Photophoresis Effect Could Lift Metal Sheets Into Exosphere
▼ Summary
– The Crookes radiometer spins when exposed to light, but its operation is often misunderstood.
– The spinning is caused by photophoresis, not radiation pressure, due to temperature differences between the blade’s dark and silvered sides.
– Photophoresis can work on various structures in low-density atmospheres, enabling potential applications in atmospheric probes.
– Researchers propose using photophoresis to lift thin metal sheets into Earth’s upper atmosphere for scientific probes.
– Common explanations attributing the radiometer’s motion to radiation pressure are incorrect because they ignore momentum from reflected photons.
The photophoresis effect, a fascinating but often misunderstood phenomenon, could revolutionize how we explore Earth’s upper atmosphere. Most people have seen a Crookes radiometer, those delicate glass bulbs with spinning vanes, without realizing the complex physics behind its movement. While commonly mistaken for radiation pressure, the true driving force is photophoresis, which exploits temperature differences to generate motion. Researchers now believe this principle could propel ultra-thin metal sheets into the exosphere, bridging the gap between balloon and satellite altitudes.
Photophoresis occurs when light heats one side of an object more than the other, creating a pressure imbalance in surrounding gas molecules. In the radiometer, dark vane surfaces absorb more photons, becoming warmer than their reflective counterparts. This temperature gradient causes air molecules to rebound faster from the heated side, pushing the vanes forward. Contrary to popular belief, radiation pressure alone can’t explain the rotation, if it did, the vanes would spin backward due to photon reflection off the silvered surfaces.
Recent experiments suggest this effect isn’t limited to radiometers. Scientists have demonstrated that engineered thin-film materials, when exposed to light in low-density environments, can experience enough photophoretic lift to reach extreme altitudes. These lightweight structures could carry sensors or instruments into regions too high for conventional balloons yet too low for stable orbital insertion. Early prototypes have already shown promise in controlled tests, hinting at practical applications for atmospheric research or planetary exploration.
The implications extend beyond Earth. On worlds with thin atmospheres, like Mars or Titan, photophoretic propulsion might enable low-cost, long-duration aerial platforms without relying on heavy propulsion systems. By fine-tuning material properties and surface coatings, researchers aim to optimize lift efficiency, potentially unlocking new ways to study hard-to-reach atmospheric layers. While challenges remain, the blend of classical physics and cutting-edge engineering could soon turn this century-old curiosity into a transformative aerospace technology.
(Source: Ars Technica)
