Literature / Project Hail Mary

"It’s a simple idea, but also stupid. Thing is, when stupid ideas work, they become genius ideas. We’ll see which way this one falls."

Project Hail Mary is Andy Weir’s third novel, published in 2021.

The main character is Ryland Grace, an astrobiologist turned school teacher who wakes up in an unknown environment, and with no recollection of who he is or how he came to be there. Also, there are two corpses with him in the room. Suffice it to say Grace has his work cut out for him.

Just like Weir’s previous novels, Project Hail Mary follows established science pretty rigorously, for the most part, with the major caveat being the astrophage.

Warning: Beware of unmarked spoilers related to the First-Episode Spoiler. If you don't know about the twist already, it will be spoiled by reading below.

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This novel provides examples of:

• Absence Makes the Heart Go Yonder: Discussed — although Eridians normally mate for life, Rocky worries that his mate ("Adrian") may have moved on while he is in space. The epilogue reveals that Adrian waited for him.
• Achievements in Ignorance:
  • The Eridians managed to create a manned interstellar ship on their first try despite lacking fundamental scientific knowledge of how space or physics works, which astounds Grace. Unfortunately, the consequences of this results in radiation killing off everybody aboard except Rocky.
  • Both Rocky and Grace manage to accidentally figure out ways of communicating with the other before they knew anything about each others' biologies. For example, Rocky built a barrier out of multiple forms of xenonite, so Grace could pick the easiest to hear through. By sheer luck, one of them was transparent to visible light, and when Grace pressed his face against it to see better, Rocky assumed he was picking it.
• Alcohol-Induced Stupidity: Morphine variant. Grace needs painkillers administered after suffering an injury, and makes somes rather questionable calls trying to fix the situation under the influence of drugs, exhaustion, and sleep deprivation. He succeeds, but the screw-ups cause Rocky to admonish Grace as stupid repeatedly and get mad at the prospect of him taking more.
• Alien Kudzu: In a manner of speaking. An energy-eating microbe starts affecting the sun’s luminosity, which kicks off the plot.
• Aliens Never Invented Democracy: Or any other form of something Earthlings would recognize as a government. Instead they sing in groups and somehow come to agreements that way.
• Aliens Never Invented the Wheel: Eridians never invented transistors. Or Einsteinian physics. Or a concept of radiation, let alone radiation poisoning. Or a dozen other things humans needed to know to manage interstellar travel. And they still managed to do it!
• Almighty Janitor: Grace isn't just More than Just a Teacher, he's an expert microbiologist with a groundbreaking paper and competent in several other fields of science- and this is before all the training he goes through. Stratt calls him out on why this is, he's a coward that couldn't handle academia or any field where he could make a difference and preferred the safety of being "the cool science teacher." He doesn't deny it.
• The Aloner: Grace thinks he is the only living person for light years around. The actual loner was Rocky, who arrived in Tau Ceti before Grace was even born.
• Alternative Number System: Eridians use base six.
• Always Night: Erid's atmosphere is so thick that no light can reach the surface. Eridians are blind as a result and evolved to "see" using echolocation instead.
• Ambiguous Situation: The Distant Epilogue shows Grace living on Erid, and finally gets confirmation that Earth's sun has returned to normal. From this, he concludes both that the Earth was saved and that humanity retained the capacity to launch space missions (which would be needed to implement the solution), but he can't know anything more than that. Grace (and by extension, the audience) has way of knowing how bad things got before the beetles arrived, and can hope that Apocalypse Anarchy didn't get too bad. He wonders about the people he knew back on Earth, and wonders how many of his old students survived, but is unlikely to ever know unless he takes the risk of returning to Earth.
• Amnesiac Hero: Grace wakes up with no memory of who or where he is. The memories come back slowly over the course of the book in the form of flashbacks. Turns out the Amnesia is the product of Sufficiently Advanced Technology.
• Amnesiac Resonance: Grace wakes up with no memories, but with enough scientific knowledge and ability to quickly conclude that he's a scientist of some sort. He notices that he naturally uses metric units when doing calculations regarding time and motion, but thinks in Imperial units otherwise. This leads him to conclude that he's American. (He mentions that he might be Liberian or maybe British, but American is clearly the most likely). He notes, with some frustration, how strange it is that he knows what countries use Imperial units, but doesn't know his own name.
• Anti-Hero:
  • Grace is a Classical Anti-Hero. While he takes risks upon himself and has to rely on his smarts and acumen to try to save Earth, he makes a number of mistakes that costs him time and effort. He also wasn't fully qualified for as an astronaut and not the most qualified biologist. He was in fact the C-team choice, but given that he knew Astrophage inside and out and was trained in the mission protocols, he was the logical choice when his predecessors managed to blow themselves up. He also makes a number of mistakes that nearly end the mission in failure, some from carelessness, some of it through sleep deprivation and painkillers, some through simple human negligence. He was also shown to be risk-adverse to the point of cowardice; when given the (polite illusion of) choice to join the crew, he made a bullshit excuse. When called on it, he tearfully refused to go on the suicide mission, and had to be shanghaied by Stratt, who wasn't going to let Grace's cowardice put humanity in peril.
  • Stratt herself, who is a Pragmatic Anti Hero, almost to the point of Anti-Villain. She is cool and calculating, domineering, ruthless, and pragmatic, and can be described as politely arrogant and dismissive. Granted, that was the exact kind of person you would want to head up the efforts to save the world, and she was driven by the simple fact that if her plans didn't pull through and if she didn't do exactly what was necessary to buy humanity more time, the species would go extinct. The world's governments lined up behind her to do as exactly as she said, with governments passing treaties exactly for the purpose of following her authority and giving her preemptive pardons. She built an entire industry of cultivating Astrophage by paving over the Saharan desert, wreaking havoc with the global climate, and nuked Antarctic ice sheets to free up methane from below the ice. She knows that her actions will be respected as far as she pushes them, but only for so long. She anticipates that as soon as the Hail Mary is on its way, she'll be on trial for a number of crimes and that she won't live long enough to see the outside of a jail cell again. She's duty-driven to the point that she considers the entire matter worth it, if it saves humanity.
• Anthropic Principle: Discussed in its original form (not the TV Tropes definition) when Grace muses on the likelihood of both humans and Eridians having similar technology levels. Rocky posits that they both must have both sufficient technology to breed astrophage and send a crew on an interstellar mission and insufficient technology to solve the issue without leaving their star systems, otherwise they would never meet and ask the question at all.
• Apocalypse How: A Class 2 is already occurring on Earth, and a Class 4–5 looms for Earth on the generational horizon, courtesy of the sun’s cooling. The same applies to Rocky's homeworld, Erid, but in their case, as the atmosphere is much thicker (and therefore retains more energy) it is a much less pressing issue.
• Apologetic Attacker: Stratt, repeatedly, while forcing Grace on the Suicide Mission against his will. She sincerely hates doing it, but the woman who nuked Antarctica to buy time against the astrophage isn't going to let one man's cowardice screw over humanity.
• Applied Phlebotinum: Xenonite, a super-strong xenon-based material that can be used to build pretty much anything except taumoeba-82.5 containment. The novel gets around how it's made by having Rocky, an engineer, not know the science despite being an expert in using it.
