Large amounts of existing space debris pose a threat to satellites, space vehicles, and astronauts aboard those vehicles. However, cleaning up the debris is problematic. For example, suction cups don't work in a vacuum, and traditional sticky substances like tape are largely useless because the chemicals they rely on can't withstand extreme temperature swings.
To address the problem, researchers from Stanford University and NASA's Jet Propulsion Laboratory (JPL) combined gecko-inspired adhesives and a custom robotic gripper to create a device for grabbing space debris. Mark Cutkosky, professor of mechanical engineering, explains that the gripper is “an outgrowth of work we started about 10 years ago on climbing robots that used adhesives inspired by how geckos stick to walls.”
The adhesives developed by the Cutkosky lab have previously been used in climbing robots and even a system that allowed humans to climb up certain surfaces. They were inspired by geckos that can climb walls because their feet have microscopic flaps that, when in full contact with a surface, create a Van der Waals force between the feet and the surface. These are weak intermolecular forces that result from subtle differences in the positions of electrons on the out-sides of molecules.
The flaps of adhesive on the gripper are only sticky if the flaps are pushed in a specific direction, so making it stick requires a light push in the right direction. This is a helpful feature for the kinds of tasks a space gripper would perform.
“If I tried to push a pressure-sensitive adhesive onto a floating object, it would drift away,” said Dr. Elliot Hawkes, a visiting assistant professor from the University of California, Santa Barbara. “Instead, I can touch the adhesive pads very gently to a floating object, squeeze the pads toward each other so that they're locked, and then I'm able to move the object around.” The pads unlock with the same gentle movement, creating very little force against the object.
The gripper the researchers created has a grid of adhesive squares on the front, and arms with thin adhesive strips that can fold out and move toward the middle of the robot from either side. The grid can stick to flat objects, like a solar panel, and the arms can grab curved objects, like a rocket body.
One of the biggest challenges of the work was to make sure the load on the adhesives was evenly distributed, which the researchers achieved by connecting the small squares through a pulley system that also serves to lock and unlock the pads. Without this system, uneven stress would cause the squares to unstick one by one, until the entire gripper let go. This load-sharing system also allows the gripper to work on surfaces with defects that prevent some of the squares from sticking.
The group also designed the gripper to switch between a relaxed and rigid state. “Imagining that you are trying to grasp a floating object, you want to conform to that object while being as flexible as possible, so that you don't push that object away,” explained Hao Jiang, a graduate student in the Cutkosky lab. “After grasping, you want your manipulation to be very stiff, very precise, so that you don't feel delays or slack between your arm and your object.”
The group first tested the gripper in the Cutkosky lab. They closely measured how much load the gripper could handle, what happened when different forces and torques were applied, and how many times it could be stuck and unstuck. Through their partnership with JPL, the researchers tested the gripper in zero gravity environments, including a small gripper that went up in the International Space Station (ISS).
Next steps for the gripper involve readying it for testing outside the space station, including creating a version made of longer lasting materials able to hold up to high levels of radiation and extreme temperatures. The current prototype is made of laser-cut plywood and includes rubber bands, which would become brittle in space. The researchers will have to make something sturdier for testing outside the ISS, likely designed to attach to the end of a robot arm.
Back on Earth, Cutkosky also hopes that they can manufacture larger quantities of the adhesive at a lower cost. He imagines that someday gecko-inspired adhesive could be as common as Velcro.