Researchers Levitated a Small Tray Using Nothing but Light
Their simulations estimated that a 6-centimeter plate could carry 10 milligrams of cargo in the mesosphere under natural sunlight. Ten milligrams may not sound like much; a drop of water weighs five times as much. But engineering advances have shrunk silicon chips into dust-sized sensors far smaller than that. These “smart dust” systems can fit a power source, radio communication, and a data-collecting sensor in cubes only a millimeter across. “Researchers can do a lot when you give them a cubic millimeter of silicon,” says Bargatin. “And a cubic millimeter of silicon weighs a couple of milligrams.”
In their vacuum chamber test, they found that when cranking the light intensity up past the power of sunlight, that extra rush of energy carried the flyer higher. But after about 30 seconds, the disk began curling up from photophoretic force, eventually collapsing. Ultrathin Mylar is very flimsy on its own, says Bargatin. The shag of carbon nanotubes makes the Mylar disk more rigid, but the force of high-speed molecular collisions eventually buckles the flyer. The team’s model can predict what disk sizes, air pressures, and light intensities cause this, and Bargatin says work to develop a lightweight frame is ongoing.
Bargatin envisions researchers one day releasing sensor-laden levitators in the mesosphere and letting them roam, like weather balloons or floating ocean sensors. “Another approach is to actually develop smart flyers that can control where they’re going,” he says. The same tilting that stabilizes the levitators could be used to steer them. And, he adds, suspending the sensor from the levitator like a parachuter hanging from a canopy would help keep the system upright when faced with wind.
Still, Marsh is not convinced that such a device could withstand mesospheric conditions. “Any instrument is going to have to operate in the extreme conditions of the mesosphere, where the average winds can easily exceed 100 mph,” he writes. Winds in the upper mesosphere can be especially shearing, temperatures can drop to 140 below zero, and space weather radiates through the mesosphere and can damage communication systems.
Paul Newman, chief scientist of Earth Sciences at NASA’s Goddard Space Flight Center, agrees that accounting for mesospheric wind will be a big technical challenge, but he can’t help but delight at the possible applications. “I actually think this is a really cool idea,” he says. One possibility would be to probe water vapor in the mesosphere, where polar clouds form so high that the sun still illuminates them at night. The mysterious clouds aren’t just beautiful, Newman says; their possible link to increased greenhouse gases means they may become more common—but researchers can’t track the mesosphere’s water content and temperature as well as they’d like. Mesospheric clouds are “another sign of climate change. And we need information to show that,” Newman says. “That’s why these could be really cool for getting data on atmospheric composition.”
Newman adds that the plates’ tininess and levitation ability could also be intriguing for Mars research. The air pressure of the Martian atmosphere is similar to Earth’s mesosphere, so perhaps light, autonomous levitators could collect temperature or composition measurements. “You can just take off once per day, and go up and then come back down and land on your little Martian lander,” he imagines. “We don’t have that information on Mars. That would just be fantastic.” (NASA is planning to test out a small helicopter called Ingenuity as part of its soon-to-land Perseverance rover mission, but that craft will be much bigger and is still in the test flight stage; it’s not ready for science missions yet.)
Bargatin says they are currently exploring applications for Mars, and that the team is also hoping to make their microflyers work at sea level on Earth. But regardless of any eventual use, Azadi will always remember seeing the Mylar creation float for the first time, exactly according to his theoretical predictions. “After that,” he says, “I called my girlfriend and I said, ‘I think I’m going to graduate soon.’”
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