Designed for flight forty-five miles above the Earth’s surface, Harvard SEAS researchers have devised a nanofabricated lightweight structure capable of sunlight-driven propulsion through a process called photophoresis, capable of monitoring one of Earth’s most challenging locations to navigate.
Stretching between 30 and 60 miles above the Earth’s surface, the mesosphere has proven extremely difficult to study, as the altitude is too high for planes and balloons, yet too low for satellites. Achieving regular direct access to this long-out-of-reach portion of the atmosphere could be a major boon to improving weather forecasts and climate model accuracy.
Now, a new breakthrough technology could make it possible, by allowing lightweight structures to reach largely unexplored heights powered by sunlight alone.
Photophoresis
Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), the University of Chicago, and other institutions worked on the project, which was revealed in a new paper published in Nature.
“We are studying this strange physics mechanism called photophoresis and its ability to levitate very lightweight objects when you shine light on them,” said lead author Ben Schafer, a former Harvard graduate student at SEAS, now a professor at the University of Chicago.
Photophoresis is a physical process where gas molecules bounce off of an object’s warmer side more forcefully than its cooler side in extremely low-pressure environments. One such environment is the difficult-to-reach mesosphere.
Designing a Mesosphere Probe
Co-author David Keith first envisioned such a photophoretic device over a decade ago as a tool in the fight against global warming. To bring the concept to life, Keith enlisted Schafer, then one of his graduate students, and Joost Vlassak, the Abbott and James Lawrence Professor of Materials Engineering at SEAS. Recent nanofabrication advances finally provided the team with the necessary precision required to make a practical photophoretic flight device.
“We developed a nanofabrication process that can be scaled to tens of centimeters,” Vlassak said. “These devices are quite resilient and have unusual mechanical behavior for sandwich structures. We are currently working on methods to incorporate functional payloads into the devices.”
Former Harvard postdoctoral fellow Jong-hyoung Kim led the device design and fabrication. The team fabricated thin membranes of ceramic alumina, covered with a bottom layer of chromium. As the chromium on the bottom absorbs sunlight, it becomes heated to a temperature greater than the top layer, producing a photophoretic lifting force powerful enough to overcome the object’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.”
Flight Testing
“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,” Schafer said.
To test their centimeter scale device, the team utilized a low-pressure chamber built in Vlassak’s lab by Schafer and Kim. The air pressure in the chamber was set to 26.7 Pascals, the same as that of 60 miles above the Earth’s surface, and the device was subjected to light at 55% of the intensity of sunlight.
“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. “It opens up an entirely new class of device: one that’s passive, sunlight-powered, and uniquely suited to explore our upper atmosphere. Later they might fly on Mars or other planets.”
Uses for their innovation are many, according to the team. Adding lightweight sensors would allow for finally investigating the wind speed, pressure, and temperature of the mesosphere. More data from this region would prove essential to fine-tuning climate models for improved weather forecasting and longer-term climate change predictions.
Additionally, the devices could form a low-latency competitor to Starlink, with their much closer proximity to the surface improving network speeds. Uses are not limited to Earth, as the devices may be perfect for exploring low-pressure environments such as Mars. However, getting to the development of such potential uses is the next step, as the team is presently working on adding onboard communications systems to their tiny airships.
“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.”
The paper, “A Bird’s-eye View of Stellar Evolution through Populations of Variable Stars in Galactic Open Clusters,” appeared in Astronomy and Astrophysics on August 13, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.