Image copyright European Space Agency Image caption A visualisation of the satellites and other debris around earth

Millions of pieces of human-made trash are now orbiting the Earth. Some are tiny, others are large enough to be seen with a telescope, but all pose a risk to space craft and satellites.

And according to experts the threat is growing as space becomes more and more crowded.

Some 23,000 pieces of space junk are large enough to be tracked by the US Space Surveillance Network. But most objects are under 10cm (4in) in diameter and can't be monitored. Even something the size of a paper clip can cause catastrophic damage.

Media playback is unsupported on your device Media caption Watch space debris grow from 1950s to now

"At the moment we're not tracking stuff that small," says Brian Weeden of the Secure World Foundation, a Washington based organisation dedicated to the sustainable use of space.

"And that's important because something as small as a centimetre can cause problems if it runs into a satellite."

Collisions are rare, but half of all near-misses today are caused by debris from just two incidents. In 2007, China destroyed one of its own satellites with a ballistic missile. In 2009 an American commercial communications satellite collided with a defunct Russian weather satellite.

As recently as 2015, the debris from that collision forced the crew of the International Space Station to evacuate to the Soyuz capsule. No-one was harmed, but the debris will likely remain in the Earth's orbit for hundreds, if not thousands of years.

Image copyright NASA

Scientists are experimenting with ways to clean up space. So far, there is no space vacuum cleaner. And debris have a nasty habit of creating more debris that get exponentially smaller and harder to spot.

More than 7,000 satellites have been put into space but only 1,500 are currently functioning. And within the next decade the number could increase to 18,000 with the planned launch of mega-constellations - large groups of satellites aimed at improving global internet coverage.

"That's going to amplify the problems we have with tracking objects, predicting close approaches and preventing collisions," says Weeden. "The problem is going to become much, much harder in the next several years."

Everything travels at the same speed relative to its altitude in space. That's not a problem if everything moves in the same direction, says Weeden, but objects often follow different orbits and can cross paths - a situation known as a conjunction.

"Think of it like all the cars on a highway are doing a hundred miles an hour. If the car next to you is doing that speed you don't really notice it. But if the car coming at you is doing that speed - you'll collide at 200 miles an hour."

Image copyright ESA Image caption An impact crater in a window of the International Space Station from debris

Lauri Newman is NASA's traffic cop at Goddard Space Flight Center, Maryland. She is responsible for using military data to decide whether the space agency's unmanned craft such as satellites need to be moved to prevent a collision with debris.

"Satellites can protect themselves from things that are smaller than a centimetre by putting up extra shielding," she says. "But the things between one and 10cm - if you can't track it there's nothing you can do."

Satellite technology is essential to almost every modern convenience - from communications to GPS navigation and downloading movies on demand. It's also vital to national security.

"It affects everything," says Lt Col Jeremy Raley a program manager at Darpa, the Defense Advanced Research Projects Agency. "So I need to be able to see everything (in space) all the time and know what it is when I see it."

That's why Darpa is leading military efforts to find better ways of tracking space debris. In October last year it delivered a massive 90-ton telescope to the US Air Force at White Sands, New Mexico.

The Space Surveillance Telescope is designed to penetrate Geosynchronous orbit (GEO) which is becoming increasingly important. Communications and television satellites in GEO can remain in a fixed position above the Earth, offering uninterrupted service.

"The telescope is a big deal because it can see more objects and smaller objects. And rather than having to take time to look at an object and then look at something else, it can keep track of things on a more persistent basis," says Lt Col Raley.

Image copyright US defence department Image caption The Space Surveillance Telescope

But that level of scrutiny costs money and also raises the question of whether the US should share its data to improve space safety overall.

That was one of the issues discussed at a recent symposium in Washington organised by the Universities Space Research Association and the Space Policy Institute at George Washington University. Experts discussed who should manage space, who should be responsible for debris and whether there should be an agreed set of international guidelines for the sustainable use of space.

"There's a classic public policy, economic question here," says Weeden. "It's like pollution. It might not be worth it for you to pick up your garbage and avoid polluting the river, but there are costs to society if you don't. How do you get people to be responsible when the costs may not be borne by them?"

No single nation or entity is responsible for space although in 1959 the UN set up a Committee on the Peaceful Uses of Outer Space (COPUOS).

"There are currently 85 countries that are members of this committee and they range from space powers such as the US, Russia and China to countries like Costa Rica that don't even have a satellite in orbit but are an end user of satellite functions," says Weeden. "Getting all of those countries to agree on this stuff is a really difficult challenge."

But with more nations and commercial organisations operating in Earth's orbit and many looking beyond, such issues are becoming increasingly urgent.

Do nothing is no longer an option.