A team of researchers from Harvard University, the University of Chicago, Pukyong National University, and Universidade Federal do Paraná has developed a new approach to study a difficult-to-reach layer of Earth’s atmosphere. This region, located between 30 and 60 miles above the surface, is not accessible by airplanes or weather balloons and is too low for satellites. Improved knowledge of this area could help refine weather forecasts and climate models.
The group’s findings were published in Nature and describe lightweight flying structures that can levitate using only sunlight. The research explores photophoresis—a process where gas molecules bounce more forcefully off the warm side of an object than the cool side—creating lift in low-pressure environments like those found in the upper atmosphere.
“We are studying this strange physics mechanism called photophoresis and its ability to levitate very lightweight objects when you shine light on them,” said Ben Schafer, lead author of the paper.
Schafer was previously a graduate student at Harvard under Professor David Keith, who now works at the University of Chicago as a professor in the Department of Geophysical Sciences and faculty director of the Climate Systems Engineering initiative.
Keith explained that this mechanism allows passive flight for devices in atmospheric regions where no other aircraft or device can operate: “It opens up an entirely new class of device—one that’s passive, sunlight-powered, and uniquely suited to explore our upper atmosphere,” said Keith. “Later they might fly on Mars or other planets.”
To achieve lift through photophoresis, researchers built thin membranes from processed aluminum oxide with a chromium layer underneath to absorb sunlight. When exposed to light, temperature differences between surfaces create enough force for these lightweight structures to float.
“This phenomenon is usually so weak relative to the size and weight of the object it’s acting on that we usually don’t notice it,” Schafer said. “However, we are able to make our structures so lightweight that the photophoretic force is bigger than their weight, so they fly.”
The concept originated over ten years ago when Keith designed photophoretic nanoparticles as possible alternatives for reflecting sunlight back into space as part of climate change mitigation strategies. In previous theoretical work published in 2010 in PNAS, he suggested these particles could last longer in the atmosphere compared to sulfate aerosols—which may negatively affect stratospheric chemistry—and be more cost-effective.
Recent advances in nanofabrication technology made it possible for Keith's team—including expert Joost Vlassak—to move from theory into practical experiments by building nanoscale devices with high precision. The researchers created small-scale structures and measured how much force acted on them under different conditions before comparing results with predictions about performance at upper-atmosphere pressures.
“This paper is both theoretical and experimental in the sense that we reimagined how this force is calculated on real devices and then validated those forces by applying measurements to real-world conditions,” said Schafer.
One experiment demonstrated a one-centimeter-wide structure levitating at an air pressure similar to what exists 30 miles above Earth when illuminated with just over half typical sunlight intensity.
“This is the first time anyone has shown that you can build larger photophoretic structures and actually make them fly in the atmosphere,” said Keith.
Potential uses include deploying sensors onboard these floating devices to gather wind speed, pressure, and temperature data from hard-to-study regions—information important for calibrating climate models used in forecasting weather patterns and projecting climate change impacts. Other applications could involve creating telecommunications arrays capable of transmitting data similarly to low-orbit satellites but with lower latency due to their closer proximity to Earth's surface.
Because Mars’ thin atmosphere shares some properties with Earth’s upper layers, these levitating disks could also support planetary exploration or communications there.
Next steps for the research team include adding communication payloads so future versions can transmit real-time data during flight.
“I think what makes this research fun is that the technology could be used to explore an entirely unexplored region of the atmosphere. Previously, nothing could sustainably fly up there,” Schafer said. “It’s a bit like the Wild West in terms of applied physics.”
Funding came from several sources including Harvard University's Star-Friedman Challenge for Promising Scientific Research; MRSEC (supported by National Science Foundation); Harvard Grid Accelerator; and CNPq (Brazil).