The comet loomed in her canopy. A vantablack mass hiding the stars. No glint on the ice, no coma, just darkness. They'd called it 1I/2077 then the Very Low Albedo Object then Ymwelydd Tywyll. It was silent but Mei's spiraling ship was not: the debris, the power failure, the comet itself. Caution. Mass Intercept. Pull up. Pull up. She flared at the last moment, desperate. Show me stars, Mei said. Show me stars. Then she screamed as proximity closed to zero. The hull buckled. A wall of shattering silica. At least some part of her might know what it was.
She awoke to emergency foam disintegrating around her and the ship still talking. Cabin Pressure. Cabin Pressure. She grasped an oxygen canister and scrambled madly from the seat only to sail twenty feet from the wreck. As she tumbled gracelessly Mei saw an impossible fissure carved hundreds of feet through the rock and ice back to the heavens. But it wasn't rock and ice: it was thick, fibrous, torn. The tissues furthest from her, at the point of impact, coiled and knit themselves together, sealing the breach in itself. The air stilled and pressed in on her ears. The atmox at her belt reported it breathable. No pathogens. She let the canister's mask fall from her mouth as she gazed into the maze of chambers and arteries and glowing bulbs spreading around her into a humid gloam.
Mei took a deep and careful breath.
What Mei crash-landed into – a vast, living structure growing in or around a comet – is a Dyson tree. First proposed by physicist Freeman Dyson, these genetically-bioengineered plants would thrive in deep space, creating self-sustaining habitats within their structures while drawing energy directly from starlight and moisture from the comet's body.
"Ice is better than rock as a basis for life," he wrote in 1997, "and comets contain not only ice but also most of the other chemical elements that are essential for biology."
Plants could grow greenhouses… just as turtles grow shells and polar bears grow fur and polyps build coral reefs in tropical seas. These plants could keep warm by the light from a distant Sun and conserve the oxygen that they produce by photosynthesis. The greenhouse would consist of a thick skin providing thermal insulation, with small transparent windows to admit sunlight. Outside the skin would be an array of simple lenses, focusing sunlight through the windows into the interior… Groups of greenhouses could grow together to form extended habitats for other species of plants and animals.
Imagine a skin- or shell-like outer layer that protects and pressurizes the interior, allowing habitable spaces within—cosmic gardens that would generate their own atmospheres, recycle nutrients, and potentially serve as living spacecraft or permanent habitations—a terrarium in deep space. If most of a comet's journey offers little photsynthetic potential, close approach to the sun brings with it boundless energy that might be stored within for generations.
Dr. David Galbraith, Head of Science at the Royal Botanical Gardens in Canada, elaborates on the concept in a recent article.
Freeman Dyson's tree would be planted on a comet to take advantage of gasses coming off it. It would produce chemical energy from sunlight through photosynthesis. Like the Dyson sphere, the engineering knowledge necessary to produce a Dyson tree lies far in our future, if we are ever able to create such a thing at all. … What would life be like on a comet? For one thing there's little gravity as comets are small objects. This could lend itself to large structures. One of the reasons Dyson's astronomical arboreal invention isn't completely mad is that comets are "dirty snowballs," containing significant amounts of water and carbon. In the vicinity of the sun they lose gasses, including water vapour, giving them the visible tails we can see from Earth. Perhaps in this rarefied, strange, and temporary environment life could be planted and survive, at least for a while. If a Dyson tree was able to capture some cometary atmosphere perhaps it could function like a greenhouse too, enabling life to spread into the more remote corners of our solar system.
Dyson, who died in 2020 at age 96, let the rigors of science feed a powerful speculative imagination, though he didn't write much fiction. His more famous sphere, a hypothetical megastructure encompassing a star to capture all its energy, is a common SF trope and the subject of serious astronomical research.
The more scientifically-challenging tree has not been visited quite so often. Rachel Pollack's short story Tree House, in 1984, might be the earliest direct excursion.
It didn't look so strange as he had feared. Trunk, branches, no leaves, just a kind of mosslike growth covering most of the bark. The moss, he knew, acted as a shield against radiation eve while it utilized all available light for photosynthesis. Even at the orbit's farthest point from the sun, the ultraviolet, unblocked by any atmosphere, was enough to keep the Tree going. And when the orbit brought them in close, the moss could store energy effectively as the solar batteries mounted in the upper branches. As he watched, it grew bigger, like a timelapse film of a tree shooting out of the ground. Soon he could no longer see the whole thing, only a part of the trunk and some lower branches. Originally grown on a research satellite, the Tree was the transplanted to its present home, where drone ships tended with accelerated growing techniques to bring it to its present size in only twenty-five years. Now the drones had left and there was only the Tree. Up close like this it looked alien, a place where human beings couldn't possibly belong.
In 1985, Carl Sagan and Ann Druyan's more hard-sciencey Comet included an early visualization of the concept by artist Jon Lomberg—you can see it and order prints at his website.
The idea flourished (beg pardon) in works like Michael Swanwick's Vacuum Flowers, which honored its inspiration by calling the trees' inhabitants Dysonians, and Donald Moffitt's Genesis Quest books. Dan Simmons' award-winning Hyperion Cantos expanded the concept to stellar scale, imagining entire star systems embraced by living matter.
I was reminded of the concept by a forthcoming short story written and illustrated by artist Simon Roy that he announced on his Instagram page. Maybe more people will be visiting Dyson Trees soon, if only in fiction.
In my own imagination, though, I can't help but think of a Dyson Tree as a vector for interstellar life that already exists. Rather than something we create ourselves to live in, it seems more likely that we encounter something that something else created for … something else to live in, lost to the deep in a form of rafting dispersal. An alien ecology with its own unthinking priorities, headed our way, ready to be warmed by a new sun.
And so within the hollow comet, Mei's wonder turns to alarm.
Deep in the chambers she saw it. A darkness that moved. That flowed. Old as the void and hungry. It had fed on the tree's flesh and slept in its arms for thousands of years. Now it sensed her. The mass pulsed with cold light as it wettened itself, each ripple marking distance lost. Time lost. Mei stood frozen. She knew there was no sanctuary here. Only a trap. Only the dark between stars made manifest, coming for her now with terrible patience.