Stellar engines and Dyson bubbles can be stable

A range of speculative space ventures envisage the use of ultra-large structures for the collection and reflection of light. Given the length-scale of such structures they cannot be considered as point masses for the calculation of gravitational and radiation pressure forces. Using a simplified model it will be demonstrated that ultra-large reflectors in static equilibrium levitating above a central star (so-called stellar engines) are always unstable if the reflector comprises a uniform disc. However, if the reflector has a non-uniform mass distribution, specifically a ring supporting a reflect or, a stellar engine can in principle be passively stable. Moreover, while it can be shown that static swarms of reflectors levitating above a central star (so-called Dyson bubbles) are unstable, in principle they can become passively self-stabilizing if arranged about the star as a dense cloud. While such ventures are clearly speculative, understanding the orbital dynamics of ultra-large structures, and in particular the conditions for passive stability, can provide insights into the properties of potential technosignatures in search for extraterrestrial intelligence studies.