There are millions of satellites in our Earth's orbit. Almost all of them are junk. How can this be cleaned up? And why?
The junk cloud
My post yesterday, CubeSats: Tiny Payloads, Huge Payoffs, about tiny, affordable satellites, raised a good question (from 
We've had space junk since 1957, the year that Sputnik 1 was launched. But it wasn't Sputnik people saw shining brightly in the night sky, it was the body of the rocket that launched it into space.
The US government currently tracks some 24,000 objects, down to 10 cm in size. 18,700 of these are listed in Space Track (you need to create an account to view). As of July 2013, more than 170 million bits of debris smaller than 1 cm and about 670,000 pieces of debris 1–10 cm are whizzing in orbit around the earth. That's a lot of junk.
We never see this debris in any pictures, of course. The earth has a circumference of just over 40,000 km and a diameter of just under 12,800 km. So no piece of space junk will even make it onto a pixel. But they do form a very thin cloud:
credit: NASA
But that doesn't mean this junk is not hazardous. Recently, in August 2018, the International Space Station (ISS) sprung a "minute pressure leak", according to NASA. They found the leak: a 2mm wide hole in the Soyuz spacecraft attached to the Russian part of the space station. The likely culprit is a piece of space junk. Tiny as it might be, when travelling at more than 28,000 km per hour, it packs a lot of energy. A hit by a 10 cm sphere of aluminium would be akin to detonating 7 kg of TNT.
Here's a picture of the hole:
credit: ESA
The Kessler syndrome
The real danger is space junk spiralling out of control as one collision breaks the debris into smaller pieces creating a cascade of exponentially more collisions. This chain of events is known as a Kessler event. This leads to a Kessler syndrome where the cascade rains down into lower altitudes and the decay timescale is such that "the resulting [low Earth orbit] debris environment is likely to be too hostile for future space use".
If you've seen the 2013 film Gravity, you'll know exactly what I'm talking about. That's why Donald Trump's idea of a Space Force that would dominate space i.e.: use weapons in space, would be absolutely self-defeating. There is no way to alter the physics: if an enemy spacecraft is destroyed, all your spacecraft stand a good chance of being destroyed in a catastrophic cloud of high-energy space debris. And all the satellites that we unknowingly use every day of our lives would be destroyed, with little chance of fixing the problem since it would be just too dangerous there.
The space junk problem is getting worse by the day. And unfortunately, the CubeSats are just adding to the problem. Researchers at the University of Southampton, have modelled that 270 CubeSats launched annually could wreak greater havoc in the orbital environment than a carefully managed constellation of larger craft, such as the thousands more planned by OneWeb and SpaceX.
So what's to be done
There are a number of initiatives to clean up the junk.
The first is to make sure that all satellites are de-orbited in a controlled way soon after their operational lifetime. SpaceX and all the other major players have committed to this. The challenge is that this de-orbiting must be done with an almost 100% success rate to avoid catastrophic collisions. In 2015, 35% of all satellites were out of compliance. Of all CubeSats launched between 2003 and 2014, a fifth didn’t meet the criteria. It costs a lot of money to be compliant, and no-one enforces it.
There is a lot of interest in actively cleaning up the junk. Just recently (September 2018) a British satellite, successfully ensnared a CubeSat simulating a piece of junk in orbit using a big net. The vehicle, known as the RemoveDEBRIS satellite, deployed its onboard net, which then captured a nearby target probe that the vehicle had released a few seconds earlier. The demonstration shows that a simple idea like a net may be an effective way to clean up all the material orbiting Earth. The idea behind the net is to capture a piece of material and then drag it down to Earth’s atmosphere, where it will burn up.
Many other systems are being researched. One of them is the laser broom. It uses a ground-based laser to ablate the front of the debris, producing a rocket-like thrust which slows the object. With repeated targeting, the debris would fall enough to be dragged into the atmosphere and be burned up.
Let's hope they will be successful.
References:
https://www.space.com/36506-cubesats-space-junk-apocalypse.html
https://www.wired.com/story/the-space-junk-problem-is-about-to-get-a-whole-lot-gnarlier/
https://www.theverge.com/2018/9/19/17878218/space-junk-remove-debris-net-harpoon-collisions
https://en.wikipedia.org/wiki/Space_debris
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