• Appropriated Appelation: A non-pejorative example: Rocky is perfectly happy for his ship's English name to remain Blip-A since the Eridians didn't actually bother to name it, not seeing the need. When you have neither a nautical nor an aviation tradition and this is your first spaceship ever, why would you give it an individual name?
• Artifact Name: The Hail Mary’s radar tags the unidentified alien ship as Blip-A when it first detects it, and Grace never bothers to give it a proper name other than that.
• Artistic License – Biology: The astrophage’s energy storage and physics-fuckery.
• Artistic License – Physics:
  • Played with, again by the astrophage’s energy storage and physics-fuckery — it's opaque to neutrinos. Before it gets explained in-universe, the conversation shifts topics.
  • Played straight with xenonite, which gets no explanation whatsoever short of "the Eridians have better materials science than we do".
  • Some back-of-the-napkin calculations show that the Hail Mary would be too heavy to carry enough fuel to maintain a constant 1.5g thrust for nearly four years, to say nothing of the Blip A.^note Rocket fuel equations are infamous for their exponential growth; adding more fuel also adds mass, which quickly leads to diminishing returns even for fuel as ridiculously efficient as Astrophage. One way around this would be if the spin drives, instead of projecting light directly into space, directed it into the spin drive mirrors, doubling the thrust (once from the light hitting the mirrors, once from the counterreaction of its reflection back out into space). The problem with that is explaining how the spin drive mirrors, which are moving parts, could be engineered to withstand the titanic amounts of force this would put on them.
• Bilingual Dialogue: Humans and Eridians are physically incapable of speaking each other's language, but since they fall into similar frequency ranges, each can learn to understand the other. By the end of the story, Grace and Rocky can converse fluently, even though each is speaking his own language.
• Bittersweet Ending:
  • Leaning toward the sweet. Despite being willing to make a Heroic Sacrifice to save Rocky and his entire species, Grace survives the trip and winds up alive and well on Erid, while the probes he sent back to Earth appear to have saved his own people as well. This does mean being stuck in an enclosed habitat on a very alien world, with high gravity taking a toll, but the Eridians go to extreme measures to make him as comfortable as possible, he gets to be near his best friend, and he appears to be very happy being a teacher again (to Eridian children, who are as excited as Earth children would be to be in a class taught by an actual alien!) Returning to Earth someday remains a possibility, but such a harrowing and risky one that it's unclear whether he'll ever do it.
  • On Earth, it can be assumed that civilization still exists, but the planet is surely dealing with problems resulting from an unstable climate and it's unknown how many people died before Project Hail Mary succeeded. On the other hand, as a minor character points out, humans now have access to astrophage technology to help them deal with that.
• Bizarre Alien Biology: Eridians are 400 pound, dog-sized, five-limbed, echo-locating spiders made primarily of metal and ceramics only loosely shot through with anything humans recognize as biological matter. They have two circulatory systems, one hot and one cold, that they use to power their muscles and doubles as their immune system. Sleep is involuntary and can last anywhere from 2 to 14 hours. The list goes on. Needless to say, they find humans equally bizarre.
• Bizarre Alien Reproduction: Eridians only have one biological sex. To reproduce, two Eridians lay eggs that absorb one another to become fertilized.
• Bizarre Alien Senses: Eridians primarily use an extremely powerful form of echolocation the way humans use sight, with Grace noting that Rocky could "see" him through the airlock wall. Rocky in turn considers being able to "hear light" an extraordinary ability. Though he also learns that humans are helpless in the dark, which is his native environment and something he doesn't have trouble with.
• Blind Jump: The Eridians didn’t know about relativity, leading to massively wrong trajectory calculations. This actually works in their favor, as Rocky arrives three years "early" and with a huge surplus of fuel that comes in handy later.
• Book Ends: Grace both begins (at least as of the first flashback) and ends the novel as a science teacher. Slight difference in location, though.
• Boring, but Practical:
  • Grace is taken aback by Stratt's reliance on data collected by amateur astronomers about the dimming of infected stars near the solar system. She explains that professional astronomers are busier with discovering and researching distant stars, which aren't relevant to the astrophage crisis. Data from amateurs, on the other hand, proves perfectly reliable due to its consistency and sheer quantity.
  • While the very nature of the Hail Mary and its propulsion is new technology, all machinery and lab equipment aboard it is existing off-the-shelf technology with many thousands of hours of consumer testing, save for the coma apparatus and medical droids. Its Centrifugal Gravity mode, though experimental, spared them from having to design a whole university's worth of new, untested scientific apparati that work in microgravity. Microsoft Office may not be flashy, but it works reliably.
  • Yáo's preferred eventual euthanasia method, a military pistol. Simple, little chance of failure or complications, and has the benefit of being able to euthanize other crewmates in case their own methods go wrong.
• Burial in Space: What Grace does with the two corpses he woke up with.
• The Captain: Yáo, though he's long dead by the time the ship reaches Tau Ceti.
• Casual Interplanetary Travel: Not quite "casual", but much more efficient travel is enabled by the astrophage, which stores energy as mass and is ludicrously efficient as starship propellant. It allows the things to swim between stars, and using them as fuel allows a constant thrust for a days-long voyage between planets, or a years-long voyage between stars, as opposed to the centuries it would take in more conventional ships.
• Catchphrase: Stratt: "It's not like that. [...] Okay, it's exactly like that."
• Central Theme: Food. Food is everything. The conflict of the story is that Astrophage is eating the sun's energy, meaning that Earth is going to cool down, disrupting the food chain resulting in famine. Stratt has a whole speech about how economies and wars have historically centered around food production. Grace and Rocky solve the problem by finding a natural predator. The problem is, once the predator escapes confinement it eats the Astrophage being used as fuel. Grace is unable to return to Earth because, after he's taken Rocky home, he doesn't have enough food to last the journey. In another book by the same author the protagonist solving the food issue is the central plot of the book. In this book, it is very clearly stated that the protagonist can't solve the food issue the way Mark Watney did. Thankfully he befriends an alien who manages a way to feed him.
• Centrifugal Gravity: The Hail Mary when she’s coasting. To be more accurate, it’s a bola design^note Similar to the "Tumbling Pigeon" design, where the ship spins itself along an axis, this design separates the ship into two pieces connected by cables, and spins that way, allowing the ship to be smaller but still be able to generate enough spin.
• Chekhov's Gun:
  • Subverted with the Hail Mary's "zero all rotation" button, which Grace takes note of during his early explorations of the ship's controls. Although it appears to be the perfect solution to his misadventure above Adrian that leaves the ship in an out-of-control spin, Grace resists the temptation to use it for fear that it will not work properly because of the ship's damage. He finds a more creative solution instead.
  • Dubois's preferred eventual euthanasia being an nitrogen asphyxiation apparatus, with a supply of the needed gas. This comes into play when the nitrogen gas is needed to help adapt Taumoeba to the atmospheres of Venus and Threeworld.
  • Averted with the actual gun that Yao took on board, which we never see.
