A Solar Moth near Novgorod in the Bourgatov system
The Solar Moth or Solar Thermal Rocket uses a system of parabolic mirrors to focus the light from the local star on a propellant tank. A typical moth design has three aluminum coated inflatable reflectors, 175 metres in diameter, which concentrate the light of the star onto a propellant tank, heating and expanding the propellant through a rocket nozzle. One suitable propellant is hydrogen seeded with an alkali metal. Other designs use arrays of half-silvered transparent spheres to heat the propellant.
Solar Moths work best in the innermost parts of planetary systems, but they rely on imported volatiles brought from the outer system. These small, relatively low tech ships are often used as private yachts and pleasure craft by hobbyists, although some polities in remote systems use them more extensively. Because of their low acceleration they are popular with clades with slow chronometric rates or contemplative lifestyles, particularly in the Sophic League.
Some craft which use more conventional drive systems have Solar Thermal Rockets as emergency back-up systems for use in emergencies when all other power has been lost; the system is compact when stored and low in mass.