Any spacecraft has to propel itself by firing something in the opposite direction — combustion gases for a conventional rocket, a plasma stream for an ion engine, reflected sunlight for a solar sail. Daedalus likes the idea of a motor which acts on the tenuous gas in space itself. A conventional propeller, of course, could never be made big enough. But Daedalus recalls the Crookes radiometer, the little ‘light mill’ in a glass bulb, which spins in sunlight. The gas in its bulb has to be so rarefied that the mean free path exceeds the thickness of its vanes, making it an ideal engine for a low-pressure environment. In such attenuated conditions, a temperature difference across a vane, set up by sunlight shining on one side of it, drives the surrounding gas past it. Each molecule that hits the edge of a vane recoils with increased thermal energy, propelling the vane.

Even at normal orbital altitudes the mean free path is many metres. So any large object with a temperature difference from front to back should be propelled through space, cold end first, by the radiometer effect. The obvious design is a modified solar sail. Daedalus proposes a long plastic sheet, perhaps many square kilometres in area but only a few micrometres thick, with a surface coating graded continuously from dead black at the ‘rear’ to fully metallic at the ‘front’. The blacker the surface, the more it would be heated by sunlight. A temperature gradient would be established along the length of the sheet, propelling it cold-end first by thrust from its whole vast area. Radiation-pressure would also act on the sheet, making it a combined radiometer craft and solar sail. Unlike a pure solar sail, it could be oriented (by a system of giant adjustable veins or rudders) to steer towards the Sun, the ideal direction for a solar-powered spacecraft.

For journeys away from the Sun, Daedalus will impregnate his radiometer craft with a graded distribution of radioactive material, to keep the rear always hotter than the front. As it slowly climbs away from the Sun in an outward spiral orbit, its solar propulsion will fade but its radioactive drive will keep going. In a mission lasting centuries, it could journey far into interstellar space. Its transmitters, powered by thermoelectric radioactive heating and aimed at the Earth by a huge parabolic section of its metallized film construction, would keep us informed of its findings.