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In 60 years of space exploration, we've placed almost 7,000 satellites in orbit. Less than a third still function. The rest is dangerous junk - and their number is growing. Here's how we might mitigate the threat.
Space debris is a bit like space itself: We know it's up there, but beyond that there's so much we know we don't know.
The statistics are fun, though, if a little misleading. It's a bit of a safari crunching the numbers, but here goes.
Since 1957, the year the Soviet Union sent Sputnik into space, there have been 4,900 space launches. So far so good.
In that time we have put 6,600 satellites in orbit and/or created "an on-orbit population of more than 18,000 tracked objects." Elsewhere, ESA cites "more than 17,000 orbital objects" being tracked and catalogued by the US Space Surveillance Network. And in a third document it's 22,000 objects. But you get the idea: It's a lot.
Of the 6,600 satellites, 3,600 remain in space, and less than a third (about 1,100) are operational.
So of the 18,000 tracked objects "only 1,100 are functional spacecraft," or six percent.
The rest is junk - but no ordinary junk.
It's the fastest junk out there, traveling at speeds between 40,000 and 56,000 km/h (25,000 - 35,000 mph). At those speeds, debris as small as one centimeter can have the force of a hand grenade upon impact with spacecraft.
Scientists estimate 700,000 objects in orbit are about a centimeter in size and a further 170 million objects are larger than one millimeter. But even a "sand grain" of debris can cause a significant rip.
Another human footprint
There's a statistic that says there are 29,000 human-made objects of 10 centimeters or larger in orbit.
And another that says about 75 percent of all catalogued objects - the ones we've mapped and tracked - are in low-Earth orbit.
Together they form a dark cloud, a constant threat on our horizon. And that's a little too close for comfort.
Low-Earth orbit (LEO) is a relatively thin band at an altitude starting at just about 160-200 kilometers and stretching to 2,000 km. But it's also the most densely populated region, hosting communications, space science and Earth observation satellites.
The International Space Station is also in LEO at an altitude of about 400 km. One of the reasons LEO is such a popular destination is that it's the easiest to get to and offers a relatively stable orbit. But because of its popularity, things like the ISS are constantly under threat from space debris - some of which the station produces itself.
During a spacewalk last month, astronauts Peggy Whitson and Shane Kimbrough were halfway through installing debris shield bags when one of the shields drifted away to join all the other trash floating about.
As the name suggest, the debris shield bag would have protected the ISS from impacts … from space debris. Instead it's become part of the problem. But the shield will eventually burn up in the Earth's atmosphere.
Not all space debris is human-made. Some of it is created when objects collide. When that happens, it can result in a cascade of even smaller bits. And the more bits there are, the more collisions there are. It's known as the Kessler Syndrome, after NASA's Donald Kessler, who devised the theory.
Now for the really fun part - it's even better than the statistics.
Space scientists are eager to clean up space, especially low-Earth orbit, before space debris renders it useless. But it's not as easy as it sounds.
One of the most imaginative ideas is using nets and "lassos" to rein in space debris. NASA was once looking at using a kind of electric lasso to capture an asteroid, and the same idea may have been considered for space debris. But the Asteroid Redirect Mission has been … somewhat redirected. The Soil Moisture Active Passive mission, on the other hand, does use a "huge golden lasso," as NASA describes it. But it's actually an antenna.
Earlier this year Japan's space agency, JAXA, tested an "electrodynamic tether," the Kounotouri Integrated Tether Experiment (KITE).
The tether was 700 meters long, made from thin wires of stainless steel and aluminum. And the idea was to extend it from an ISS cargo ship, slow down orbiting debris, and then drag it to a lower orbit, where it could eventually burn up.
But in February, JAXA scientists said the mission had failed.
Space mist, solar sails and aerogel
There are other technologies that may or may not be under serious consideration, such as solar sails launched as a secondary payload with a satellite. Theoretically, the solar sail would catch the force from the sun's radiation and use that to guide a derelict object out of orbit.
In the 1990s NASA's Ames Research Center developed the idea of spraying a form of mist in an object's path to slow it down and get it to deorbit.
A substance called Aerogel has been considered for mitigating space debris. Aerogel is very low density material that can slow small particles down from orbital velocities and capture them without destroying them.
Currently, remediation-related technology development is with the NASA HQ Space Technology and the Office of the Chief Technologist - so watch that space.
Lasers and robots
European space scientists are likewise working on the idea of tethers and nets, but also robotic arms. In fact all the prospective technology is fairly universal. The e.Deorbit program aims to launch in 2023. It will target an ESA-owned derelict satellite in low orbit, capture it and then burn it up in a controlled atmospheric reentry.
Meanwhile the German Aerospace Center (DLR) has its sights on laser technology - first to seek, and eventually destroy, space debris. Zapping space debris with a laser beam would theoretically make the material on its surface evaporate. That would start to slow the object down and make it fall. By the time it's reduced to a speed of 200 meters per second, it would only be a few years before it hit the Earth's atmosphere and burnt up.
The elephant in space
Ironically, some of the biggest bits of space debris are the things we can almost see from Earth, like the International Space Station. No, I'm not saying the ISS is trash just yet. It's just that when the ISS is decommissioned, whenever that is (if it is), it's projected to fall into the ocean.
Its predecessor, the Soviet/Russian space station MIR is also "buried at sea," as are countless bits of rockets used to launch our satellites in the first place.
Whether it's debris in space or debris on Earth, it's all space waste. And what are we doing about that?
The NEO and Debris Detection Conference runs from January 22-24 in Darmstadt, Germany.