In 2014, the International Space Station had to move three times to avoid lethal chunks of space debris. The problem also threatens crucial and costly satellites in orbit. So what is the scale of the space junk problem, and what can we do about it?

Image copyright Science Photo Library Image caption More than 5,000 launches since the start of the space age have left Earth orbit increasingly congested and contested

Forty-five years ago the associate director of science at Nasa's Marshall Space Flight Center, Ernst Stuhlinger, an original member of Wernher von Braun's Operation Paperclip team, was asked by Sister Mary Jucunda, a Zambia-based nun, how he could suggest spending billions of dollars on spaceflight when many children were starving on Earth.

Today, Stuhlinger's response still provides a powerful justification for the costs associated with space research.

"It is certainly not by accident that we begin to see the tremendous tasks waiting for us at a time when the young space age has provided us the first good look at our own planet," he said.

The systems on the space station that provide vital life support are also responsible for its unique vulnerability to a debris impact - a pressurised module in a vacuum might behave like a balloon if punctured

"Very fortunately though, the space age not only holds out a mirror in which we can see ourselves, it also provides us with the technologies, the challenge, the motivation, and even with the optimism to attack these tasks with confidence."

In the intervening years, the maturing space infrastructure has supported our new and ongoing efforts to tackle global health, hunger, poverty, education, disaster risk reduction, energy security and climate change.

Indeed, we have made great use of Stuhlinger's "mirror" to meet many of society's biggest challenges.

Sadly, the space environment has borne the brunt of our increasing reliance on satellites and our long-lived belief that "space is big".

More than 5,000 launches since the start of the space age, each carrying satellites for Earth observation, or communications, for example, have resulted in space becoming increasingly congested and contested. The issue has been examined for a BBC Horizon documentary on BBC Two.

Image copyright US AIR FORCE / Bennie J Davis III Image caption The US has a network of sensors, such as this 3.67m telescope in Hawaii, to track satellites and debris

Now, the US Space Surveillance Network is tracking tens of thousands of objects larger than a tennis ball orbiting above us, and we suspect that there are one hundred million objects larger than 1mm in the environment.

Due to their enormous orbital speed (17,000 mph), each one of these objects carries with it the potential to damage or destroy the satellites that we now depend on.

Red Conjunction

Perhaps the most visible symptoms of the space junk problem are the regular collision avoidance manoeuvres being performed by the International Space Station (ISS), and the increasingly frequent and alarming need for its occupants to "shelter-in-place" when a piece of junk is detected too late for a manoeuvre.

The systems on the ISS that provide vital life support are also responsible for its unique vulnerability to a debris impact - a pressurised module in a vacuum might behave like a balloon if punctured.

The recent "red conjunction" (where a piece of debris comes close enough to pose a threat to the space station) involving a fragment from a Russian satellite on 17 July this year was yet another demonstration of the growing threat from space junk.

Image copyright NASA Image caption Astronauts aboard the ISS shelter in the Soyuz capsule when a piece of junk is detected too late to manoeuvre

Thanks to the hit film "Gravity", and the Oscar-nominated performance of Sandra Bullock, we can now readily appreciate the anxiety that must be felt by the astronauts and cosmonauts aboard the International Space Station whenever they receive such a "red conjunction" call.

In spite of these occurrences, the space station is actually orbiting at an altitude where the number of debris is relatively low.

At higher altitudes the amount of space junk is substantially greater, but only robotic spacecraft are exposed there. Nevertheless, these satellites are some of the most valuable for understanding our planet. Due to this congestion, there is an increasing chance that the space junk population could become self-sustaining.

That is, more junk could be created by collisions than is removed through the natural decay caused by atmospheric drag. Indeed, we already have some experience of this: in February 2009 two relatively small satellites collided over Siberia creating about 2,000 new fragments that could be tracked, with many still orbiting today and regularly passing close to other satellites.

Space junk in numbers

Image copyright NASA Image caption In 2007, a chunk of space debris punched this hole in the radiator panel of space shuttle Endeavour

500,000 pieces of space debris between 1 and 10cm

More than 21,000 pieces larger than 10cm

More than 100 million pieces below 1cm

Most orbital debris is within 2,000km of the Earth's surface

The greatest concentrations of debris are found at 750-800km

Travel up to speeds of 28,163 km/h (17,500 mph)

Only 7% of space junk is functional

Sources: Nasa, Esa

The Kessler Syndrome

Self-sustaining collision activity is something else that the film Gravity showed us. Dubbed the "Kessler Syndrome" after the Nasa scientist Don Kessler (now retired) who recognised and described this process with Burton Cour-Palais in 1978, such a scenario is a real - albeit often exaggerated - possibility.

