Close to Critical
A novel of a hellworld and its sentient natives.
Close to Critical, a science fiction novel by American author Hal Clement, first appeared as a three-part serial in *Astounding Science Fiction* in 1958, with its initial hardcover release in July 1964.
The story takes place on Tenebra, a planet orbiting the star Altair. This "hellworld" has 27 times Earth’s mass, three times its diameter (38,226 kilometers), and surface gravity three times that of Earth (29.43 meters per second squared). Its escape velocity is 33.558 kilometers per second, and it rotates once in just under four Earth days. Tenebra’s atmosphere is mostly water, heavily mixed with oxygen and sulfur oxides. Surface temperatures approach the critical point of water (374°C, 22.1 megapascals), so at night the atmosphere condenses into raindrops 9 to 15 meters wide. While the expected surface density would be about 322 kilograms per cubic meter, Clement establishes that the actual surface pressure is 800 atmospheres, compressing the air to nearly the density of liquid water (1,182 kilograms per cubic meter). This density ensures the huge raindrops fall slowly despite the high gravity; otherwise, they would strike with lethal force.
Life on Tenebra does not rely on photosynthesis, as the thick atmosphere blocks most of Altair’s light. Instead, plants use chemosynthesis, converting sulfur oxides in a process similar to that of Earth’s deep-sea hydrothermal vent organisms. Ultraviolet radiation from Altair, hitting the upper atmosphere, replenishes the sulfur oxide balance. The planet’s lakes and small oceans are filled with sulfuric acid. Because water’s heat of vaporization is near zero under these conditions, nightly rainfall and daytime evaporation cause extreme tidal fluctuations, like those in the Bay of Fundy. This constant acid ebb and flow weakens the crust, triggering frequent earthquakes and landscape changes. Humans have observed Tenebra for two decades from the space station *Vindemiatrix*, which orbits synchronously every 96 hours.
The plot begins with human researchers sending a semi-autonomous robot to Tenebra’s surface. It descends on rockets until atmospheric pressure exceeds the combustion chamber pressure, causing the engines to overheat and melt. The robot jettisons them and completes its fall under a parachute. After acclimating to the dark, cloudy world, it spends months mapping and observing native life.
Quick Facts
- Author
- Hal Clement
- Cover Artist
- Paul Lehr
- Country
- United States
- Language
- English
- Series
- Mesklin
- Genre
- Science fiction
- Publisher
- Ballantine Books
- Release Date
- July 1964
- Media Type
- Print (magazine, hardback and paperback)
- Pages
- 190 (paperback)
- Preceded By
- Mission of Gravity
- Followed By
- Star Light
Facts from the source article.
Lore & Background
The novel begins with human researchers sending a semi-autonomous robot to the surface of Tenebra. The robot descends on rocket boosters until atmospheric pressure exceeds the pressure in the rockets' combustion chambers, causing the rockets to overheat and melt. The controllers jettison the engines, and the robot completes its descent under a parachute. After months of exploration, the robot encounters a sentient native resembling a large, scale-armored fir cone with four pairs of limbs and lensless eyes. The robot steals ten of its eggs and stone knives, then returns to its own base. Sixteen years later, Nick Chopper, a human raised by the robot (now called Fagin), encounters other humans like himself and gains the enmity of a native chief named Swift. A battle ensues, and Fagin negotiates a truce.
Reader's Guide
Close to Critical is significant as an example of Hal Clement's hard science fiction, featuring a meticulously constructed alien world. The planet Tenebra is described with detailed physical parameters: 27 times Earth's mass, three times Earth's diameter, three times Earth's gravity, and an atmosphere near the critical point of water. The story explores the consequences of such an environment, including raindrops 9 to 15 meters wide that descend slowly due to atmospheric density. The novel also introduces chemosynthetic plant life, replacing photosynthesis because of the thick atmosphere. The plot involves a space station, a bathyscaphe, and a cattle drive across Tenebra's surface. The work reflects Clement's interest in plausible alien biology and physics, and it remains a notable entry in the genre for its rigorous worldbuilding.
Did You Know?
- Tenebra has an atmosphere consisting of water heavily laced with oxygen and oxides of sulphur.
- The planet's surface temperature comes close to the critical point of water, causing raindrops 9 to 15 meters wide.
- Tenebra's plants use chemosynthesis based on the transformation of sulphur oxides, not photosynthesis.
- The robot in the story is named Fagin, a reference to the character in Charles Dickens' novel Oliver Twist.
Frequently Asked Questions
Who wrote Close to Critical and when did it first appear?
American science fiction author Hal Clement wrote the novel. It debuted as a three-part serial in Astounding Science Fiction in 1958, and a standalone hardcover edition followed in July 1964.
Where is Close to Critical set?
The story takes place on Tenebra, a planet in orbit around the star Altair. It is one of the most physically extreme worlds featured in science fiction, often described as a classic hellworld.
What makes Tenebra such a hostile environment?
Tenebra packs 27 times Earth's mass into a diameter of roughly 38,226 kilometers, producing about three times Earth's surface gravity. Its atmosphere is overwhelmingly water vapor, and the planet spins once in just under four Earth days, compounding the challenges for any life there.
What is the central premise of Close to Critical?
The novel follows the sentient native species of Tenebra as they attempt to develop the technology to escape their punishing world. Their biology, culture, and engineering all revolve around overcoming an escape velocity of 33.558 kilometers per second.
Why do fans consider Close to Critical a landmark in the genre?
It is widely regarded as a defining hellworld story because it grounds an entire civilization in the physics of an extraordinarily massive, high-gravity planet. The sentient natives and their struggle toward spaceflight give the novel a distinctive focus on non-human intelligence shaped by extreme conditions.
More in Space exploration novels 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced