Researchers are figuring out how to use a nature-inspired "gecko gripper" technology to solve the mounting problem of space debris.
Watch your backyard closely on a warm Florida morning, and you can see geckos climbing the fence to greet the Sun, looking for an insect snack before the mid-day heat sets in. The same gripping technique that geckos use to scale smooth surfaces such as fences could soon be grappling with space junk in orbit.
Hao Jiang (Stanford University) and colleagues recently proposed using “gecko grippers” to grab space debris in an effort to clean up near-Earth space. The concept was published in the June 28th Science Robotics.
An estimated half a million objects larger than a pea orbit Earth, of which 23,000 objects larger a baseball are currently being tracked by the U.S. Joint Space Operations Command. The vast majority of even the larger objects are defunct. You need only look up various designations for satellites on Heavens-Above to notice that most of them are actually discarded boosters. And the numbers are only growing — space junk is a mounting problem.
The idea of space junk wasn't a big concern early in the Space Age, and satellites were frequently placed in orbits that would last for centuries. Vanguard 1, for example, was launched in 1958 and is still in orbit as the oldest human artifact circling Earth. Now, satellites are frequently required to incorporate de-orbit capability as standard hardware — NanoSail-D2 tested just such technology in 2011, when it deployed a solar sail to reenter Earth's atmosphere. Satellites in low-Earth orbit typically reenter due to atmospheric drag, while farther-out satellites, such as those in geostationary orbits, are typically pushed out into “graveyard orbits” for disposal.
Despite those recent efforts, two events in the past decade exponentially increased the amount of space debris in low-Earth orbit (LEO): the 2007 Chinese anti-satellite missile test and the 2009 collision of Iridium-33 and Kosmos 2251.
Single collisions are not the real issue, though; the fear is that one collision has the potential to cause more. The grim scenario of a runaway cascade of collisions is known as the Kessler Syndrome, recently brought into vogue by the film Gravity, starring Sandra Bullock and George Clooney. Also sometimes referred to as an ablation cascade, such a chain reaction — while maybe not as dramatic as depicted by Hollywood — is a serious concern. The International Space Station now has to perform periodic Debris Avoidance Maneuvers (DAMs) when space junk gets too close to call, and more than once, ISS crew have had to wait out a pass in their Soyuz capsule "life boats," ready to return immediately to Earth if necessary.
Geckos in Space
While removing space debris is an obvious solution, grappling with and moving objects in space presents a challenge. Some have proposed using nets or harpoons to capture space junk, but those methods lack fine control. Grippers inspired by gecko's webbed feet, on the other hand, can grab a large object, move it, and then release it without imparting a large amount of unwanted momentum.
A gecko's toes contain rows of microscopic hairs, which increase their contact surface area as the toe presses down. This action makes use of what's known as van der Waals forces, weak forces between molecules that fade in strength as the molecules get farther apart. Though it's counterintuitive that such a weak force could stick so strongly, the contacts of millions of hairs on a gecko's toe really add up, ultimately allowing a gecko to stick to a pane of glass by a single toe.
Gecko-inspired grippers have been studied for years, both to understand how they work and to develop commercial adhesive products.
Now researchers are finding, gecko grippers are also ideal for space. In a vacuum, suction doesn't work (no air), and chemical adhesives typically lose their stickiness in the extreme temperature and radiation conditions. Magnetism also won't work on glass surfaces, such as those on solar panels. But the van der Waals forces employed by gecko grippers work fine in a vacuum. In fact, it's possible to turn their "stickiness" on and off, said Aaron Parness (NASA Jet Propulsion Laboratory) in a recent Facebook live chat, which makes the grippers reusable.
The grippers designed by the team use tiny, wedge-shaped fibers that mimic those on a gecko's foot. With a small amount of evenly applied force, the surface area and attraction of the pads increase. But if the pressure is applied slightly unevenly, the seemingly magical attraction disappears.
Early tests in the team's "robodome" were encouraging: the grippers could easily snare a 850-pound object and move it about. Jiang and colleagues also demonstrated the concept during short, zero-g parabolic flights aboard NASA's “Vomit Comet,” where the grippers grabbed and released a cube, cylinder, and a sphere easily.
Soon, gecko robots could be crawling around the exterior of the International Space Station (ISS). Early tests of the grippers were carried out aboard the ISS in 2016, and researchers envision future use of the technology on the exterior of the ISS for repairs and inspection. "We've sent a total of five grippers to space," Parness said on Facebook live.
The technology may find use on Earth in industrial applications as well, as it could carry large, delicate loads (think, for example, of large glass windshields in a factory) without exerting a lot of pressure.
Another intriguing idea that Jiang's team is looking at is to impart a tiny electric charge on the gecko pads, creating a hybrid of van der Waals forces and electrostatic adhesion. This technique could augment the system and possibly overcome some of its limitations, such as degradation by moisture and minute quantities of molecular oxygen.
So, when will we see robotic geckos grabbing satellites in orbit? Well, a technology demonstrator may fly with NASA's Restore-L mission, which aims to test refueling and repair technology for satellites in in low-Earth orbit in mid-2020. Researchers agree that early stage grippers would be better for handling large objects such as defunct satellites or large boosters versus “sweeping” up shards of smaller debris.
Pretty soon, what works for backyard lizards on a sunny day could end up providing a solution for cleaning up low-Earth orbit as well.