- Iain M. Banks, the Player of Games (1988)
An Orbital is a rotating megastructure consisting of an enormous band of material arranged into a ribbon-like ring measuring millions of kilometres in diameter. The entire megastructure is spun to create day-night cycles and produce artificial gravity on the structure’s inner surface. As the Orbital spins, centrifugal forces hold the lithosphere, hydrosphere and atmosphere against the structure’s inner surface to support the desired type of ‘planetary’ environment. In this article, I will describe an archetypical Orbital whose entire inner surface is made to duplicate roughly the same conditions as on the Earth’s surface.
To satisfy the first condition necessary to support an Earth-like environment, the Orbital has to orbit the Sun at around the same distance as Earth is from the Sun. This ensures the insolation received is just right. The next condition is to provide a 24 hour day-night cycle and an Earth-like gravity on the structure’s inner surface. This requires the Orbital to have a diameter of 3.71 million kilometres and the whole structure must be spinning at a rate of once every 24 hours. At that rate of spin, the structure’s rim is moving at 135 km/s. An Earth-like atmosphere is held against the structure’s inner surface by spin-induced centrifugal forces. Walls that are a few hundred kilometres high line the edges of the Orbital’s inner surface. These gigantic walls along the structure’s edges keep the atmosphere within the Orbital’s inner surface by preventing it from slipping off the edges into space.
Constructing the Orbital will be a challenge because there is still no known material strong enough to withstand the titanic stresses found within the structure of the Orbital. This yet-to-be-discovered material or “unobtanium” will be required to construct the Orbital’s stress-carrying structure. The lithosphere, hydrosphere and atmosphere will all be laid onto the inner surface of this unobtanium-based stress-carrying structure. Other structures such as the towering walls along the edges of the Orbital’s inner surface can be made using known materials with extremely low density and very high strength such as self-supporting diamondoid foam. In addition, diamondoid foam can also be used to sculpture the desired topography for the environment on the structure’s inner surface.
The entire circumference of the Orbital measures 11.66 million kilometres and it takes light almost 40 seconds to traverse that distance. If the Orbital has a width of 40000 kilometres, the total habitable area on the Orbital’s inner surface will be a mind-boggling 466.4 billion square kilometres. An area like this is equivalent to 912 times the total surface area of the Earth (including the oceans) or 47500 times the area of the United States of America. If each square meter of surface area requires 12800 metric tons of material, the entire Orbital will be about as massive as the Earth. This is equivalent to a 1.63 kilometre column of solid iron for each square meter of surface. Nevertheless, such a structure will have 912 times more habitable surface area per unit mass than a planet like Earth.
An observer standing on the Orbital’s inner surface will see a sky that is rather similar to one seen from Earth’s surface as the atmosphere overhead is entirely open to the vacuum of space. However, the observer will be constantly aware of a marvellous sight where the world at both spinward and anti-spinward horizons will appear to curve upwards and eventually join overhead at a great distance of 3.71 million kilometres away. As the structure rotates once every 24 hours, the observer will be able to see the approach of dawn and dusk along the great arc of the Orbital.
Nights on the inner surface of the Orbital will be spectacular as ‘ring shine’ from the illuminated portion of the Orbital will appear thousands of times brighter than the full moon on Earth. Due to the large amount of ‘ring shine’, astronomers on the structure’s inner surface will have a great difficulty trying to observe the nigh sky. Then again, the dark and pristine vacuum of space is never too far away as astronomical observations can be performed from the outer surface of the Orbital, which is just tens of kilometres ‘underground’ from the habitable inner surface.
The Orbital sweeps across the entire sky like a grand firmament. Observing it should be an interesting pastime for surface inhabitants. Looking overhead with unaided eyes at the great arc of the Orbital, the innumerable oceans and continents will appear as mere speckles. Even a full scale recreation of all of Earth’s continents will be barely noticeable on such an enormous scale. Given the sheer immensity of the Orbital, the effect of surface curvature is a lot less than on any planet-size globe. A surface inhabitant can see all the way to the base of distant mountains, unlike on a planet where the base of distant mountains tends to get hidden away by the planet’s curvature.
To create day-night cycles, the Orbital is tilted at an angle with respect to the Sun. If the Orbital has no tilt, it will eclipse the Sun all the time from the perspective of an observer on the structure’s inner surface. The tilt also creates two warm seasons and two cool seasons each year. The middle of each warm season is marked by a midsummer eclipse of the Sun and the middle of each cool season is marked by the Sun’s lowest position in the noon sky. Unlike on Earth, the length of daylight on the Orbital will not vary, resulting in no long summer days or long winter nights. If the orbit of the Orbital around the Sun has some eccentricity, it can cause one warm season to be warmer than the other and one cool season to be cooler than the other.
