Space Mirrors and Solettas
Mirrors 1
Image from Alex Mulvey
Space mirrors are space-based structures designed for redirecting sunlight. Such mirrors have a wide range of applications, including but not limited to solar amplification, terraforming, and supplemental lighting. Only small fractions of sunlight ever reach sources that benefit from it; space mirrors are utilized to redirect extra sunlight towards environments that may otherwise not receive as much sunlight as desired.

Sunlight is most efficiently absorbed when it hits perpendicular to the absorbing surface, efficiency which drops as the impact angle increases, due to several factors. On spherical bodies this is easily observed when the equator is much warmer than the poles, however any surface will suffer from this phenomenon. The venerable Stanford Torus design uses a tilted mirror to direct sunlight to the inner surface, and Bishop Rings are generally oriented edge-on to the star, with some designs incorporating a system of mirrors to collect sunlight. Solar power arrays will see decreases in efficiency when not pointed directly at the sun. Space mirrors are advantageous for these scenarios because they are able to reflect light directly perpendicular to a surface from many ranges and directions.

Energy Redirection

Sunlight is a widely utilized energy source for both organic and inorganic production; solar power arrays, along with organisms that photosynthesize, convert sunlight into energy. Space mirrors can be used to not only redirect but to focus light on particular targets, in order to supply them with more energy than they would otherwise receive. Point-focus mirrors are arrays of a generally concave shape, which take sunlight from a wide surface and focus it onto a much smaller area.

Some worlds use an array of mirrors known as a soletta to funnel sunlight towards the planet; these arrays are placed between the planet and the local star, often at or near the L1 Lagrange point, and appear to form aring around the star as seen from the planet's surface.

The receiving end of light redirection can take many forms; it could be photosynthetic life on the surface of a habitat or other environment, or solar power arrays located there. A single mirror array could also focus all of the light to a central point of the array itself, where the power can be converted into energy and then beamed elsewhere when necessary.

Mirrors 2
Image from Alex Mulvey


Space mirrors and solettas are a common terraforming tool. Large bands of orbital mirror arrays are placed in regular succession to reflect heat down to the surface of the world, increasing its surface temperature. Typically these mirrors are wide-focused; flatter and more distributed, they are able to provide general and even heating over a wide surface. Concave mirrors are utilized less for terraforming as the focused heat can be damaging if used improperly, though they can be used to melt surface ice if needed.

Very lightweight mirror arrays are often configured as statites, balanced between the stellar light pressure and the gravity of the planet. However these lightweight mirrors require fine control, otherwise they will drift too close to the planet and burn up, or drift away from the planet and be lost.

These terraforming mirror arrays can either be disassembled if the world is eventually able to maintain a stable temperature on its own, or they may become a permanent component, helping to continuously stabilize the worlds' temperature.

Supplemental Lighting

Space mirrors have other uses beyond energy and heating. It may be desirable to simply add light to otherwise dark areas. O'Neill cylinders are enclosed livable surfaces; those with entirely solid walls will see very little natural light on their interior. Mirrors at the ends of the cylinder can redirect natural light down the cylinder, negating the need for artificial lighting or careful positioning. Mirrors can also be used to light the night side of spherical or other bodies. Other surfaces which may by design or necessity not receive much or any natural lighting, such as the opposite side of Dyson swarm components, ringworlds, or any other solar-oriented structures, can all benefit from this application.

Space Mirror
Image from Steve Bowers
A lightweight mirror made from thin foil, rotating slowly to retain its shape

Design and Engineering

Though space mirrors generally require very minimal energy to operate beyond reorientation, they still have optimal designs for a given application of their various features including material cost, ease of maintenance, positioning, size, and distribution. Design requirements and goals will be different for every application. An array for terraforming a spherical body may be many smaller mirrors in a polar orbit, providing regular and constant heating evenly distributed across a wide surface. Point-focus mirrors are by nature intended to focus light on a specific point, which may be impossible to do at all times due to orbital dynamics, whether the point is on a spherical body, ring, or other habitat.

Mirrors illuminating Venus
Image from Steve Bowers
Mirrors illuminating Venus
With enough surface area and sufficiently fine control, space mirrors can be used to direct concentrated energy towards selected areas of a planet or megastructure. Uses of this capacity include melting ice, heating the atmosphere to control weather systems, and even excavating canals and ditches.

A similar technology on a somewhat smaller scale is the weather machine concept, which uses very large numbers of small, balloon-supported adjustable mirrors in the atmosphere or in low planetary orbit.
Related Articles
Appears in Topics
Development Notes
Text by Alex Mulvey
extra material by Todd Drashner and Steve Bowers
Initially published on 21 April 2020.