The atmospheric mesophere, between about 50 km and 100 km altitude, is too high for aircraft or balloons but too low for satellites. It borders the ionized layers which reflect radio waves, and generate aurorae. Daedalus now wants to explore it.

He has been inspired by the Crookes radiometer, that little windmill in a glass flask which spins in sunlight. Each vane is bright on one side but black on the other. The sunlight warms the blackened side of each vane, so that gas molecules hitting it rebound with extra momentum. This thermal transpiration exerts thrust on the vane. The effect peaks at around 5 pascals or so, just about the atmospheric pressure at 70 km altitude. So Daedalus is designing a big Crookes radiometer in the form of a helicopter rotor. Each angled aerofoil blade will be black on its upper surface, but reflective underneath. The blades will be angled so that sunlight shining down on them will drive them in that rotational sense that generates lift.

This huge frail craft, designed for tenuous high-altitude air, will have to be self-stabilizing. To keep its rotor horizontal, it will have a payload slung axially beneath its central hub. It will have enough lift to stabilize above its height of maximum efficiency. If it descends a little, it will then gain power and climb back up again. Oblique sunlight will not merely power it, but will urge it sideways, away from the Sun.

Thus, in the Northern Hemisphere, it will be pushed west and north during the morning, and east and north during the afternoon. At sunset it will lose lift; during the night it will spin lazily downwards like a sycamore seed. With luck it will still be within its operating altitude when the Sun rises next day; it will climb again, and start another leg of its zig-zag progress north. In summer, its odyssey will ultimately bring it into permanent polar sunlight, where it will orbit the pole steadily. When the changing seasons replace this cheerful buoyant radiance by winter darkness, it will fall to Earth and be lost.

Daedalus's altoradiometer will be a cheap, simple, long-lived research tool for polar aerospace. A balloon could just about lift it high enough for it to climb the rest of the way under its own power. Its solar-powered instrument package would then sample and transmit radiation fluxes, ion densities and magnetic data, for months on end. Astronomers could even observe stellar occultations through its fast-spinning blades.