• Chekhov's Skill:
  • Grace being a science teacher greatly comes in handy for explaining things like relativity and radiation to Rocky, and later gives him a purpose when he's living on Erid.
  • Despite their lack of advancements in other areas, Eridians are geniuses at material sciences. When Grace goes to Erid, they're eventually able to synthesize vitamins and nutrients for him.
• Cincinnatus: Stratt's job in a nutshell, as she has essentially unlimited political power regarding Hail Mary that will vaporize the instant it launches. And for all her hard work she has a dismal future to look forward to.
• Cluster Bleep-Bomb: Implied, Rocky's Anger Born of Worry tirade against Grace's questionable actions includes all sorts of negative words, including at least one he doesn't know the meaning of yet. Since they never taught each other swears..
• Comically Missing the Point:
  • The First Contact with Rocky goes remarkably well, until Grace tries teaching Earth units of distance with a tape measurer. Rocky has no idea what to do with the device and just pulls and releases the tape repeatedly like a child playing with a toy. This is what leads Grace to figure out that Eridians can't see like he can.
  • Also, before the actual First Contact, when Grace receives a strange canister from the Blip-A, he spins up the Hail Mary's rotational gravity to use the science equipment to analyze the canister. The next time he looks through the scope, he sees that the Blip-A is spinning as well, clearly misinterpreting what the Hail Mary is doing as an attempt to communicate.
• Competence Porn: Downplayed in comparison to The Martian, but still present. Grace is a capable scientist and Rocky is an expert engineer.
• Cool Teacher: Grace deliberately cultivated this for himself, styling his classroom halfway to a mad scientist’s laboratory.
• Cowardice Callout: Towards the end, the amnesiac Dr. Ryland Grace discovers that he didn't nobly volunteer for the Suicide Mission to another solar system after all, but tried to duck out by claiming he was more use teaching science. Stratt, who was in charge of the titular project, cuts this excuse dead by asking if he'd do more good by training a few kids, or saving all of them. Stratt calls him out for his cowardice and sends him on the mission against his will after drugging him to induce the amnesia. Remembering this exchange later prompts him to go back for a stranded Rocky, giving up the possibility of returning to Earth to save an alien species.
• Cryonics Failure: The Hail Mary is fitted with an experimental coma-inducing system to keep its crew in suspended animation for the trip to Tau Ceti. Grace is the only one who wakes up, and deduces that his crewmates had been dead for a long while before he came to.
• Culture Clash: Downplayed. Differences between Earth and Eridian cultures lead to some misunderstandings between Grace and Rocky, but they don't significantly affect the overall plot^note Both of the main characters are understanding and intelligent enough to comprehend that anything that seems odd (or even insulting) is almost certainly due to the massive differences in not only their culture, but their biology, and they both do their best to be understanding and accepting.
  • Grace is initially creeped out by Rocky's insistence on watching him sleep. Later, Grace learns that watching each other sleep is simply a part of Eridian culture, and that Rocky was doing so because he couldn't imagine wanting to sleep without being watched.
  • While eating is a social activity for humans, Eridians consider it disgusting and don't even like to talk about it. After Rocky agrees to eat one meal in front of him for science's sake — an operation that involves Rocky splitting open his abdomen, ejecting the waste from his previous meal, and then putting the new meal into the same hole before closing it again — Grace finds himself in agreement and drops the subject.
• Cunning Linguist:
  • Stratt showed no difficulty working through paperwork in six languages and four alphabets.
  • Rocky and Ryland managed to learn each other's language in a few weeks. Justified on Rocky's end by Eridians' eidetic memories and on Grace's end by creating and studying from a dictionary, which he updates as he learns new words. Also justified in that they neither party is fluent in the other's language; they've just picked up enough to communicate important ideas in simplified terms. In the Distant Epilogue, Rocky and Grace are able to communicate much more fluently, but that's due to Grace having been immersed in Eridian language for 16 years.
• Cyanide Pill: Since the trip to Tau Ceti is one-way, the crew of the Hail Mary were given the supplies to commit suicide via their method of choice rather than slowly starve to death once they'd completed their mission. On screen options include nitrogen asphyxiation, a Chinese military pistol, and a kilogram of heroin. The fact that Grace didn't note his own choice is an early clue that he was a last-minute replacement.
• Death World:
  • Erid is like this for anyone from Earth. Almost twice Earth's gravity, with an atmosphere almost 10 times thicker (read: 10 times more atmospheric pressure and heat) and mostly made of ammonia.
  • The reverse is also true. Earth's environment is almost a vacuum as far as Eridians are concerned, and its high oxygen content causes their lung-equivalent to spontaneously combust just from their natural body heat. Rocky in particular is incredulous that human survival depends on a gas (O2) that slowly kills them via oxidation, and that they just accept it as a fact of life.
• Determinator: Stratt. Anything is on the table if it increases the odds in humanity’s favour.
• Dirty Coward: It turns out that Grace didn't volunteer for the mission; Stratt literally had to tie him down, drug him, and temporarily wipe his memory to get him on board, correctly predicting that by the time his memory returned, he'd be too committed to the mission to stop.
• Distant Epilogue: Set on Erid, a few decades after the events of the main story. Grace is still alive and well, though his body is starting to deteriorate under the planet's 2g gravity, and he's teaching physics to young Eridians. He and Rocky are still good friends, and Rocky delivers the news that Sol has returned to its normal brightness, meaning humanity is saved, and based on speed-of-light delay, they got their solution working very quickly after receiving the beetles. The question of whether or not Grace will eventually return to Earth - or whether Earth will send visitors who arrive before Grace dies - is left unanswered.
• Drink-Based Characterization: During the celebration of the Hail Mary being launched and assembled Yáo has a German beer, Ilyukhina has a large thermos full of vodka, and Dubois has a red wine (perfectly aerated).
• Due to the Dead: Ryland dresses his deceased crewmates in their uniforms before performing a Burial in Space for them. When he does this he hasn't yet recovered his memory of who they were (or even their names) but still feels the need to handle their bodies respectfully.
• Eloquent in My Native Tongue: Implictly; the translation of Rocky's language is crude, but Grace figures that's just because of his own lack of skill and assumes he sounds the same to Rocky. The last chapter, set on Erid after almost a decade and a half, shows the translation of Rocky's speech as perfectly ordinary English to reflect that Grace can fluently understand it.
• The Engineer: Ilyukhina in the flashbacks, though she dies during the transit to Tau Ceti. Rocky takes over the role in the main plot.
• Enhance Button: Grace is repeatedly in awe at the software running the Hail Mary. Not unexpected given he was never intended to fly her.
• E.T. Gave Us Wi-Fi: Inverted, as Grace gives Rocky a laptop filled with the entire sum of human knowledge, presumably allowing the Eridians to make a massive leap forward in space-travel. Rocky returns the favor with samples of xenonite, but subsequent events prevent Grace from bringing it back to Earth.
• Extremophile Lifeforms: Humans and Eridians are this respective to each other; one’s comfortable temperature range boils the other alive, and the normal atmospheric pressure for one is close to vacuum for the other.
