It's been 53 years and over 4,500 launches since the dawn of the space age, and Earth's orbit is a junkyard. Our orbit is littered with spent rocket stages, lens caps, broken-up satellites, frozen urine, the odd glove, bits of foil, and the tool kit dropped by astronaut Heide Stefanyshyn-Piper during a spacewalk in 2008. You name it; the low Earth orbit has probably got it.
Millions of pieces of this space debris orbit the globe at break-neck speeds, and the spacecraft that pass through orbit are in jeopardy from even the smallest objects. But while the problem is evident, the solution remains elusive. Will Earth's orbit forever resemble a scene from WALL-E? Many scientists have now turned their attention to cleaning up the clutter.
Every satellite that goes up to orbit is the pride and joy of some company, lab, or nation. But once it has outlived its purpose, it's nothing but junk.
Attaching a hi-tech kite-tail to satellites might seem too simple to work. But electrodynamic tethers could provide a low-cost, lightweight method of debris removal. Best of all, they operate without fuel. Tethers take advantage of the Earth's magnetic field, interacting with the ionosphere via 3-mile-long wires to produce a drag effect, pulling defunct satellites down to lower orbits until they burn up in the atmosphere.
Tethers Unlimited Inc. has a working prototype called the Terminator Tether that could de-orbit large satellites in a matter of months after being deployed. Pictured is an artist's concept of NASA's ProSEDS tether, a variation on this model designed as a source of propulsion.
In 2007 China conducted an anti-satellite (ASAT) test, shattering an aging Chinese weather satellite with a missile. This led to the creation of over 3,000 new debris fragments--all potentially catastrophe-causing should they collide with a shuttle or space station.
But surprisingly, for debris at low altitudes, fragmentation by ASAT device can actually be helpful in speeding orbital decay. In February of 2008, the United States used an ASAT interceptor to destroy a failed satellite at an altitude of 150 miles (compared to 537 miles for the China test). Because of greater atmospheric drag at that altitude, 99 percent of the debris re-entered the atmosphere within one week (and burned up on re-entry). While controversial, this technique can still be used to remove defunct satellites from Low Earth Orbit at altitudes below 180 miles.
As this remarkably detailed painting illustrates, ASAT weapons are very destructive. Some space agencies are pushing for an outright ban at high altitudes.
By far the greatest tool for preventing collisions is careful monitoring. NASA and the Department of Defense currently track over 21,000 objects using ground- and space-based surveillance systems, following pieces as small as 2 inches in diameter.
This detection will soon be twice as sensitive; the U.S. Air Force "pathfinder" satellite expected to launch this year will be the first-ever satellite devoted solely to tracking debris. It should help fill in holes in the data, and hopefully prevent collisions like the much-publicized crash of two satellites in 2008.
The Long Duration Exposure Facility (LDEF) satellite, shown here docking above the Space Shuttle Challenger, was another important asset, having tracked debris for six years before being decommissioned in 1990.
Among the more unconventional ideas for eliminating debris are foamy "nerf balls" or panels made of aerogel, shown here.
A man-made, super-light solid, aerogel was used as the capture medium in the Mir Environmental Effects Payload (MEEP) Orbital Debris Collector, to capture debris particles over 18 months in 2009. Its porous properties and low density make it an ideal candidate for picking up debris before being rocketed back to Earth--much like putting a layer of Jell-O on your windshield to catch bugs.
The downside? Aerogel can only be used for small debris--paint chips, metal slag and dust--most of which aren't a big threat to large spacecraft.
In space, what goes up doesn't necessarily come down. Or at least, not for many years. Anything launched above 620 miles will usually stay in orbit for a century or more. Designing satellites to re-enter the atmosphere at the end of their active life could therefore be a viable option for reducing the buildup of non-functional debris.
This isn't without its complications. While most objects that re-enter the atmosphere will burn up high above our heads, some objects can survive the heat of re-entry and can pose a risk to humans. For example, take this 150-pound titanium motor from a Delta II third stage, which landed in Saudi Arabia in 2001. The chance of falling debris causing a casualty is less than one in 10,000, since most land in the ocean or uninhabited regions. But since NASA and ESA can be held liable for damage, they'll likely be wary of any strategy that would increase re-entry rates.
Another device that could speed up the decay of orbiting clutter is the solar sail. Proponents of this technology suggest that a folded-up sail could be attached to satellites, rocket stages, or any other piece of space-going technology. When that technology reaches the end of its working life, the sail would pop open.
Once deployed, these carbon fiber sails would harness solar winds to propel objects back into the Earth's atmosphere. In March of 2010, a working prototype called the Cubesail was revealed by the University of Surrey, and is expected to be tested in a demonstration mission next year. Further afield, researchers say the Cubesail could be sent into orbit to remove existing debris.
Any spacecraft planning to spend time in the atmosphere will need to cope with hurtling clutter or be quickly destroyed. To deal with micrometeoroids (objects smaller than 10cm) shields are a necessity. For decades, the ISS and others have relied almost entirely on the Whipple shield: a multi-layered armor that absorbs the impact of small debris.
But a new shield type may soon join the armory: metallic foam. Consisting of solid metal with a high volume of open-cell, gas-filled pores, these low-density materials are advantageous for their lightweight cellular structure relative to other shield types.
Want NASA-quality protection for your skull? Metal foam in a closed-cell conformation is also used in bicycle helmets.
You can't talk space junk without mentioning lasers. Laser-based clean-up ideas range from satellite-mounted lasers that zap debris out of orbit, to the aptly named "laser broom" that sweeps debris out of the path of the International Space Station. Always cool--always costly.
On Earth, lasers help measure the effects of micrometeoroid collisions at the Hypervelocity Ballistic Range at NASA's Ames Research Center in California. Projectiles launched at speeds of up to 17,000 miles per hour pass through two laser curtains, which detect their velocity. The "energy flash" seen here is caused by the high-speed projectile striking a solid surface.
At speeds averaging 6 miles per second, even the smallest fragments can punch holes in the armor of spacecraft, and anything larger than a few inches will cause total destruction on impact, creating even more debris. In 1978, NASA scientist Donald Kessler predicted that these types of impacts would give way to something more serious--a domino effect of collisions in Earth's orbit that would make space flight impossible for generations.
Scientists are now scrambling to undo the damage caused by decades of space littering--designing satellites that withstand impacts, spacecraft that dump additional propellant to minimize explosions, and upper stages that maneuver into storage orbits at the end of their active lives. Whether these and other measures can keep us from Kessler's future, however, remains to be seen.