Areas adjacent to the towering walls along the edges of the structure’s inner surface will experience a more pronounced seasonal variation from the tilt of the Orbital. As the Orbital goes around the Sun, the area adjacent to one rim wall will be in the wall’s shadow for approximately half a year while the area adjacent to the other rim wall will receive extra light from sunlight reflected off the rim wall itself. This means that during first half of the year, the area adjacent to one rim wall will tend to be the coolest place on the Orbital and during the second half of the year; the same area will tend to be the warmest place on the Orbital. The opposite is true for the area adjacent to the other rim wall.
Overall, seasonal variations on the Orbital will not be as large as those that occur on the Earth. The Coriolis Effect will not be significant on the Orbital because the only form of Coriolis Effect is the rise and fall of air within the troposphere where most of the weather occurs. With a thickness of around 10 kilometres or so, the depth of the troposphere is insignificant compared to the 3.75 million kilometres diameter of the Orbital. Without major temperature gradients and without the Coriolis Effect, the weather on the habitable inner surface of the Orbital will be gentler and more localized than weather on Earth.
Without a moon, tidal effects on the Orbital will be weaker than those on Earth since the only form of tides on the Orbital will be those generated by the Sun. With no contrasting cold polar oceans and warm equatorial oceans like those found on Earth, natural circulation between surface waters and deep ocean waters cannot be established. Without such a circulation system, deep ocean waters will become anoxic. In order to maintain rich oxygen-bearing waters throughout the entire depth of the oceans, artificial heating can be applied to the deep ocean waters at specific locations on the ocean floor to keep the circulation running. Additionally, the underwater topology can be carefully sculptured to enable the mixing of surface water with deep ocean water from Sun-driven tidal effects alone.
Towering walls similar to those found rimming the structure’s edges or exceedingly high mountain ranges called ‘bulkhead ranges’ can be used to contain and isolated alien environments and ecosystems that are very different from the standard Earth-like environment on the Orbital. This is because the immense heights of these walls or ‘bulkhead ranges’ keep in the atmosphere that they surround. Entire pristine prehistoric worlds that are populated by once extinct creatures can also be enclosed within these colossal barriers. Panoramic views of the surroundings from these ‘bulkhead ranges’ will be especially breathtaking as these mountains tower hundreds of kilometres above the surroundings. In comparison, Earth’s
is only 8848 meters in height. On the immense scale of the Orbital, mountains
that far surpass the height of Earth’s Mount Everest or Mars’ Olympus Mons will
appear as almost indistinguishable bumps on such an exceedingly vast landscape.
‘Bulkhead ranges’ and other mountains of comparable heights rise far above the atmosphere and their summits are exposed to the silent vacuum of space. These mountains are rather interesting as they rise through the full extent of the troposphere, stratosphere and mesosphere. In fact, these mountains are so high that they rise well above the ozone layer and even above the high-flying noctilucent clouds. The summit environments of these mountains are basically bare rock exposed to the vacuum of space. In order to reduce the mass of material required for such mountains, the interior bulk of these mountains can be made mostly hollow and be supported by ultra-strong diamondoid foam or other forms of exotic materials that have very low densities and very high strengths.
Journeying to a distant part of the Orbital will be a challenge due to the sheer size of the Orbital. Even for someone cruising at a speed of 10 km/s onboard a high speed vacuum tube maglev train, it will still take almost 2 weeks to circumnavigate the entire Orbital. As a result, rapid transit to far-off places on the Orbital will require technologies similar to those employed for large scale commercial interplanetary space travel. Furthermore, interplanetary space voyages disembarking from the Orbital’s rim will be much simpler since a spacecraft released from the outer surface of the Orbital will already be travelling at a speed of 135 km/s.
An advanced technological civilization is likely to view the construction of an Orbital as a very attractive mega-engineering project. Apart from the stunning vistas, an Orbital provides hundreds of times more habitable area than an Earth-size planet for an Earth-mass worth of building material. This makes an Orbital a lot more ‘efficient’ than a planet since it offers many times more habitable area per unit mass of material.
Although building such a megastructure is far beyond current technology, the search for Orbital-like megastructures around other stars can be a form of non-conventional search for extraterrestrial intelligence. NASA’s Kepler space telescope is a planet-hunting instrument which monitors the brightness of over a hundred thousand stars simultaneously and looks for tiny dips in a star’s brightness that may be indicative of a transiting planet. An Orbital-like structure transiting a star should produce a transit lightcurve that is different from one produced by a transiting planet.