There have been numerous ideas floated for the best way to clean up space debris, and the latest comes from researchers at Stanford University and NASA's Jet Propulsion Laboratory (JPL), who have come up with a robotic gripper that's based on the sticky pads of the gecko. The difference is that instead of helping a lizard to stick to window panes, this gripper is designed to gently latch onto dead satellites and other debris.

There are over 500,000 pieces of debris or "space junk" orbiting the Earth, ranging in size from old launch vehicles and dead satellites down to flecks of paint. Traveling at tens of thousands of miles per hour, even the smallest of these can strike with the force of a meteor, and if a large one should hit a satellite or, worse, a manned spacecraft, the impact could turn them both into deadly clouds of shrapnel.

One major problem with managing this hazard is that in order to move something, for example, a dead communications satellite, you need to get hold of it first. However, a dead spacecraft will very likely be tumbling, not in very good shape, and certainly won't cooperate. Snares, nets, and harpoons might cause the target to react by moving in who knows what direction and magnetic tugs only work on magnetic materials.

Adhesives seem like a good alternative, but the Stanford/JPL team says that sticky stuff doesn't do very well in space. Some adhesives freeze solid in the cold of space or melt in the sun's rays. They can also boil away, evaporate, dry out, or denature into a useless goo or powder. Worse, many adhesives only work if they're pressed against an object, and in space Newton's first law means that you may just end up pushing a dead satellite around.

Researchers combine gecko-inspired adhesives and a custom robotic gripper to create a device for grabbing space debris(Credit: Stanford University)

The Stanford approach is to use gecko-inspired adhesives that were first developed for climbing robot experiments about 10 years ago. Similar to a gecko's foot pads, the adhesive is a sheet of rubbery polymer embossed with little flaps. Though these flaps are 40 micrometers across compared to a gecko's 200 nanometers, they still have a surprisingly strong grip due to the weak intermolecular forces caused by differences in the positions of electrons on the outsides of molecules, known as the Van der Waals force.

At the end of the day, this means that when the adhesive material is gently placed on an object and the flaps pushed in a specific direction, it grips as the flaps press together. Another gentle motion, and it neatly releases. The trick is to press the adhesive against the object in such a way that the forces are applied evenly. Otherwise, the uneven stresses would cause it to come away and drop the object.

This is why the space debris catcher the team has developed is a robotic hand rather than just a sticky pad. The gripper has a grid of adhesive squares on its front and the arms that are controlled by a microprocessor. This allows the gripper to evenly press against both flat and curving objects in a hugging fashion that evenly distributes the forces, resulting in a final firm, stiff grip.

So far, the gripper has been subjected to laboratory tests to measure how well it can handle various loads under different forces and torques. It's also been tested at JPL's Robodome, which is basically a giant air hockey table that simulates a two-dimensional weightless environment.

"We had one robot chase the other, catch it and then pull it back toward where we wanted it to go," says visiting professor Elliot Hawkes. "I think that was definitely an eye-opener, to see how a relatively small patch of our adhesive could pull around a 300 kg (661 lb) robot."

The gripper was then tested on a special airplane making parabolic trajectories that briefly produce free fall conditions that allowed the device to grasp and let go of a cube and a beach ball in zero-g. This was followed by tests aboard the International Space Station.

The team now hopes to test the gripper in the vacuum of space on the end of one of the station's robotic arms, but this will require building an improved version because the current plywood and rubber band construction would soon fail in that harsh environment.

Meanwhile, the team will concentrate on how to manufacture the adhesive in industrial quantities that could make it as common tomorrow as Velcro is today.

"There are many missions that would benefit from this [technology], like rendezvous and docking and orbital debris mitigation" says Aaron Parness, group leader of the Extreme Environment Robotics Group at JPL. "We could also eventually develop a climbing robot assistant that could crawl around on the spacecraft, doing repairs, filming and checking for defects."

The research was published in Science Robotics.

The video below discusses the gecko-based robotic manipulator.

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