Researchers at Harvard University have demonstrated the first centimetre-scale aircraft that can hover in near-space using only sunlight, according to a peer-reviewed study published in Nature. The prototype consists of two thin, perforated aluminium-oxide membranes spaced 125 microns apart. When the lower membrane, coated with chromium, absorbs light it becomes hotter than the upper layer, generating an upward airflow through thermal transpiration—a photophoretic force first described in the 19th century. In laboratory tests the 1-centimetre-wide device levitated at an air pressure of 26.7 pascal when illuminated with 750 watts per square metre, roughly 55 percent of natural sunlight. The team’s models indicate that enlarging the flyer to a 3-centimetre radius could lift a 10-milligram payload at about 75 kilometres, a region of the mesosphere often called the “ignorosphere” because traditional balloons and aircraft cannot reach it. The authors say swarms of such ultralight vehicles could provide long-duration platforms for climate monitoring, communications relays and even Mars exploration. Several of the researchers have launched a start-up, Rarefied Technologies, to develop the concept further.