An Alderson disk is a colossal circular structure built around a sun, with the star in its exact center. They were named after Dan Alderson, who originally proposed the idea. The idea occasionally pops up in science fiction, most notably in the short-lived Malibu superhero line Ultraverse, where an Alderson Disk called the Godwheel played a very central role.
Documentation for this idea is extremely sparse; much of this article is speculation on the part of the author.
The Disk in its original conception would measure thousand miles through its cross section, and extend from within the orbit of Mercury out to the orbit of Mars or beyond. A wide variation on these dimensions is possible, depending on purpose or design.
Such an immense construct would likely out-mass its central star, and the gravity would pull toward the central plane of the disk. In other words, no matter what side of the disk you stood on, your ‘down’ would always be oriented toward the disk under your feet. Because of this, the disk does not have to rotate to produce artificial gravity, and can avoid some of the structural stresses a Ringworld and other similar rotating megastructures may have to endure to produce Earth-like gravity.
Even so, an Alderson disk would still experience vast structural stresses just from its mass spread across such enormous dimensions, requiring material strength of a magnitude completely unattainable by today’s technology. Ultra-Tensile Strength materials, such as scrith described in the novel Ringworld or the force-field enhanced metals of Iain M. Banks’ Culture novels, may be needed to even contemplate building something as mind-boggling immense as an Alderson Disk. And like with other megatructures of its size, an entire solar system or more would have to be dismantled for the resources needed for construction, and raw mass may need to be drawn off of entire stars.
How much of the Disk would be habitable would depend exactly on one’s definition of the term. It could mean habitable solely by humans with minimal technology (For average temperature, that would require about 20 degrees up through 110 degrees Fahrenheit, depending on environment) or whether it means habitable by any kind of life, including extremophiles (which would extend from –70 degrees to more than 140 degrees Fahrenheit.)
Taking the latter definition, and figuring for a star like the Sun, the habitable zone would stretch from approximately 88 millions miles out to about 127 million miles from the sun. The human-comfortable ‘Eden’ zone would probably be within a small variance of Earth’s natural distance from the sun, say from about 90 million to 100 million miles distant.
Using greenhouse techniques, say by having segmented sections with different gas mixtures, or by just producing millions of large domes around the outer edge of the Disk’s ‘natural’ habitable zone, the builders could extend the habitable zone to perhaps more than 150 million miles from the sun.
Taking into account both sides of the disk, this creates the potential living space on the Disk of approximately 30 million times the surface area of Earth.
In order to maximize available sunlight for both habitation and energy gathering, the Disk may be built on a very gentle slop from its inner edge to outer. In order to keep the atmosphere from seeping out into space along its edges, enormous walls would be built along the inner and outer edges of the habitable zone. In fact, in order to maximize the amount of livable space, different concentric sections of the habitable zone may be walled off from each other in order to create individualized gas mixtures and surface pressures and biospheres optimized for the amount of sunlight received at that point. Inhabitants may have to pass through tunnels or enormous airlocks to get from one section to another. Seen from space, the Disk’s habitable zone on both sides may look like dozens or hundreds of different stepped circles, like a vast rounded ziggurat.
A day-night cycle could be created by having the central sun ‘bob’ up and down along its vertical axis. The gravity of the disk would keep pulling it toward its center of gravity, which would be right in its empty center zone where the star would reside. So when its momentum would carry it up, gravity would slow it down and eventually pull it back toward the enter, and its momentum would carry it over to the other side of the disk before it was pulled back again. It would be very much like a super massive pendulum, allowing the sun to rise and set over each surface of the disk over a 24 hour or similar period.
Another solution would be what was described in Malibu Comics’ Godwheel, which involved having a binary star at its center, instead of a single sun. The stars would revolve around each other, and the Disk would be built at an angle where the stars would be hidden from view for a least a short time in their cycle, allowing a true night.
Still a third option would be to have a set-up similar to Niven’s Ringworld, where a ring of gigantic connected ‘shadow squares’, made up of solar-sail-like material millions of miles wide, would rotate around the star’s equator within the central open space of the disk, to block out the sun on the disk’s surface for hours or days at a time.
The environment on an Alderson disk would have a profound twilight character. No matter what day-night system, if any, is used, the landscape would look like it is perpetually undergoing sunset or sunrise. Most plants and trees would adapt and invariably grow on slants, bending toward the light.
Because a life-supporting Alderson Disk would not rotate, it would not have the same progressional instability problems of some other megastructures. The central star would also be able to center itself on the Disk’s center of gravity, avoiding the danger of brushing against the megsatructure, as existed in the case of the Ringworld. However, a means of attitude adjustment may still be needed, as the star and the disk’s revolution through the galaxy can still lead to imbalances in the system in the very long term. However, with an Aldeson Disk, this can prove very difficult, as unlike a ringworld or dyson sphere, it may actually mass more than its parents star. In fact, it would probably be ‘easier’ to adjust the position of the central star than the disk, perhaps by using the disk to generate a massive magnetic field that could nudge the star along to a proper position.
This all describes only a ‘simple’ life-supporting Alderson Disk. The super-technology needed for this construct, here estimated at level 23 of the Tech Level scale, could actually make the entire surface area of the Disk potentially habitable, as well as a large portion of its interior. This would involve technologies not easy to describe or comprehend by us in the 21st Century.
An alternate version of an Alderson Disk could be built around stars that have powerful magnetic fields. This may include neutron stars, in which case the disk would likely be much smaller than the solar-system-sized one described above. These generator disks would uninhabitable, and would spin as fast as they could tolerate.
These Disks would have a built-in lattice of conductive (or better, super-conductive) material, and their motion through the star’s magnetic field would generate enormous amounts of current. The disk can therefore be used as an enormous power generator, like a vastly oversized homopolar generator. Unlike the habitable disk, gravity would not be a major consideration, and could be made hollow and much lighter, allowing it to be spun up or down much more easily as needed.