• Exposed Extraterrestrials: Averted. Rocky first appears wearing a sort of green sweater/jumpsuit, with a hole on top. When he and Grace are celebrating their breakthrough with the taumoeba, Rocky breaks out his "special clothing for celebration," complete with jewel decorations and gloves.
• First Contact: Grace runs into an alien from the 40 Eridani system, much to the surprise of them both. Fortunately, they're quite friendly.
• First-Contact Math: Grace is a science teacher, of course that’s his approach.
• First-Episode Spoiler: The introduction of the Eridians occurs about a quarter of the way into the book. As you can see from the rest of the page, this greatly effects the following events.
• Five-Second Foreshadowing: Some of Grace's returning memories show up right before they become relevant, such as the flashback about Stratt dismissing a copyright lawsuit for putting copies of all of Earth's software aboard the Hail Mary without paying royalties, which appears right before Grace starts looking for a way to translate Rocky's speech. Thanks to Stratt, he has a plethora of sound-analysis programs to choose from.
• Flashback: Grace’s amnesia subsides with these, providing him — and the reader — with not always relevant information.
• Foreshadowing:
  • The first thing Grace notices about Rocky's spaceship is that its flat-panel design is all wrong for holding in pressure. The mission to Tau Ceti is the Eridians' first-ever attempt at space travel. They used flat panels because they were easier to make, and their incredible material science capabilities more than made up for their lack of experience with pressure vessels.
  • In flashbacks, Grace is regarded as Stratt's right-hand man, and knows the details of the mission almost as well as she does. Stratt later reveals that this is because he was on the back burner as the mission's tertiary biologist all along, thanks to being one of the world's leading experts on Astrophage and having the all-important coma resistance gene. Grace is not happy about this.
  • Dubois and Sharpiro, the primary and secondary biology specialists for the mission, begin an active and...um, very public relationship during training. When confronted with the fact that one of them will be going on a Suicide Mission within a few years, they reassure Grace that they are well aware their relationship will end in death. And indeed it does...when both are blown sky-high by an Astrophage experiment gone wrong just days before launch. This forces Stratt to fall back on her tertiary biologist, Dr. Grace. To say this does not go over well is an understatement.
• For Want Of A Nail: If even one of Grace's crewmates had survived the voyage to Tau Ceti, the story would have been very different thanks to them not having Laser-Guided Amnesia.
• From Bad to Worse: Earth's hypothetical demise is explained in detail, that starting with crop failures, things would compound to rapidly accelerate how fast humanity would die off. Ecologically, species going extinct will further the climate decline, driving more species extinct, etc. Stratt points out that dying crops is only the first step, since humans will inevitably start fighting over the dwindling food supply, resulting in wars that will probably do more damage than the climate change itself does.
• Gadgeteer Genius: Rocky is no scientist, and Eridian science in general lacks a lot of theory that humans take for granted. Nonetheless, he proves adept at producing pretty much any kind of device that his materials will allow, and they always work the first time.
• Glacial Apocalypse: What Earth will face if the astrophage doesn’t get stopped.
• Global Warming: Actively accelerated to buy time against the sun's dimming. This goes as far as nuking Antarctica to dump methane into the atmosphere and keep the Earth warm for as long as possible.
  Stratt: Humanity has been accidentally causing global warming for a century. Let's see what we can do when we really set our minds to it.
• Godzilla Threshold: Stratt has been crossing every line in order to give humanity the best chance of survival. Forcibly recruit anyone who has even a sliver of applicable knowledge for the project? Pave over the Sahara desert to generate enough fuel for an interstellar spaceship?Nuke Antarctica to generate enough greenhouse gases to keep Earth warm for a few more years? She'll stop at nothing to prevent humanity's extinction. Even forcing a civilian onto a suicide mission against his will, because he was the best choice.
• Go Mad from the Isolation:
  • With astrophage taking care of propulsion issues, this is the new limiting factor for interstellar travel. That's why a completely untested (in humans, anyway) hibernation system was the single piece of experimental equipment that Stratt allowed on the Hail Mary.
  • Averted with Rocky, who despite having nobody to talk to or watch him sleep for many years, still never went mad or gave into despair. Grace speculates whether Rocky is abnormally mentally resilient or if all his kind is like this, though this never gets answered.
• Gone Horribly Right: When Grace and Rocky realize that astrophage is part of the native ecology of Tau Ceti, they set out to find cells that prey on on it. Their entire fuel supply also consists of astrophage, and they soon realize that the predator cells are really good at killing them.
• Gosh Dang It to Heck!: After years of teaching junior high students, Grace has fallen into the habit of replacing swear words with softer substitutes even when there's no real need to censor himself. If he breaks out an actual swear word, you know things are going south.
• Grew a Spine:
  • Grace is so heavily affected by the forecasted fate of his students and their families that he marches back into the inital lab and demands Stratt to let him continue astrophage research. She relents.
  • After the reveal that Grace had to be forced onto the mission and spending most of the novel dreading his eventual death in space, Grace finally proves once and for all that he's no coward when he willing chooses to give his own life (seemingly) to rescue Rocky and the Blip-A, and by extension the Eridians.
• Had to Be Sharp: Grace speculates that a high-gravity environment like Earth or Erid might be necessary for sapient life to evolve, since a heavyworlder species will need to think and react quickly in order to evade predators or capture prey. A species from a low-gravity world, by contrast, will be used to moving and thinking slowly, without natural selection putting pressure on them to evolve intelligence.
• Heavyworlder: Erid has roughly double Earth’s gravity with twenty-nine times its surface air pressure, and its inhabitants are built like tanks to compensate.
• Heroic Sacrifice:
  • It's known from the beginning that the Hail Mary will be a Suicide Mission, as there isn't enough fuel to return to Earth from Tau Ceti. The three crew members are giving up their lives for humanity. It turns out that Grace's sacrifice wasn't actually voluntary.
  • After Hail Mary gets stuck in a spin, Grace is pinned beneath a chair and can't breathe. Rocky leaves his enclosure to pull the chair off him, exposing himself to Grace's atmosphere in the process. The result is that Grace is saved, but Rocky is left unconscious and badly wounded due to essentially having suffocated and burned at the same time. Thankfully, he recovers.
  • Near the end of the book, Grace gives up his chance to return to Earth and save himself, instead opting to go back to rescue Rocky and the Eridians, knowing full well he'll starve to death when his supplies run out in a few years. Fortunately, it turns out he can survive on taumoeba for a while, and the Eridians eventually figure out how to make proper food for him.
• Hidden Depths: Stratt reveals she used to be a history major, and beneath her no-nonsense extreme utilitarian attitude is an interest and understanding in why humans do what they do. The reason she's so driven on the Project Hail Mary mission is that she's well aware of how famine is just the start of their problems, and how humans will likely wipe themselves out if the mission fails.
• Hive Mind: How Eridian "thrums" work. By singing in a group, Eridians are able to combine their intelligence to come to conclusions and decisions that no individual Eridian would be capable of making. Unlike the typical application of the trope, joining or leaving is voluntary and can be done at any time.
• Human Resources: Played for laughs in the epilogue. When figuring out how to feed Grace on Erid, the scientists made meat by cloning muscle tissue from the only Earth mammal they had: Grace himself. Grace calls them meburgers and has a good sense of humor about the whole thing.
