As space junk soars, science turns to nature for ideas
It was satellites that launched the space age, and it鈥檚 satellites that could bring it all crashing down.听
More than 1,300 circle above our heads, providing navigation signals, studying the planet, and beaming TV to millions. Few communications pass directly through space networks, but if not for the聽nearly three dozen atomic clocks providing reliably precise timestamps to anyone with an antenna, financial markets and cell service would quickly fall apart.
It doesn鈥檛 take much to reduce these finely tuned machines to thousand-pound wrecks. With life expectancies聽, the engineering marvels are surprisingly disposable. And as new arrivals and old relics crowd the useful, near-Earth orbits, governments and researchers seek ways to restore space鈥檚 once infinite promise. A smorgasbord of proposals includes slowing the depleted spacecraft with ground-based lasers so that they fall to a fiery end, and flinging them down directly with , and whips. As yet, though, no one鈥檚 reaching for the checkbook.
鈥淓verybody recognizes that this is a problem, and that the problem is getting worse, but it鈥檚 not clear exactly whose job it is to clean it up,鈥� says Jet Propulsion Laboratory (JPL) robotics researcher Aaron Parness.
Now, a collaboration between Dr. Parness鈥檚 JPL group and a team of Stanford engineers suggests this man-made problem might have a nature-inspired solution: gecko-like pads sticky enough to grab objects in the harsh vacuum of space.
Orbital minefield
What goes up must come down, unless it鈥檚 traveling more than 25,000 miles per hour. Of those objects that make it into orbit, air resistance eventually drags down most, but some trajectories can keep others whizzing around for decades.
The Department of Defense catalogs tens of thousands of artificial objects around Earth, but fragments too small to track . Any one of these hyper-speed projectiles could cause impact damage ranging from divots to explosions.
One big target is the International Space Station, which routinely takes hits from sub-millimeter sized particles and actively dodges larger bits .听
And smaller spacecraft are in danger too.听In 2009, a US commercial satellite tore through a Russian military satellite, each that encircled the planet.
Statistical analyses predict these crashes every five to ten years, suggesting the tipping point at which satellite shards destroy other satellites faster than the atmosphere can swallow them up . NASA astrophysicist Donald Kessler first analyzed this phenomenon, now called the Kessler Syndrome, nearly 40 years ago, and predicted that the emergence of a 鈥溾�� around Earth could someday halt space activity.
Fortunately, we won鈥檛 overnight.
鈥淭hat鈥檚 a popular misconception, that something will trigger it,鈥� Dr. Kessler explains. 鈥淚t鈥檚 more like a very slow chain reaction that plays out over tens of years.鈥澛�
Focus on 'the big stuff'
NASA鈥檚 Orbital Debris Program Office predicts the amount of softball-sized space junk to double within two centuries. Right now, collisions occur once every 10 years or so. But, as Kessler points out, such rare events don鈥檛 strictly abide by expected averages: 鈥淭here鈥檚 a significant probability that you could have four collisions within ten years, or you could have none.鈥�
And even one extra breakup makes a huge difference. Two events, the 2009 collision and a 2007 Chinese anti-satellite weapons test, account for more than a quarter of the currently tracked fragments.听
Retrieving such shards is nearly impossible, leaving only one practical solution. 鈥淕et rid of the big stuff. Get rid of the source,鈥� Kessler urges.听
Space agencies have a narrow window in which to act, and they鈥檙e trying 鈥� to some extent, at least. NASA has almost completely put a stop to spent rocket explosions, .
Thirteen agencies joined together to form the (IADC) in 1993, which issues for satellites. But these are suggestions rather than requirements, and compliance varies.
鈥淭he European Space Agency is very strict at enforcing their guidelines,鈥� Kessler says. But since NASA often grants US satellites exemptions, 鈥渢he US has not been doing very well.鈥�
The good news is, when it comes to active cleanup, some well-placed prevention could be worth thousands and thousands of cures.
Sticking points
The splintering nature of collisions means preventing even a few can significantly slow fragment growth.
NASA simulations estimate removing just two high-risk satellites per year starting in 2020 could halve the two-century growth rate; removing five could stabilize the debris cloud at current levels.
But first engineers have to overcome at least two technical hurdles. The typical defunct satellite is spinning rapidly, and lacks handles to latch onto. Any would-be space trash collector has to be able to stop the object, and then grab it.
While some engineers focus on challenging-to-control harpoons and nets, a Stanford鈥揓PL collaboration takes inspiration from some of nature鈥檚 best climbers to solve the latter with a , featured Wednesday in the journal Science.
Most sticky mechanisms break down in extreme vacuums, but geckos exploit to create a friction-like force between micro hairs on their feet and smooth surfaces. The team鈥檚 grabber mimics that behavior, resulting in a device that can a variety of shapes without pushing them away.
But deorbiting satellites is an expensive business, and Kessler worries too much focus falls on selling concepts rather than buying them. With private aerospace companies and the diminutive 鈥渃ubesats鈥� driving down the cost of space access, sharing cleanup fees has never been more feasible.
鈥淎ll you need is something like a 5 percent or 10 percent tax on each launch,鈥� he says. No country has agreed to such a fee so far.
Parness suggests the murky legal status of space complicates cleanup too. 鈥淚f there鈥檚 a Chinese satellite or a satellite from the Soviet Union and NASA goes up there and pulls on it, right now that would break international law,鈥� he explains. It鈥檚 every country for themselves.听
For now, he looks forward to testing the gecko gripper outside the ISS, and eventually collaborating with and private cubesat projects on strategies to address the space trash problem more directly.
鈥淲e鈥檙e building the technology so that whenever the decision makers decide 鈥榠t鈥檚 time,鈥� we鈥檒l be ready to go,鈥� Parness says.
