PHOTOPHORETIC FLIGHT IN THE IGNOSPHERE, THANKS TO CROOKE’S RADIOMETER, SORTA

THERE ARE SEVERAL NEAT WORDS ABOVE gleaned from Annika Inampudi’s article “Featherweight Flyers Could ‘Levitate’ Above Earth Indefinitely,” AAAS Science, August 13, 2025. Let’s consider each of these words, then put them together to describe how “future versions of these tiny flying saucers, unveiled today in Nature, could help monitor the effects of climate change high above Earth—or even explore the atmosphere of Mars.” 

Photophoresis. From the Greek, “photophoresis” is literally “light-induced motion.” Inampudi observes, “Best known for powering the Crookes radiometer, a scientific curio in which a metallic paddle spins within a glass globe, this effect uses light to induce a temperature difference between the two sides of a thin surface. In a rarefied atmosphere, the temperature difference can be enough to propel air molecules around the surface, generating a net force.”

Crookes Radiometer. In 1873, Sir William Crookes invented this device, also known as a light mill. Wikipedia notes that it “consists of an airtight glass bulb containing a partial vacuum, with a set of vanes which are mounted on a spindle inside. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity.”

Video by Nevit Dilmen from Wikipedia.

As noted in Wikipedia, Crookes devised his light mill “as the by-product of some chemical research. In the course of very accurate quantitative chemical work, he was weighing samples in a partially evacuated chamber to reduce the effect of air currents, and noticed the weighings were disturbed when sunlight shone on the balance.”

In fact, Crookes misunderstood the physics of his light mill’s operation. Wikipedia describes “Incorrect theories,” “Partially correct theories,” and the “Currently accepted theory.” For the latter, see the YouTube “How a Crookes Radiometer Works.” And, of course, the currently accepted thesis is photophoresis.

Video from Applied Science.

The “Ignoroscope”? The atmosphere from 50 to 80 kilometers (31 to 50 miles, 164,000 to 262,000 ft) is known as the mesosphere. Science’s Inampudi says, “it’s called the ‘ignorosphere’ because it is so difficult to get hardware into that area and keep it there. The mesosphere is too dense for orbiting satellites but not dense enough to fly an airplane or a balloon.”

This and the following image by Ben Schafer and Jong-hyoung Kim from Harvard News.  

“Though understudied,” Inampudi notes, “the mesosphere is seen as a ‘miner’s canary’ for the effects of greenhouse gases on climate, because clouds within this extremely cold region are very sensitive to changes in water vapor and temperatures.”

Game Changers. To explore this region, researchers have turned to photophoresis: Inampudi quotes Ruth Lieberman, a heliophysicist who worked on early proposals for the technology though not involved in the Nature paper: The most efficient objects ever developed that can fly using this obscure physical effect, “they are the ultimate green ‘perpetual motion’ devices.”

Perforated Sheets of Aluminum Oxide. “To push photophoretic flight further,” Inampudi recounts, “Schafer and his team [of the Nature paper] made a device that consists of two perforated sheets of aluminum oxide, held together by a series of small pillars, that absorb light on one side and transmit it through the other. The resulting temperature differences are enough to propel the thin air at high altitudes around and even through the small holes in the device, keeping it aloft.”

Photophoretic thrust. 

Inampudi continues, “The mesosphere is uniquely suited to sustaining such devices: It gets enough sunlight to power the temperature difference, and it has just the right amount of air.” 

Sorta like a Crooke’s radiometer. 

And On To Mars, Maybe…. Inampudi writes, “The team also envisions sending its miniscule flyers to Mars, whose tenuous atmosphere resembles Earth’s mesosphere. University of Chicago geophysicist Edwin Kite, who was not involved with the paper, says current orbiters on Mars have done a poor job of probing the lower atmosphere. Swarms of these featherweight flyers could fill that gap, he says, by beaming back continuous information on martian weather, including wind speed, water vapor, and other climate conditions.”

Image by Schafer et al., Nature from Science. 

Alas, Inampudi recounts, “The device faces headwinds on Earth, however. Months ago, President Donald Trump’s administration asked NASA to terminate multiple satellite missions that monitor the mesosphere to gauge how greenhouse gases contribute to climate change.”

Quelle stupid near-sighted ignorant action.

“Like many other scientists facing cuts in climate and planetary research, Inampudi reports, “Schafer is looking to the private sector for support. In 2023 he started a company, Rarefied, that hopes to launch these devices into Earth’s mesosphere and develop them for commercial purposes.”

And recall all this evolved from Sir William Crooke’s light mill. ds 

© Dennis Simanaitis, SimanaitisSays.com, 2025