Photophoretic forces could levitate thin 10 centimeter-scale structures in
Earth′s stratosphere indefinitely. We develop models of the thermal
transpiration lofting force on a bilayer sandwich structure under stratospheric
conditions driven by radiative fluxes in the thermal-infrared and solar-band.
Similar structures have been levitated in the laboratory. Lofting is maximized
when the layers are separated by an air gap equal to the mean free path (MFP),
when about half of the layers′ surface area consists of holes with radii <
MFP, and when the top layer is solar-transmissive and infrared-emissive while
the bottom layer is solar-absorptive and infrared-transmissive. We describe a
preliminary design of a 10 cm diameter device that combines a levitating
structure made of two membranes 2 μm apart with the support structure
required for stiffness and orientation control. We limit the design to
components that could be fabricated with available methods. Structural analysis
suggests that the device would have sufficient strength to withstand forces
that might be encountered in transport, deployment, and flight. Our models
predict a payload capacity of about 300 mg at 25 km altitude and our analysis
suggests it could support bidirectional radio communication at over 10 Mb/s and
could have limited navigational abilities. Such devices could be useful for
atmospheric science or telecommunications, and similar devices might be useful
on Mars. Structures a few times larger might have payloads of a few grams.Comment: Main: 14 pages, 4 figures. Supporting information: 7 pages, 13
figure