Are genetically engineered trees employing bio-nanotech, designed to function as a biological space habitat. Freeman Dyson first proposed them in the early Information Age.
A genetically engineered plant designed to survive and grow on the nucleus of a comet or carbonaceous asteroid with enough water. Such plants produce oxygen atmospheres within subsurface spaces in the nucleus, or even within the plant, by utilizing solar energy and organic materials within the comet itself. Typically, Dyson trees are composed of several main trunks which radially branch out from the surface, and which intertwine and form a spherical structure, that may be up to a hundred kilometers across, though fifty kilometer diameters are more common. The surface of the plant is a non-permeable layer, often impregnated with carbonaceous material for solar environment protection, and to prevent water and gas loss.
Growth of a new tree begins when a suitable comet is diverted into a close solar orbit and a seed is planted on it. Over several years the seed extends a root system around and through the comet and then begins the growth of the primary trunk systems. Depending on the make up of the comet and the distance from the star, full growth may take up to a century or more. Average tree lifetimes run to the millennia even without life-extension techniques, and a mature may support a population in the millions.
A fully grown tree is a spherical structure, It generally consists of 4 to 8 trunk structures growing out from a comet nucleus. Branches grow from the top of each trunk and intertwine and merge with each other to form a single structure. The trunks and primary branches are hollow and contain a breathable atmosphere and symbiotic ecology, as well as a space-adapted ecology on their exteriors.
Sub-species have been engineered to survive at a variety of distances from a star and under various wavelengths. Generally they can be found at distances ranging from .5-4 AU around stars in the G, K and M class. Dyson trees are popular habitats for the space adapted, and two Cargo Cults have been found inhabiting lost Dyson trees and there may be others out there as well.
The greatest difficulty with Dyson trees is running infrastructure through them without damaging them or making them unlivable. Early versions had to rely entirely on wireless communications or broadcast power. Since then, genetic engineers developed trees that can be coaxed into growing a thin layer of bark over mains, sewer lines and the like with the application of the proper chemicals. Combined with grafting to provide fast repairs it is a very workable solution.
Another issue was power generation, which was a minor issue – bio-nanotech was used to produce solar panel leaves, to generate power. Later, the same techniques were used to create programmable surfaces to serve as solar panels, which worked much more efficiently. For more intensive power needs, inversion beam created antimatter provides more than enough energy.
In systems where the trees have been long established entire 'orbital forests', consisting of hundreds of trees spread across millions of cubic kilometers, may be found in the most desirable orbits. Heavily populated trees are easily identifiable, with dense swarms of shuttles, ships, and other vehicles weaving among breaks in the foliage. With docking stations, control towers, manufacturing faculties, clusters of space homes, water-filled diamondoid spheres glowing in the dark like blue pearls.
Since the Dyson tree seeds are relatively cheap, compared to most technological solutions, and more reliable than a lot of nanotechnology, growing Dyson trees has become a common way of building a habitat. There are even feral versions of Dyson trees. These propagate by sending seeds that are propelled by solar sails, chemical rockets or magnetic sails. Whole feral ecologies have spring up in some star systems.
Tech Level: Biotech 8, Nanotech 5
Designed to thrive in vacuum or minimal atmosphere conditions, Yggdrasil bushes may be planted on any suitable body with at least 1/100G surface gravity. During its growth phase, the Yggdrasil bush mines minerals, atmospheric gases and water out of the soil and stores them within its trunk. If insufficient volatiles are available on the parent body, they may be imported for the use of the bushes. Using a portion of the stored gases, it ejects a set of seed packets from itself once every few weeks. Each bush broadcasts its position using bioluminescent cells on its trunk and leaves. When a sufficient number of bushes have accumulated, their bioluminescent signals naturally form a primitive local network. The direction and trajectories of future seed packets are determined using data shared by this network in order to maximize plant growth and area coverage for the forest as a whole.
When upward growth is complete, each bush begins to extend the growth of its canopy of leaves. Canopy growth continues until each bush has thoroughly intertwined its canopy with that of its neighbors. At this point the joined canopies fuse together into a tough, plastic-like sheath capable of holding integrity against standard atmospheric pressure. Completion of the canopy across the entire planetary surface triggers a reflex within the bushes, causing them to release the atmospheric gases and water stored during the growth phase. This release not only creates a livable space under the canopy but also frees up the storage spaces inside the trunks for habitation. As a result Yggdrasil bushes can provide for much more living volume than a normal Dyson tree but you have to wait until its final phase of growth to move in.
In the final phase of their growth, the Yggdrasil bushes alter their biochemistry to maintain and support the newly created atmosphere. Symbiotic soil gardening organisms and other basic elements are budded off of the bush root system generating a self-sustaining ecosystem. Starting from a single seed, a complex of Yggdrasil bushes can convert a smaller world entirely into a living habitat in as little as twenty years, but of course the larger the world is the longer it will take.
This speed is achievable because the Yggdrasil bush is a variant of the Dyson tree with a shorter and narrower trunk. However, while the bush itself only grows to approximately a kilometer in height, its need for at least 1/100G surface gravity means the objects they are planted on already have a greater diameter than a full grown Dyson tree. There are also variants of the Yggdrasil bushes that can be more easily seeded on smaller ice moons and halo objects. They escape the surface gravity restrictions by not ejecting seeds for new growth and instead send out surface roots to cover the object.
This is not to say the seed ejection system is not still in place, only that it is used in a different way. Each bush still broadcasts its position using bioluminescent cells on its trunk and leaves but in this variant it is the surface roots that receive the signals. When a surface root grows beyond it's host body's horizon, and can thusly no longer receive those signals a response is triggered to grow a new trunk. Also, on these lower gravity objects the seed ejection organ functions as a seed-launching organ - allowing the bush to colonize new host objects.
Tech Level: Biotech 8, Nanotech 5