The Crookes radiometer is a familiar device that can be identified by its spinning blades. When placed in light, it produces a measurable change, making it a potential tool for launching metal probes into the upper atmosphere. However, many people misunderstand its physics.
Researchers have discovered that photophoresis, the force driving the blades to spin, can be applied to various structures as long as they’re in low-density atmospheres. The idea is to use this effect to send probes beyond the reach of balloons and satellites. While testing prototypes closer to Earth’s surface, the team has made progress.
A common misconception about the Crookes radiometer involves radiation pressure. This explanation claims that photons hitting one side create a push, making it spin in the opposite direction. However, this is incorrect due to photons bouncing off the other side, adding momentum and reversing the expected rotation.
The correct mechanism behind photophoresis relies on temperature differences between the dark side of the blade (which absorbs light) and the silvered side (which reflects light). This difference creates an excess of absorbed photons on the dark side, driving the spinning motion.
Source: https://arstechnica.com/science/2025/08/misunderstood-photophoresis-effect-could-loft-metal-sheets-to-exosphere