Ammonia Circulatory System — the "cool" blood of an Eridian. Liquid ammonia at 210°C circulates throughout the body, absorbing waste heat and carrying it to the carapace for rejection. Eridians would die if ACS temperature exceeds 223°C.

The gas giant orbiting Tau Ceti where Astrophage breed. Named by Ryland Grace after Rocky confirms the planet's identity. Astrophage travel from Tau Ceti to Adrian at 0.92c to reproduce using the planet's carbon dioxide atmosphere.

A single-celled photosynthetic organism, approximately 1 micron in diameter, that feeds on starlight and stores energy as matter — violating conservation of energy as understood by humans. It breeds on planet Adrian at Tau Ceti, and has infected both Sol and 40 Eridani, dimming their output and threatening all life on Earth and Erid.

An early unmanned probe sent toward Venus to investigate the Petrova line anomaly. Its data — including the discovery of Astrophage cells — triggered the global Project Hail Mary programme.

Small autonomous probes — John, Paul, George, and Ringo — stored in the Hail Mary's nose cone. Each beetle is powered by a small Astrophage engine and used for tasks including EVA, sample collection, and as supplemental propulsion engines when the main drive is compromised.

A proposed large-scale solar energy capture panel designed to be deployed across the Sahara Desert to generate enough power to breed Astrophage as fuel. Ryland's key contribution to Earth's survival strategy before the mission.

Rocky's ship. The name given by Ryland Grace after Rocky's ship first appeared as an anomalous radar blip — "Blip-A" being the first contact designation. The ship is constructed entirely from xenonite and is a highly sophisticated engineering achievement, though lacking computer systems.

Rocky's home planet, formally designated 40 Eridani A b. It orbits 40 Eridani at roughly one-fifth the Earth–Sun distance, meaning its "year" is very short. Surface gravity is 2.09g. Atmospheric pressure is 28 atmospheres. Rocky uses the word "Erid" when speaking English; the full Eridian name is a musical chord.

The sapient species from the planet Erid. Pentadextrous (five-limbed), approximately the size of a large dog, mass ~167 kg. Eyeless — they navigate entirely by sonar. They communicate using musical chords produced by five independent gas bladder systems. Their blood is liquid mercury, their body temperature over 200°C.

The interstellar spacecraft carrying Ryland Grace (and originally two other crew members who died during the journey) to the Tau Ceti system. Powered by an Astrophage-fed spin drive. Equipped with centrifuge for artificial gravity, scientific laboratories, and a xenonite partition tunnel for alien contact. 12.27 light-years from Earth at the story's opening.

Hot Circulatory System — the "hot" blood of an Eridian. Mercury at 305°C circulates to the muscles, providing the heat energy that powers muscular contraction. Without the HCS, an Eridian cannot move.

A microscopic tool used to inject a substance directly into a single cell. Ryland uses a nanosyringe to puncture an Astrophage cell to study its internal structure — the moment he first sees that Astrophage is translucent when breached, revealing its stored energy.

A telescope instrument tuned to the Petrova wavelength (25.984 microns), allowing detection of Astrophage and Astrophage-powered engines across interstellar distances. Ryland uses it aboard the Hail Mary to find Rocky's ship, and later — during the lonely solo years of Ch. 26 — as a daily ritual to watch Blip-A's engine flare and know Rocky is still alive.

An arc of infrared light (at 25.984 microns — the "Petrova wavelength") emanating from the sun's north pole and bending toward Venus, first documented by Russian astrophysicist Dr. Irina Petrova. The line is caused by Astrophage cells travelling from the sun to Venus to breed. Observing it was the first evidence of Astrophage's existence.

25.984 microns — the specific infrared frequency at which Astrophage emits light when active, and which Astrophage-fuelled engines glow most brightly. Named for Dr. Irina Petrova, who first documented the wavelength. Used as the basis for the Petrovascope.

The desperate, last-resort interstellar mission to send a crew to the Tau Ceti system to discover why that star is unaffected by Astrophage — and bring back a solution. Named for the American football play: a long, desperate pass with no real hope. The project is organised and ruthlessly controlled by Eva Stratt under international mandate.

The Eridian engineer and sole survivor of Blip-A's crew. Rocky is not his real name — his actual name is a musical chord in the Eridian language. Ryland gave him the name "Rocky" and Rocky reciprocated by calling Ryland the Eridian word for "grace." He is a repair specialist, not a science Eridian — which makes his problem-solving ingenuity all the more remarkable.

Middle school science teacher, amateur astrobiologist, and — against his will — the sole surviving crew member of the Hail Mary. Selected by Eva Stratt because his unpublished research paper first identified the mechanism of Astrophage's relationship with Venus. He wakes up with complete amnesia at the start of the novel and pieces together his identity and mission through memory flashes.

The Hail Mary's propulsion system. Astrophage is fed into a chamber where it is activated, emitting Petrova-frequency photons in a directed beam that provides thrust via photon pressure. Extraordinarily efficient — a small amount of Astrophage provides enormous thrust. The spin drive allows interstellar travel within a human lifetime.

The shared airlock chamber built into the xenonite tunnel between the Hail Mary and Blip-A. A square-cross-section compartment where both Ryland and Rocky can place objects for exchange across the atmospheric barrier — Earth air on one side, Eridian atmosphere on the other. Rocky built and installed it in the tunnel.

Eva Stratt — a Dutch logistics expert appointed by the UN with unlimited authority to run the Project Hail Mary programme. Cold, ruthless, and almost certainly correct about every decision she makes. She recruits Ryland, organises the entire mission infrastructure, and personally selects the crew. She accepts the moral weight of her decisions without flinching.

A star 11.9 light-years from Earth, almost identical in type to Sol but slightly smaller and cooler. The destination of the Hail Mary mission, because Tau Ceti is the only star in the vicinity showing a Petrova line but no dimming — meaning something in the system is keeping Astrophage in check. That something turns out to be Taumoeba.

A single-celled organism native to planet Adrian that feeds on Astrophage. Named by Ryland Grace (Rocky: "Earth culture. You find. You name."). Taumoeba is the reason Tau Ceti's star is not being dimmed — it is the natural predator of Astrophage. The entire mission succeeds or fails on whether Taumoeba can be cultivated to survive in Venus's or Erid's atmosphere.

The atmospheric simulation used for Taumoeba testing that approximates the conditions of Erid — a blend of Erid's high-pressure, ammonia-rich atmosphere. A Taumoeba strain that can survive both Venus atmosphere and Threeworld atmosphere can be deployed to save both Earth (via Venus) and Erid simultaneously.

An alien material developed by the Eridians — transparent, extremely strong, chemically inert, and thermally non-conductive. It is used to construct Blip-A's hull and internal structure, and to build the tunnel between the two ships that allows Ryland and Rocky to interact without mixing their incompatible atmospheres. Xenonite should not exist according to any known materials science.