More than 500,000 pieces of space debris orbit Earth, traveling at speeds up to 175,000 mph. A small piece of space debris traveling at these velocities could significantly damage a spacecraft or a satellite. It could also pose a threat to the lives of astronauts living on the International Space Station. Scientists at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., are developing a gripping system that could grip objects like orbital debris or old satellites that are difficult to handle. What is the inspiration behind this specific grappling system? Gecko feet.
Not to be confused with the five geckos that died aboard a Russian satellite back in September, these geckos will not be going to space. However, a gripping system modeled after the tiny hairs found on the bottom of geckos’ feet could be going into space one day. This new system of adhesive gripping tools, or gecko gripper project, could potentially make it easier to capture dangerous space debris.
“Orbital debris is a serious risk to spacecraft, including the International Space Station,” said Aaron Parness, a JPL robotics researcher and principal investigator for the grippers. “This is definitely a problem we’re going to have to deal with. Our system might one day contribute to a solution.”
The systems of tiny hairs on geckos’ feet help it cling to walls and rough surfaces with ease. According to NASA, “geckos’ feet have branching arrays of tiny hairs, the smallest of which are hundreds of times thinner than a human hair. This system of hairs can conform to a rough surface without a lot of force. Although researcher cannot make a perfect replica of the gecko foot, they have put ‘hair’ structures on the adhesive pads of the grippers.”
Wedge-shaped with a slanted, mushroom-shaped cap are the synthetic hairs, also known as “stalks.” When the gecko lightly touches part of an object with its foot, only the very tips of the hairs on the gripping pad make contact with the surface.
Force is applied to the gripping pad material in a way that makes the hairs bend. This creates more room for contact between the hairs and the surface of what it’s sticking to, resulting in greater adhesion.
“The stickiness of the grippers can be turned on and off, by changing the direction in which you pull the hairs,” Parness said.
The stickiness turns off when force is withdrawn and hairs are no longer bent. This means that gecko’s feet are not permanently sticky. This is because of a phenomenon called van der Waals forces.
Van der Waals forces, named for physicist Johannes Diderik van der Waals, explain how these forces happen. NASA states “these temporary adhesive forces happen because electrons orbiting the nuclei of atoms are not evenly spaced, creating a slight electrical charge. Such forces persist even in extreme temperature, pressure and radiation conditions.”
According to a Stanford publication on gecko feet structure, it wasn’t until 2000 that it was discovered that the adhesion was due to van der Waals forces created between the spatula and the surface of a gecko’s toepad. Researchers hypothesized that suction, friction, and electrostatic forces were the origin of cohesion, until Robert Full of the University of California, Berkeley, determined it was van der Waals forces.
The publication explains: “van der Waals forces are intermolecular forces created by induced polarizations of molecules. Though weak and negligible in most considerations, van der Waals forces become significant on the micro and nanoscale. In the case of gecko feet, the spatulae are so small and get so close to the surface that an attractive van der Waals force of around 0.4 µN develops between a single spatula and a surface. While a seemingly insignificant number, the combined force of the millions of spatulae on a single gecko foot produce an adhesion force of around 10 N, or 2.25 lbs. Considering a gecko foot has an area around only 100 mm^2, it was inevitable that scientists would attempt to mimic the power and efficiency of such a material.”
Parness believes the system could grab space debris that is spinning or tumbling, and difficult to hold on to.
“The reliability of van der Waals forces, even in severe environments, makes them particularly useful for space applications, “ Parness said.
The project was selected for a microgravity test flight through the Flight Opportunities Program of NASA’s Space Technology Mission Directorate in August. The grippers were used in short periods of weightlessness aboard NASA’s C-9B parabolic flight aircraft.
The grippers were able to latch onto a 20 lb. floating cube. They were also capable of grappling a scientist wearing a vest representing a 250 lb. object, made of spacecraft material panels. Members of the research team held the device with adhesive pads. Eventually the grippers will be integrated into a robotic arm or leg.
The grippers have performed successfully on a number of tests. They have been tested on more than 30 spacecraft surfaces at JPL. They have been tested successfully in a thermal vacuum chamber, with conditions like temperature of minus 76 degrees Fahrenheit (minus 60 degrees Celsius) to simulate the environment of space. The gecko grippers were tested in over 30,000 cycles of “on” and “off,” with the adhesive working perfectly. These separate tests took place while Parness was in graduate school at Stanford University in Palo Alto, Calif., and several new prototypes have been designed since then.
There are more than 21,000 pieces of space debris larger than 3.9 inches (10 centimeters) in Earth’s orbit traveling at speeds up to 17,500 mph. Approximately 500,000 pieces of debris are the size of a marble or larger. Many millions of pieces of space debris are so small they can’t be tracked. NASA warns the non-trackable space debris’ are often the most dangerous.
“The greatest risk to space missions comes from non-trackable debris,” said Nicholas Johnson, NASA chief scientist for orbital debris, in a September 2013 NASA news release on Space Debris and Human Spacecraft.
The U.S. Space Surveillance Network tracks the space junk orbiting our planet. It is impressive so few collisions have occurred considering the amount of debris orbiting Earth.
In February 2009, a defunct Russians satellite destroyed a functioning U.S. Iridium commercial satellite after colliding with it and added over 2,000 more pieces of trackable space debris orbiting Earth.
In 2007, China’s anti-satellite test, using a missile to destroy an old weather satellite, created over 3,000 pieces of space debris.
In 1996, debris from a French rocket that had exploded a decade earlier hit and damaged a French satellite.
NASA points out that space shuttle windows have been replaced due to damage caused by tiny pieces of space debris. The debris collected was later analyzed and proven to be paint flecks. Paint flecks traveling at these velocities can damage a spacecraft.
The gecko gripper project at NASA JPL is a forward-thinking technology that could be used in a variety of tasks. The grippers could grapple orbital debris, as well as perform spacecraft inspections or help dock small satellites to the ISS. The gripping system is a technology that can make space travel a safer experience for humans.