Scientists have devised tiny featherweight disks that could float freely in Earth’s mesosphere or the thin air of Mars, theoretically even while carrying payloads. Our mesosphere, which extends about 50 to 85 kilometers above the planet’s surface, is sometimes called the “ignorosphere”—it’s too high for aircraft and weather balloons to reach but too low for access by satellites, making it one of Earth’s least-studied regions. Versions of the researchers’ light-powered fliers could potentially carry sensors through this zone.
The new centimeter-wide prototype disks are made from two thin, perforated membranes of aluminum oxide connected by minuscule vertical supports. They are kept aloft by a force called photophoresis: the light-induced movement of small particles at very low atmospheric pressures. In laboratory experiments described in Nature simulating mesospheric air pressure and illumination, the researchers showed that their devices could float passively without any power source.
Gas molecules bounce more forcefully off the light-warmed side of an object than they do off the cooler one, creating airflow. In this case, the research team coated the bottom of each disk with chromium so it would absorb light and heat up more than the top. Thus, gas molecules pinging off the lower part gained more momentum than those at the top, generating lift, similar to the way a rocket’s jet produces upward thrust. Carefully calibrated holes in the disk’s structure increased this thrust, using an effect called thermal transpiration to passively channel the air from cooler to warmer regions.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
“The air is not only moving around the sides of the structure—it moves through the structure, too, creating these little jets,” says materials scientist Benjamin C. Schafer, co-lead author of the paper. This enhancement boosted the disks’ performance enough to surpass that of previous photophoretic fliers demonstrated by other groups, which had required illumination several times brighter than that of sunlight.
Photophoresis was first demonstrated in the 1870s by English physicist William Crookes. He developed what came to be known as a Crookes radiometer, a toylike device that spins outstretched fins when exposed to sunlight. But because photophoresis works only at very low pressures and generates a very weak force, the phenomenon was long seen as a mere novelty. That began to change a couple of decades ago, Schafer says, as advances in nanofabrication let researchers make devices light enough to levitate with the meager force of photophoresis alone.
Using a laser to mimic sunlight, Schafer and his colleagues demonstrated photophoretic levitation on their centimeter-scale structures in a low-pressure chamber. They also designed a six-centimeter-wide version of the disk to carry a 10-milligram payload—which, in theory, would be enough to power a small communications system with a radio-frequency antenna, a solar cell and integrated circuits. The team calculates that this larger version of the disk could stay aloft at the rarefied altitude of 75 kilometers during daytime; in summertime at polar latitudes it could even fly continuously in the mesosphere.
Ruth Lieberman, a heliophysicist who worked on earlier attempts at photophoretic technology, calls it a brilliant design. “As long as the sun is shining, these things will work,” she says. “They are also made out of very inexpensive materials. Once you get past the prototype phase and can figure out how to manufacture [at scale], it strikes me as a really potentially fantastic solution for observing the atmosphere at very low cost in a way that gets you very good spatial temporal coverage.”
Schafer envisions a future in which swarms of these structures collect atmospheric data and relay telecommunications not only in Earth’s mesosphere but also in the tenuous atmosphere of Mars, which exhibits similarly low pressures. Schafer has co-founded a company that is developing new versions of the disks, and he hopes to launch payload-free atmospheric test flights soon.
Actually creating the larger disks that can carry payloads in the mesosphere or beyond is a more formidable task—perhaps a five- to 10-year project, Schafer says: “I think it’s certainly doable, but it’s going to take a lot of time and work.”