• Humans Are Divided: Deliberately subverted. The world's leaders quickly grasp that working together is the only hope for survival. Accordingly, Stratt is given unilateral and virtually unchecked authority over whatever she deems necessary for the project. She gets to issue orders to virtually anyone and her budget is YES.
• Humans Through Alien Eyes:
  • Rocky roughly describes humans as leaky space blobs and, by his people's standards, he's basically correct. Humans are extremely porous and fragile compared to Eridians and frequently ingest and excrete water, yet due to evolving on Earth, have some innate radiation resistance and more adaptive immune systems by necessity.
  • Rocky finds the concept of sight utterly bizarre, as Eridians never evolved eyes on their pitch-black planet, relying on echolocation instead.
  • By Eridian standards, human memory and ability to do mental math is laughably bad—so bad, Rocky finds it faster to convert from Eridian units to Earth units in his head than to wait for Grace to punch the numbers into a calculator.
  • Computers go both ways. Rocky is amazed by the concept of a "thinking machine," but considers the direct-current batteries used in Grace's top-of-the-line laptops laughably inefficient.
  • Played for laughs when Grace accidentally "drops" a screw into the depths of space. Rocky chides him to use a third hand to hold onto the screws, only to stop mid-sentence when he remembers that humans only have two.
• Humorless Aliens: Averted. 'Sarcasm' is one of the squishier words, but it gets added to the dictionary. Grace also notes that Rocky uses a specific tone when he's joking around, though it takes him a while to pick up on it.
• I'm a Humanitarian: At the end of the book, Grace is living on Erid, which has a biosphere never meant for humans or their nutritional needs. Luckily they had an example of earth-evolved biota on hand and eventually worked out how to clone muscle tissue in-vitro. (You see where this is going.) By the time of the Distant Finale, a large part of Grace's diet is made up of what he likes to call "me-burgers". He is fully aware of how disturbing it is objectively, but you try living off of taumoeba slurry and vaguely sweet protein shakes for a few years and see how gross you find it then.
• I Need a Freaking Drink: Grace's old colleague had several whiskeys before dinner upon hearing about the sun-dimming data; after Grace finds out and grasps the enormity of the problem, he needs some too.
• In Medias Res: The story starts when the Hail Mary arrives at Tau Ceti's system and the background is explored through Grace's flashbacks.
• Innocent Aliens: This might be a result of who survived, but both are quite benevolent to each other. The Eridians also make every effort to help Grace when he's living on their planet.
• Insectoid Aliens: Eridians for us, somewhat. For them humans are leaky space blobs of sorts.
• Interface Spoiler: Averted in the ebook version. The table of contents shows the chapter titles as regular numbers ("Chapter 1", "Chapter 2", etc.), while in the book itself, the very last chapter number is Eridian numerals ("Chapter ∀ℓ") to show Grace's long time on Erid
• Jerk with a Heart of Gold: By his own admission, Grace is a Dirty Coward who wasn't willing to sacrifice himself for humanity. But Stratt recognizes that deep down, he's a fundamentally good person who will do the right thing when push comes to shove, and forcibly inserts him into the Suicide Mission against his will. Once that particular memory resurfaces, Grace grudgingly admits that she was right, and proves it later on with his What You Are in the Dark moment.
• Laser-Guided Amnesia: Grace wakes up with this; his memory is completely gone, though his scientific knowledge and skills remain untouched. His memories quickly begin to return, however, and he chalks it up to a side-effect of being in a medically-induced coma for years. In fact, the amnesia was deliberately induced by a drug designed for that purpose, used as a tool of last resort by Stratt after Grace refused to go on the mission voluntarily. She just didn't count on his crewmates both dying on the voyage and being unable to fill him in when he woke up.
• The Law of Conservation of Detail: The fuel bays’ menu has the option 'Jettison'. Naturally it comes up later in the novel.
• Layman's Terms:
  • Any time scientific discussions occur on Earth, the important details will be explained in simpler terms for the benefit of characters who aren't scientists (namely Stratt) or don't have the specialist knowledge to understand a more in-depth explanation. Conveniently, this also keeps important exposition from getting too technical for the audience to follow.
  • Most of the scientific talk aboard the Hail Mary happens like this, thanks to Grace and Rocky's limited vocabulary in each other's languages. Prior to running into Rocky, Grace's internal narration provides the layman explanation by habit due to him being a middle school teacher.
• Long-Lived: Eridians have a life expectancy of about 700 years.
• Loophole Abuse: The medical droid on the Hail Mary uses the onboard clock to strictly regulate the dosage of painkillers. Grace simply tells it to change the clock time and asks it for more drugs when he needs them.
• MacGyvering: Thanks to the circumstances they're in, Grace and Rocky have to create a number of elaborate devices from what they have on board their ships, ranging from a simple pendulum to 10 kilometers of chain to a Taumoeba breeding farm.
• Meaningful Name:
  • Hail Mary, full of [Ryland] Grace.
  • The ship and project (and, by extension, the book) are named for the Hail Mary pass - a desperate, last-second play with little chance of actual success. Even more fittingly, the play in question involves throwing the ball as far as possible in the slim hope that someone on your team will catch it and save you, which basically describes the mission.
• Mercy Kill: When Yao requests a gun as his chosen method of suicide he also states that if Ilyukhina and DuBois's methods fail he can use the gun to do this for them before he kills himself.
• Minovsky Physics: The astrophage has very well-defined properties, and the constant and rapid scramble to discover and apply these properties drives the plot.
• Mistaken for Pedophile: Hilariously, Grace does this to himself. Due to slowly recovering from amnesia, there's a brief moment where all he knows about himself is that he's a single man with no children who really likes children. Thankfully, it doesn't take long for that particular gap to fill itself in.
  Grace: Oh thank God I'm a teacher.
• More than Just a Teacher: Grace was a scientist in cellular biology working in academia until a "rebel phase" caused him to publish a paper postulating that life didn't require water, and hence concepts like the Goldilocks zone were incoherent. This caused a scandal which made sure that he was pushed out of academia, and he switched to being a high school teacher. At least that's the excuse he gave in his narration. Stratt points out to him that he left that career voluntarily when he got negative feedback, and he chose not to risk that emotional hardship again. When the astrophage was discovered, his background and outlandish idea saw him given a seat at the table in the international effort to solve the Astrophage problem.
• Mr. Fixit: Rocky is so capable in his ability to fix things, that Grace would (jokingly) trust him with open heart surgery.
• Multi-Armed Multitasking: Grace hypothesizes that Eridians normally use two of their limbs for manipulation (since their numerical base is two Eridian hands worth of digits), but true or not, they are capable of using more, especially when microgravity removes the need to stabilize themselves. At one point, Rocky gets frustrated when he tells Grace to use his third arm during a difficult EVA task, only to remember that humans only have two.
• Named After Their Planet: Grace names Rocky's species "Eridians" after their star, 40 Eridani A, and their planet "Erid" to differentiate from the star.
