Scientists often refer to the mesosphere as the “ignosphere” — a region that is too high to be studied by aircraft or weather balloons, but too low to be studied by satellites. Despite our technological advances, we have not yet found a viable way to monitor this large layer of air, which is located about 37 miles (60 kilometers) above the surface. But engineers are slowly getting closer to solving this problem — inspired by a 19th-century invention that resembles a toy.
An article published today in Nature magazine presents conceptual confirmation of an extremely lightweight disc-shaped structure that levitates using sunlight, without the need for fuel. Made from chrome-based ceramic aluminum, the device floats using photophoresis, which literally means “movement caused by light.” When sunlight hits the device, the difference in temperature and pressure around the disc creates an upward airflow that keeps the disc in the air. The pressure difference creates a photophoretic lift force sufficient to keep these small devices in the air.
Although this particular device was designed to study the mesosphere, the physical principles underlying its flight can be easily applied in future missions beyond Earth, including the extremely thin atmosphere of Mars, provided there is sufficient sunlight, the researchers note.
“Photophoresis does not require fuel, batteries, or photovoltaic cells, so it is essentially a sustainable flight mechanism,” Ben Schaefer, lead author of the study and research associate at Harvard University, said in an email to Gizmodo. “We could use these devices to collect revolutionary data about the atmosphere that would be useful for meteorology, telecommunications, and space weather forecasting.”
The first idea appeared back in 1873, when physicist William Crookes invented a radiometer that worked on sunlight. Subsequent projects attempted to build on Crookes’ invention, but with limited success, as explained by Igor Bargatin, a mechanical engineer at the University of Pennsylvania, in an accompanying News & Views article. (Although Bargatin was not involved in the new research, Schaefer cited his work as one of the main sources of inspiration for the device.)
However, Schaefer and his colleagues used previous research and the latest advances in nanotechnology to create their project, producing samples of “shiny, thin squares with very small holes,” as Schaefer described them. Researchers from different countries joined the project, combining theoretical and experimental stages. Typically, photophoretic force is weak compared to the size and weight of the object, so it is almost impossible to notice, Schaefer explained.
But the new device is so thin and tiny — about half the size of a coin — that the photophoretic force actually exceeds its weight, causing it to levitate. To test their calculations, the team built a low-pressure chamber in the laboratory to simulate atmospheric conditions and sunlight in the mesosphere. To their delight, the tiny discs remained in the air.
Sheffer, who is now CEO of Rarefied Technologies, is moving quickly toward commercializing these devices. His team wants to refine the manufacturing element so that the discs can carry communications technology that can collect and send back weather data, Sheffer said. “We plan to use passive devices that can be tracked remotely using lidar or radar to collect weather data in the upper atmosphere; this could reach the pilot phase in a few years,” he explained.
“If we can realize the full potential of this technology, swarms or arrays of such photophoretic aircraft will be able to collect high-resolution data on temperature, pressure, chemical composition, and wind dynamics in the mesosphere,” Bargatin added. “What began as a Victorian curiosity may soon become a key tool for exploring the most mysterious region of the atmosphere.”
