Between 30 and 60 miles above Earth’s surface lies a largely unstudied stretch of the atmosphere.
It’s too high for airplanes and weather balloons, too low for satellites and nearly impossible to monitor with existing technology. But understanding this layer of the atmosphere could improve the accuracy of weather forecasts and climate models.
A new study published in Nature by researchers at Harvard University, the University of Chicago, Pukyong National University and Universidade Federal do Paraná introduces a novel way to reach this unexplored near-space zone: lightweight flying structures that can levitate using nothing but sunlight.
“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 formerly a graduate student at Harvard in the research group of Professor David Keith, who is now a UChicago professor in the Department of Geophysical Sciences and the faculty director of the Climate Systems Engineering initiative.
Keith said this mechanism allows the structures to passively float in a region of the atmosphere where no other aircraft or device can sustain flight.
“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.”
A lift from light
Photophoresis occurs when gas molecules bounce more forcefully off the warm side of an object than the cool side, creating continuous momentum and lift. This effect only happens in extreme low-pressure environments, which are exactly the conditions found in the upper atmosphere.
The researchers built thin membranes from processed aluminum oxide, with a layer of chromium on the bottom to absorb sunlight. When light hits this structure, the heat difference between the top and bottom surfaces initiates a photophoretic lifting force, which exceeds the structure’s weight.
“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 more than a decade ago when Keith invented new designs of photophoretic nanoparticles that might be an alternative to using aerosols for sunlight reflection methods to reduce climate change caused by greenhouse gas emissions. In that 2010 paper published in PNAS, Keith hypothesized that photophoretic particles could have much longer lifetimes in the atmosphere and could also be more cost-effective than sulfate aerosols, which can have negative impacts on stratospheric chemistry.
But that previous work was all theory, and Keith was looking for ways to do something more practical. He recruited Ben Schafer as a graduate student and enlisted his colleague Joost Vlassak, an expert in nanofabrication, to help move the concept from theory to experiment. Their collaboration became feasible through recent advances in nanofabrication technology, which allow researchers to build low-mass, nanoscale devices with greater precision.
Using these fabrication methods, the research team created small-scale structures and directly measured the photophoretic forces acting on them under various conditions. They then compared those results to predictions of how such a structure would behave under the low-pressure conditions of the upper atmosphere.
“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.
A key experiment detailed in the paper shows a 1-centimeter-wide structure levitating at an air pressure of 26.7 Pascals when exposed to light at just 55% the intensity of sunlight. These conditions are similar to those found 30 miles above the Earth’s surface.
“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.
The team envisions a range of possible applications for their new device, especially in climate science. If equipped with lightweight sensors, this device could collect key data such as wind speed, pressure and temperature from a region of the atmosphere that has long remained a blind spot. This data is critical for calibrating the climate models that build the foundation of weather forecasting and climate change projections.
Other potential applications include telecommunications for defense and emergency response scenarios. Using a fleet of these devices could enable a floating array of antennas with data transmission capabilities comparable to low orbit satellites like Starlink, but with lower latency due to their closer proximity to the ground, the researchers said.
Since Earth’s upper atmosphere shares key characteristics with the thin atmosphere of Mars, the device could facilitate new modes of planetary exploration and communication in that environment as well.
The team’s next step is to integrate onboard communications payloads that would allow the device to 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.”
Citation: “Photophoretic flight of perforated structures in near-space conditions.” Schafer et al., Nature, 2025.
Funding: Star-Friedman Challenge for Promising Scientific Research at Harvard University and Harvard University MRSEC, which is funded by the National Science Foundation, the Harvard Grid Accelerator and the CNPq (Brazil).