Concerns of an uncontrollable growth of the space junk population and the loss of key satellites that enable us to address our society's problems have prompted scientists to look for ways to remove junk from space: If we can remove the problematic junk, then we can stall or even prevent the Kessler Syndrome.

This is no easy task, however, requiring new technologies, potentially new laws and - crucially - financial investment. The European Space Agency (Esa) is taking the lead, working on a mission it calls "e.Deorbit" that has the objective of removing a large European satellite from space.

Image copyright Warner Bros Image caption The 2013 film Gravity, starring Sandra Bullock, depicts a collision cascade in orbit

The mission is ambitious; numerous technologies have been developed and assessed, including a solution based on a harpoon proposed by UK engineers from Airbus Defence and Space. It is also not without risk, but a successful outcome will surely show the space-faring world that a technical solution to the space junk problem exists, even if the political, legal and financial issues have yet to be solved.

The e.Deorbit mission will face key hurdles in 2016: its systems requirements review and the Esa Ministerial Council meeting, where approval (and funding) to proceed with the mission will be debated.

Tracking systems

Objects as small as 3mm can be detected by ground-based radars

Assessment of the particles smaller than 1mm is predicted by examining the impact surface of returned spacecraft - limited to those at altitudes below 600km

Air Force space junk surveillance currently tracks 21,000 pieces of space debris greater than 10cm

The US federal government has invested approx. $1bn on a new space tracking device - the Space Fence radar system, which can track up to 200,000 pieces of smaller debris

Small satellites: the future?

Against the background of an increasing space junk problem, a renaissance is now taking place in space; what was the principal domain of governments and space agencies, with their large, multi-billion dollar satellites, is becoming the province of an emerging industry that is revolutionising the use of space.

Diminutive companies and start-ups, in particular, are showing how small budgets do not necessarily mean small ambitions. For example, San Francisco's Planet Labs, are using "cubesats" to redefine the market for Earth imagery. Their Dove satellites are smaller than a briefcase, yet have the capability to deliver high-resolution images of the Earth for a multitude of purposes.

Image copyright Science Photo Library Image caption There are concerns that the proliferation of small, low-cost satellites could exacerbate the problem

With plans by other companies, including SpaceX and OneWeb, to develop large constellations of small, low-cost satellites, there is some concern within space agencies about the long-term consequences of the ubiquitous and rapid commercialisation of space. In particular, these concerns focus on the abrupt increase in the number of satellites orbiting the Earth, which could substantially increase the need for collision avoidance manoeuvres and hasten the onset of the Kessler Syndrome.

'Super wicked problem'

In 2014, Brian Weeden, a technical adviser for the Secure World Foundation, described space junk as a "super wicked problem." Such problems, he explained, are particularly challenging to solve because time is running out, there is no central authority providing guidance or support, those seeking to solve the problem are also causing the problem, and the solutions are left for future generations to find.

The critical first step in tackling super wicked problems is to expand the group of people who support measures that reduce the risk. Indeed, there are encouraging signs that both old and new space actors understand the need to mitigate negative impacts of their activities in space and to limit the consequences for other space users.

Several companies, including Planet Labs and OneWeb have affirmed their commitment to tackle the space junk problem in the public domain. However, much work is still needed to fully understand the problem, develop technologies (such as e.Deorbit), remove legal and political barriers, and to increase awareness. The Kessler Syndrome remains an ever-present threat.

The space age has enabled global solutions to some of society's biggest challenges, just as Ernst Stuhlinger described in his letter to Sister Mary Jucunda. It has also held out a mirror and shown us that a continuing disregard for the space environment will surely affect our ability to deliver these solutions, with potential consequences for millions of people.

Hugh Lewis is a Senior Lecturer in Aerospace Engineering at the University of Southampton. He is also a member of the UK Space Agency delegation to the Inter-Agency Space Debris Coordination Committee and a member of the UK delegation to the United Nations Committee on the Peaceful Uses of Outer Space.

HORIZON: The Trouble With Space Junk is on BBC Two at 2000 on Wednesday 5th August.