• Namedworld and Namedland: The third planet from 40 Eridani A has no formal name^note Because the Eridians are blind, they never romanticized their neighboring planets enough to associate them with mythology.; Rocky just calls it "Planet Three". Grace decides that's boring and calls it "Threeworld" instead.
• Necessarily Evil: Stratt is never malicious, and certainly not self-interested, but she's utterly committed to saving humanity from destruction and in that role, she's quick to disregard moral objections to anything she considers necessary for the mission. This includes nuking the Antarctic, paving over the Sahara Desert, and impressing people into the project against their will and by force, up to and including forcing someone to go on a suicide mission. Importantly, she predicts that she'll ultimately be punished for all of this and probably spend the rest of her life in prison, but regards her own well-being as unimportant, compared to the mission.
• Next Sunday A.D.: Chronologically, the story begins pretty much in the present day. Nearly all the human technology used is currently in existence.
• Nice Job Breaking It, Hero: After Rocky is injured, Grace tries to help him heal by blowing soot out of his radiator organs. When Rocky wakes up, he explains that the "soot" was actually healing tissue — Grace had nearly re-opened all of his wounds. Fortunately, he'd healed enough by then to recover anyway.
• The Nicknamer: Grace has to come up with names for Rocky, his ship, his planet, etc. because the Eridians call those things what they are. Rocky for his part is confused about this quirk of Earth culture, e.g. why do ships and stars have names assigned, but random equipment doesn't.
• No Antagonist: Unless you count the single-celled organisms eating the sun. The central conflict of the novel is dealing with this existential crisis and every character we see is committing to doing so. Both humans and Eridians share this common. All the instances of interpersonal conflict are about how to fulfill the mission, no one ever questions that they're all on the same side.
• No Biochemical Barriers: The "Alien Food is Edible" variant is discussed between Rocky and Grace, who note that their food does have proteins and sugars in common (adding to the Panspermia theory), but have a bunch of additional components that the other can't digest or would straight up kill them. Rocky's diet in particular contains many metals toxic to human consumption.
• Non-Malicious Monster: Despite being an apocalyptic threat, the Astrophages have the cognitive capabilities that should be expected from a unicellular life form, and aren't capable of being truly malicious.
• No Plans, No Prototype, No Backup:
  • Played with. The Hail Mary is the first of her kind, but the lives depending on her success lead to most necessary components being off-the-shelf with millions of man hours proving they work, sans astrophage of course. Only optional parts can be more experimental.
  • The Eridian mission has it even worse due to their inexperience with crewed space travel. Their ship, the "Blip-A", is very crude by human standards, with only its construction material (xenonite) and fuel storage capacity being superior to the Hail Mary. They also had no idea that relativity and cosmic rays would be issues, causing no small amount of confusion and heartbreak on their way to Tau Ceti.
• Number Two: Grace is so busy working on the project as Stratt's scientific advisor, that he didn't even realize everyone considers him second in command.
• Obstructive Bureaucrat: Stratt was given extraordinary powers specifically to be able to slice through bureaucracy and get the job done at all costs, and she aggressively wields that power every time anything threatens to slow her down. In one memorable scene, she's sued for uploading all available data and software onto the mission computers with no regard for IP laws. She shows up in court for the sole purpose of explaining her authority to the judge, tells the plaintiffs to stop wasting her time with this nonsense, and dares the bailiff to take her into custody (while surrounded by soldiers).
• Oh, Crap!:
  • Grace doesn't take the fact that he is now the third crewmember of the Hail Mary very well.
  • Grace comes to a sudden and horrifying realization: He had just discovered that the specially-bred taumoeba can now travel through Xenonite, which isn't a problem for him since he can make a container of something that the taumoeba can't penetrate. But Rocky's ship is made entirely of taumoeba-permeable Xenonite, meaning there's nothing stopping the evolved microbes from eating all of Rocky's fuel, stranding his ship, and dooming the Eridians. A glance at his telescope only supports his fears.
• Omnidisciplinary Scientist: Averted. While Grace has a foundation in a wide field of science courtesy of his science teaching requirements and enthusiasm, he’s a cell biologist by trade.
• One-Gender Race: Eridians are hermaphrodites, although Grace refers to Rocky as "he/him" for simplicity.
• One of the Kids: Grace understands and gets along very well with children. Might be partly because he's in something of a state of arrested development himself.
• Only Friend: It dawns on Grace that Rocky is the only real friend he's ever had, being single on Earth and not wanting to risk putting himself out there and get a life. This is a major contributing factor to him choosing to return to the Blip-A over going back to Earth.
• O.O.C. Is Serious Business: If Grace drops his usual Gosh Dang It to Heck! habit in favor of a Precision F-Strike, you know the situation is bad.
• Opposites Attract: Dubois and Shapiro couldn't be more different (profession aside), but are apparently in a passionate relationship.
• Panspermia: How astrophage spreads from star to star. Discussed later on by Grace and Rocky, who speculate that one of its ancestors might have seeded life on Earth and Erid billions of years ago.
• Pardon My Klingon: After Ryland did a particularly dumb thing, Rocky is none-too-pleased and throws all manner of negative words at him ("idiot", "incompetent", the likes) and one which the narration notes Rocky has not yet gotten around to translating for him...
• Photographic Memory: The Eridians have this trait, which is part of the reason why they never invented computers.
• Plug 'n' Play Technology: Played with. Human and Eridian technology and protocols aren’t compatible with each other, not without some notable efforts to bridge the gaps.
• Possession Implies Mastery: Subverted. Grace and Rocky acknowledge they can work with each other's tools, but don’t know the particulars of how they work.
• Post-Scarcity Economy: Discussed as a possible future thanks to astrophage. Assuming the mission succeeds and Earth is saved, astrophage's stupidly-efficient energy storage literally makes the concept of a power grid obsolete.
• Precision F-Strike: Grace's reaction to seeing an alien starship holding position two hundred meters from his own.
• Press-Ganged: How Stratt recruits specialists for the Project. You have something to contribute? You're on the team now, whether you like it or not. Taken to its logical conclusion when Grace refuses to go on the mission.
• Promoted to Scapegoat: What Stratt envisions her future to entail. She knows that the minute the Hail Mary is away, she will probably spend the rest of her life in court and/or prison.
• Retro Rocket: The Hail Mary is a sleek and aerodynamic craft. Not because it will ever enter an atmosphere, but because at the speed she travels the interstellar medium behaves like an atmosphere. Grace describes it as looking like something out of a Heinlein story.
• Sacrificial Planet: Just about every star in our vicinity — Sirius, Wolf 359, Epsilon Eridani — is infected with astrophage and has dimmed by 10% as a result. If Sol and 40 Eridani A dim that much, their respective planets will experience a Glacial Apocalypse.
• Sadistic Choice: Near the end of the book, Grace has to choose between returning to Earth as a hero, or going back to save Rocky and the Eridians with the knowledge that he'll starve to death when supplies run out in a few years. He chooses the latter. Thankfully, the Eridians manage to figure out a better menu for him to survive on their planet, and it's confirmed that the beetles made it back to Earth and Sol was saved.
• Schizo Tech: Vice versa –- one race has incredibly advanced materials science, the other has incredibly advanced computation. And besides that Eridians seem to have no concept of any of the physics that would make Philipp Lenard mad with antisemitic rage (i.e. relativity, quantum mechanics and anything to do with ionizing radiation and its effects).
