mary is a credible engineering compromise, balancing physiological constraints with the practical challenges of constructing interstellar spacecraft. 

Astrophage: biology built on particle physics 

The Astrophage is arguably one of Weir’s more imaginative creations in his book. Unlike many fictional alien organisms, its biology is built around one of the biggest open questions in modern particle physics: the nature of neutrinos. Neutrinos are lightweight, neutral particles that hardly interact with matter. 

The first indication of the Astrophage is seen in the mysterious Petrova line: a faint arc of infrared light of a specific 25.984 micron wavelength bending to form an ellipse from the north pole of the Sun and down to Venus. The danger of the Petrova line is revealed to us very early on, when scientists discover that it is causing enormous energy losses in the Sun. 

To explain this precise infrared signature, Weir depicts the Astrophage as microscopic particle accelerators with the ability to convert between energy and matter. Specifically, Weir proposes that excess kinetic energy from proton-proton collisions is converted by the Astrophage into neutrinos. At this point, Weir makes the theoretical leap that neutrinos are majorana particles, meaning that each neutrino is its own antiparticle, allowing neutrinos to annihilate and produce photons at the Petrova wavelength. Whether neutrinos are actually majorana particles is an open question being investigated in experiments around the world, such as LEGEND-200 in Italy, that search for a signal from neutrinoless double beta decay — a theorised interaction that could only happen if neutrinos are majorana. 

However, neutrinos being majorana is not necessary for the reaction Weir describes. Since the proton collision begins with no leptons, an important law in physics — conservation of lepton number – requires that the neutrinos produced must be neutrino-antineutrino pairs. Neutrinos being majorana would only make the internal process more efficient, since this would increase the number of possible annihilation partners surrounding each neutrino inside the astrophage. 

The largest scientific hurdle is how the astrophage manages to contain the neutrinos, such that they annihilate within their bodies. Neutrinos interact so rarely with ordinary matter that billions pass through every square centimeter of Earth with no deflection or interaction. An Astrophage-sized organism would have no hope of retaining them long enough to annihilate. Weir resolves this by introducing “super cross sectionality,” meaning that the rate at which the neutrinos interact with the Astrophage’s body is very high, so the Astrophage can keep the neutrinos inside until they annihilate. Here, Weir steps in completely new territory, as no known mechanism can produce a boundary that is opaque to neutrinos. 

Could a species like the Eridians exist? 

From the tall, human-like inhabitants of the Moon in Francis Godwin’s The Man in the Moone (1683) to the familiar “little green men” of 20th-century science fiction, fictional aliens possessing intelligence comparable to our own have often reflected human physiology. However, as astronomers have begun discovering planets vastly different from Earth, authors increasingly imagine species shaped by different environments. Rocky, Weir’s alien from the fictional planet Erid, is a distinctive example of this approach. 

Rocky’s appearance initially seems singular and bizarre: a spider-like, symmetrical body with five limbs arranged around a pentagonal torso, no eyes and communication through musical chords. He is a far cry from what we typically expect of an intelligent, alien species capable of building advanced technology like spacecraft. Yet, many of these characteristics can be understood as adaptations to Erid’s environment, rather than some arbitrary genetic makeup. 

Erid’s ammonia-rich atmosphere is approximately 29 times denser than Earth’s. At such high pressures, Erid would constantly be under an extremely dense fog that greatly reduces visibility, making vision far less useful than it is on Earth. By contrast, echolocation becomes increasingly effective since sound can travel efficiently through dense clouds and gases. Rocky’s reliance on sound to communicate and gauge his surroundings reflects a natural adaptation to Erid’s environment. 

Rocky survives in a mainly ammonia-filled atmosphere, making his and Grace’s natural environments completely incompatible with each other. There have been speculations about ammonia-based life forms, which would undergo similar metabolic processes as humans, using liquid ammonia as a solvent rather than water. Erid is too hot for ammonia to remain a liquid, but it can sustain liquid water. Grace discovers that Eridians, like us, use water as a solvent in their bodies despite breathing in ammonia-rich air. Weir combines an exotic atmosphere with water to keep Rocky’s physiology closer to established biochemistry. 

Rocky’s five fold rotational symmetry and lack of five fingers on each limb appears to be another major obstacle to technological advancement. Humans rely on our bipedal nature, which frees us to use our arms and hands to hold and manipulate objects, while opposable thumbs allow us to perform delicate tasks ranging from writing to handling precise scientific equipment. Rocky’s physiology, however, demonstrates his species to be capable of major advancements: His five limbs end in three digits capable of fine manipulation, allowing him to construct instruments, repair systems and use equipment.

Through Rocky, Weir portrays a uniquely exotic yet realistic alien, with each of its features reflecting the demands of life on Erid. 

Could humanity have been saved in time? 

Humanity’s quest to stop the Astrophage is ultimately a race against time. Early in the novel, scientists estimate that the Sun’s decreasing luminosity will cause the Earth to enter a global ice age in approximately 20 years. Tau Ceti, the only nearby system unaffected by the Astrophage, is 11.9 light-years away from Earth. With our current technology, it would take thousands of years to reach the system, making saving humanity appear an impossible task. 

Weir overcomes this problem by using the Astrophage as a method of propulsion. Each Astrophage stores an enormous amount of energy and releases an intense beam of photons at the Petrova wavelength. Although photons have no mass, they do carry momentum. By directing the photons behind the spacecraft the Hail Mary behaves as a photon rocket, accelerating forward by conservation of momentum. 

The photon rocket is a theoretical propulsion concept first proposed by Eugen Sanger in 1953, but the idea is limited in practice by the immense amount of energy required to produce sufficient thrust. The Astrophage provides enough energy for this propulsion system to send the Hail Mary forward at ultrarelativistic speeds to Tau Ceti in 13 years. Since the craft travels near the speed of light, as per Einstein’s theory of special relativity, the craft experiences time dilation. Therefore, Grace only experiences 4 years and 8 months travel time, making him fit and well enough to work fast on finding a solution to overcome the Astrophage. 

Grace eventually finds the solution to combatting the Astrophage — the Taumoeba, which is an alien amoeba that consumes Astrophage — and launches return probes to carry live samples of the Taumoeba back to Earth. Although the return journey would take approximately 13 years, the preventative measures adopted on Earth may have delayed the Sun’s energy depletion long enough for the Taumoeba to arrive in time. It is therefore plausible that Grace ultimately succeeded in saving Earth. 

Countless science fiction novels ask readers to blindly accept impossible technology, but “Project Hail Mary” is unique in Weir’s attention to detail, ensuring all the science stays grounded in reality. While some aspects, such as the Astrophage, extend physics concepts beyond what is currently known, most of the novel follows well established physics principles realistically. By creating a narrative that blends ingenious fiction with scientific accuracy, Weir’s story is both believable and deeply engaging.