• Science Hero: Ryland Grace is tasked with saving the Earth from catastrophe by analyzing the astrophage phenomenon at Tau Ceti. Rocky was part of a crew to do the same thing. With Grace's science lab and Rocky's engineering know-how, they work together to save both worlds. Although "hero" might be stretching it with Grace, at least initially.
• Screw the Rules, I'm Doing What's Right!: Stratt runs roughshod over the law to get the manpower and resources to stop the astrophage, and to delay humanity's extinction as long as possible to buy time for Hail Mary to find a solution. She fully expects this to come back to bite her the moment the ship is launched and she's no longer needed, she simply accepts whatever consequences she'll face as unimportant, given that the fate of the entire world is at stake.
• Shout-Out:
  • Hail Mary carries four probes to bring its findings back to Earth. Referred to as "Beetles" due to their insectoid shape, they're named John, Paul, George, and Ringo. The prototype, used for testing and left on Earth, is called Pete.
  • Grace names Rocky “Rocky” because of his rock-like carapace, but ends up making jokes about this later:
    • When Rocky decides to name the planet Tau Ceti e after his mate, Grace decides to translate Rocky’s mate’s name as “Adrian”.
    • Near the end, when showing Rocky the results of one of their experiments, Grace shouts, “Hey, Rocky! Watch me pull a Taumoeba out of a hat!”.
  • "He led me through a maze of twisty little passages, all alike".
  • There are multiple references to the Borderlands series, including the phrase “Aaaaaaand open!”
• Shown Their Work: Both Erid and Adrian are real planets, known in real life as "40 Eridani A b" and "Tau Ceti e", respectively. Erid has the same mass and orbital period as its real world counterpart, which Weir used as the basis for designing its Eridian inhabitants.
• Silicon-Based Life: Discussed, but ultimately averted. Astrophage was originally thought to be something like this since it lives on stars, but is soon revealed to be an organic water-using life form that survives (and feeds) in such extreme heat using mass-energy conversion. Eridians, meanwhile, are mostly made of various metals and use mercury as blood, but this metallic body is actually a non-living “hive” used by a colony of water-using, carbon-based cells.
• Sleeper Starship: The Hail Mary, as a first of its kind.
• Sliding Scale of Idealism vs. Cynicism: Leans far on the hopeful and optimistic side.
• Small, Secluded World: The Hail Mary’s 125 cubic meters of living space are all that Grace has. It's made a bit tighter when Rocky moves in with his equipment, since Grace's ship has all the useable science equipment.
• Sole Survivor: Grace and Rocky are the sole survivors of their respective missions. Grace's crewmates died of unspecified medical complications during their comas; Rocky's died of radiation poisoning because the Eridians didn't know that was a thing (Rocky survived because his "quarters" were between the ship's astrophage tanks, which happened to block radiation).
• Space Elevator: On Erid, enabled by xenonite. Presumably this is their only way off the planet, as its high gravity and thick atmosphere would make launching traditional rockets impossible.
• Space Isolation Horror:
  • Downplayed for Grace, but strongly implied for Rocky. 46 years of isolation undoubtedly helped give him extra motivation to cooperate with and befriend Grace even if they hadn't shared the goal of saving their planets from the astrophage.
  • Grace is concerned enough about this to consider putting himself into a coma (something that killed his crewmates) for his trip back to Earth.
• Space Station: The ISS is still around and kicking, and used as a construction site for the Hail Mary.
• Spanner in the Works:
  • Just days before the mission was to launch, a massive explosion rocks the complex. Stratt was very careful to never let the primary and backup crews for the Hail Mary take the same transportation, so a single accident couldn't wipe out both crews. Unfortunately, she didn't apply that to keeping the crews separate on the ground. By bad luck, the primary and backup biologists are both killed in the explosion, forcing Stratt to find a new biologist ASAP.
    • That accident itself came about because some lab tech accidentally gave a milligram instead of a nanogram of astrophage, due to simple human error and the fact that the naked eye can't see the difference. As Stratt said in the aftermath, if you request a firecracker and get given a truck full of plastic explosive instead, the error will be obvious and rectified before anyone dangerously messes around with it, but the absurdly small quantities of astrophage they deal with made them entirely reliant on reading their equipment correctly.
  • Even knowing that the life-support system that would be keeping the crew of the Hail Mary in medically-induced comas all the way to Tau Ceti was almost completely untested on humans, Stratt still didn't envision a scenario where Grace would become the Sole Survivor of his crew. This is a problem because she arranged for him to wake up with Laser-Guided Amnesia after press-ganging him onto the mission at the last minute and was counting on someone else being around to fill him in.
• Standard Alien Spaceship: Played with. The Blip-A is decidedly unsleek, being made of flat panels and panes with straight edges and odd discolourations all over. The tendency for flat edges to be serious failure points for pressurized environments is mentioned before Grace gets around to finding out that xenonite is such a strong building material that even Rocky's far, far higher atmospheric pressure doesn't bother it.
• Star-Crossed Lovers: Dubois and Shapiro are aware that their fling will only end in heartbreak (either one will go on the Suicide Mission or the other one will), which doesn't stop them. Unexpectedly, they both die together in an accident right before the mission.
• Starfish Aliens: Examined in realistic detail: while Eridians are very different from humans, on the cosmic scale of things Grace and Rocky are astonished that they're still reasonably close enough in terms of biology and technology levels that they're able to work out how to communicate with each other relatively well. The comparable technology levels are explained by Rocky pointing out that a pre-spaceflight civilization couldn't even try to stop the astrophages, and anything more advanced than they are could have solved the problem without launching an expedition to the astrophage homeworld in the Tau Ceti system. In terms of interaction, Eridians evolved in total darkness so they have no eyes, but their "sight" is a highly developed form of echolocation. Grace is thankful to discover that Eridian language is based on sound in the audible spectrum for humans, and vice versa, after which it's just a matter of learning (they don't have the mouthparts to make the sounds of each others' languages, but they can understand them). Weir published extensive notes about the Eridians biology:
  • Their homeworld has a thick ammonia-based atmosphere, with 29 times Earth's atmospheric pressure (which is 1 atm) and twice the gravity. No light reaches the surface, so the entire biosphere is similar to an ocean: light from their sun is captured by free-floating atmospheric microbes that act like plankton, which are in turn consumed by filter feeders and then predators up the food chain, and finally the land-level animals like the Eridians are like a sea floor biome (they actually are "Starfish Aliens"). The absence of light means that their sight is based on echolocation, and they have a poor understanding of all electro-magnetic rays, including cosmic radiation.
  • Because the boiling point of water changes with atmospheric pressure, at 29 atm the boiling point of water is 230 C (not 100 C like at Earth's 1 atm). Thus Eridians are most comfortable at a temperature of around 210 C. Air pressure of under 19 atm is fatal to them, because the boiling point of water would correspondingly drop, so their own internal temperature of 210 C would literally boil them alive.
  • While their ammonia-based atmosphere blocks out all sunlight from their surface, the Eridians themselves actually don't "breathe" at all, either ammonia or oxygen. That is, they don't use "cellular respiration" for gaseous exchange: they do cycle air through tubes, which is also how they make noises audible to humans, but this is purely used to regulate internal temperature. Thus the pressure differential, not difference in chemical composition, is what makes them unable to live in an Earth-like atmosphere. Their cells do respire oxygen and carbon dioxide, but this is obtained through internal reactions like a closed biosphere and replenished with food, not air, when necessary.
  • Eridians are covered in heavy carapaces to survive the high atmospheric pressure, giving them the appearance of rocky spiders: this takes up over 90% of their mass and is made of inorganic metals and ceramics (i.e. how human bone is made of calcium, except it's almost all of their body). Despite this, Eridians are not silicon-based lifeforms, but carbon-based. Indeed, the basic sugars and proteins of their biosphere are essentially compatible with Earth's (they even have matching chirality), leading Grace and Rocky to speculate that both of their biospheres share a common origin through microbial panspermia. Eridian bio-matter is not edible to humans, however, due to being saturated with a large number of additional heavy metal toxins.
• Starfish Language: Human and Eridian languages are this to each other. Both fall into similar audible frequency ranges, so each can learn to understand the other's language, but neither is physically capable of reproducing the other's sounds.
• The Stars Are Going Out: Downplayed. Sol, 40 Eridani A, and many other Local Stars are dimming to about 90% of their usual brightness, which will unleash a Glacial Apocalypse on their planets in the coming decades.
• Stock Star Systems: The mission takes place at Tau Ceti; Rocky's homeworld orbits 40 Eridani A.
• Straight Man: Either DuBois is the straightest man who has ever straightened, or he is entirely unaware. Case in point: TMI.
• Surprisingly Realistic Outcome:
  • The Eridians managed to brute-force an interstellar mission despite having no experience with space travel, but this has a myriad of unintended consequences — some good (having a huge surplus of fuel thanks to not knowing that Time Dilation was a thing) and some bad (radiation killing most of the crew because they didn't know that was a thing either).
  • Astrophage is an incredibly potent way to store energy, but that also makes it incredibly dangerous to work with, as Dubois and Shapiro find out the hard way.
  • No, Grace, you can't decline going on the Suicide Mission, not while Stratt is in charge.
• Technology Levels: Discussed quite a bit. While humans and Eridians both have the technology for space travel (and were able to use astrophage to cross interstellar distances), their technological development is remarkably different. Eridians have materials science that humans can only dream of, but haven't even evented the transistor, and were able to build a starship, despite knowing nothing about radiation or general relativity. It's argued that the differences between their environments and personal characteristics shaped this (for example, Eridian memory and skill and mental calculation are such that there wasn't as much drive to invent computers in the first place).
• Tears of Joy: Grace, when Rocky offers him some surplus fuel so he can return to Earth. Again in the Distant Epilogue, when Rocky reports that Sol's luminosity is returning back to normal.
• These Questions Three...: Grace is awakened from his coma with three questions to test his recovering cognition, two rudimentary math problems and asking for his name. The last one is needed to progress to the ship's cockpit, as anyone who still can't remember who they are is presumably in no condition to control anything.
• Time Dilation: Naturally, this comes up on interstellar voyages. The Hail Mary took thirteen years to reach Tau Ceti, but the (comatose) crew only experienced three years and nine months. The Eridians didn’t know about this, nor general relativity, and assumed Newtonian physics held true. This caused no shortage of confusion during Rocky's trip, which passed in half the time he'd calculated.
• Too Much Information: DuBois and Shapiro believe that there should be no secrets within the project. They extend this to not only being open about being in a relationship, but also freely discussing the details of their sex life, without a hint of embarrassment.
• Two Guys and a Girl: Discussed, both the primary and backup crews consists of two men and a woman. Stratt hates this arrangement, and openly advocated for sending only heterosexual men to prevent the risk of romantic complications among the crews. This was a sufficiently impolitic position (especially considering the small talent pool available) that it's one of the few times she was overruled.
• Two of Your Earth Minutes: Since Grace is 'bad at math', Rocky always accommodates him. It gets deconstructed though, as he is usually converting a rough estimate in base six to its exact base ten counterpart, giving oddly precise numbers when Grace is expecting the imprecise estimate. Since Grace is in fact bad at mental math by Eridian standards, it takes him awhile to pick up on that.
• Unbroken Vigil: Rocky insists on observing Grace whenever he sleeps. This appears to be a quirk of Eridian culture, as Rocky is later shown to be extremely uncomfortable with the idea of sleeping without someone to "observe" him.
• The Unfettered: Stratt
  I don't care about morality. I care about saving humanity.
• Unobtanium: The astrophage, which required an unmanned probe mission to Venus to even collect and Grace's breakthroughs to breed on earth as rocket fuel. By human standards, xenonite is an extremely high quality material that our current materials science can't create or work with (which the Eridians consider ubiquitous enough to basically be plastic).
• Verbal Tic: Rocky ends every question with "question"; presumably a quirk of the Eridian language.
• Weaponized Exhaust: An astrophage-fueled spin drive will annihilate anything behind it, even from hundreds of kilometers away. During the Hail Mary's sample-collection over Adrian, the exhaust scorches enormous swathes of the planet and heats up the atmosphere enough that the reflected infrared light burns holes in the fuel tanks.
• Well-Intentioned Extremist: Stratt. And every 'evil' she does is justified. She’s made her peace with the likelihood that she'll eventually be Promoted to Scapegoat, and only cares about the project's success.
• Wham Line:
  • Grace tries to measure the distance from the sun by calculating how fast a sunspot is growing bigger. Then he sees it's moving far too quickly, does a few more calculations, and realizes that he's orbiting Tau Ceti, not Sol.
  • In universe, the Hail Mary project leadership finding out that humanity has 17 years before half of humanity will die out. Upon finding out that they have even less time than they thought, they start weighing drastic measures.
• What You Are in the Dark: Grace's Sadistic Choice near the end. He could easily abandon Rocky and his people to their fate and return to Earth a hero, and no one would ever know the whole truth. Instead, he goes back for his friend, dooming himself to starvation (or so he thought at the time).
• Window Love: Grace and Rocky do this after revealing to each other that they've both lost their entire crew.
• You and What Army?: Stratt demonstrates with a group of armed U.S. servicemen that even if she didn't have her blanket pre-emptive pardon from the U.S. President, no court of law can physically compel her anywhere if she doesn't want to go.
• You Are in Command Now: Grace and Rocky, being the Sole Survivors of their respective missions.
• You Need to Get Laid: Grace is horrified to hear that everyone thinks he and Stratt are sleeping together, given their closeness on the project. When he denies it, someone tells him maybe it'd do them both some good.
• You Wake Up in a Room: How the novel starts. Grace wakes up to find he's hooked up to medical equipment in a small room alone with no memory of how he got there, or even what his name and history are.
• Your Normal Is Our Taboo: Eridians consider eating in someone else's presence, or even talking about eating, to be extremely offensive. They also consider not watching each other sleep the entire time to be a gross violation of trust, which humans of course don't need or want to do.

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