By Paul Rincon
Science reporter, BBC News
The sat collision in February spawned thousands of pieces of debris (red)
On 10 February this year, a defunct Russian communications satellite crashed into an American commercial spacecraft, generating thousands of pieces of orbiting debris.
At the time, some observers put the odds of such an event occurring at millions, maybe billions, to one.
But experts had been warning for years that useable space was becoming crowded, boosting the possibility of a serious collision.
They have argued both for better monitoring of the space environment and for policies aimed at controlling the production of debris.
European space officials have now initiated a project which has the ultimate aim of protecting Europe's space-based assets against such threats.
Over the past two years, a number of incidents have drawn attention to the problem of space debris.
In January 2007, China tested an anti-satellite weapons system by destroying one of its own spacecraft.
According to the US military, the A-sat test created 2,500 new pieces of debris which have been jeopardising satellites in the vicinity ever since.
In February 2008, the US used a sea-launched missile system to shoot down a wayward spy satellite loaded with fuel.
Then on 12 March this year, a close approach by a piece of debris measuring about 1cm (0.3in) forced the crew of the International Space Station (ISS) to shelter in their Russian Soyuz escape capsule.
"It's a bit like a swarm of bees in a beehive up there," said Nasa spokesman William Jeffs at the time, "the trajectories can sometimes be erratic."
There are thought to be some 18,000 objects larger than 10cm orbiting Earth, but millions more that are smaller.
Intact satellites share Earth's orbit with everything from spent rocket stages, tools lost on spacewalks and spacecraft wreckage to paint flakes and dust. They are the flotsam and jetsam of more than half a century of human activities in space.
The Russian sat involved in February's collision looked something like this
At orbital speeds of 27,000km/h (17,000mph), even tiny pieces of debris can knock out a satellite or kill a spacewalker. And as the number of pieces of debris grows, so does the threat of collisions.
Satellite shielding is effective for objects below 1cm. But beyond that size collision avoidance - commanding the satellite to move out of the way of debris - may be the most prudent option.
America has the most sophisticated system for tracking objects in orbit. Its military operates 25 centres around the world to track objects in space; together they comprise the US Space Surveillance Network (SSN).
Until now, Europe has been largely dependent on the US for knowing what is going on in space. But European observers have for some time regarded this situation as inadequate.
Space-based systems, which provide accurate weather data, telecommunications and satellite-navigation services, play an increasingly vital role in Europe's economy.
Referring to the threats facing these space-based assets, Jean-Jacques Dordain, director-general of the European Space Agency (Esa) says: "We cannot continue to develop operational space infrastructure and ignore these other elements."
The US Haystack and HAX radars track cm-sized debris
He added: "We really have to understand the environment in which our space infrastructure operates."
Thus, in November 2008, space ministers approved a 49.5-million-euro proposal to prepare the way for a European system which will stand watch over orbital debris, near-Earth objects (NEOs) and solar activity.
Together these phenomena could threaten lives and infrastructure in space and on the ground. An advanced capability to monitor such threats is known as Space Situational Awareness, or SSA.
Officials will spend three years assessing what Europe needs to develop its capabilities in space situational awareness. They will need to formulate a data security policy as well as consider what infrastructure has to be built from scratch and how existing sensors might contribute.
Radars are generally used to track objects in low-Earth orbit, while optical telescopes are often used to observe objects further away from the Earth. Electronic eavesdropping can be used to assess whether or not satellites are active - a discipline known as signals intelligence, or SIGINT.
"In 2010 and 2011, we will deploy and validate the preliminary elements of this system. This will hopefully allow us to deliver some precursor services," Nicolas Bobrinsky, proposal manager for Esa's SSA Preparatory Programme, told BBC News.
"It is not our aim to have a fully operational system by 2011, but one which will act as an 'advanced demonstrator'."
Gaele Winters, Esa's director of operations and infrastructure, adds: "There are member states in Europe with their own facilities. Esa also has some facilities. If you combine [those resources] in an intelligent way, you can reach a point where it is possible to deliver precursor services."
Comprises manmade "orbital debris" and "incident debris"
Incident debris includes particles from short-period comets
About 18,000 orbital debris objects larger than 10cm
The number between 1cm and 10cm in size estimated at 200,000
Number smaller than 1cm exceeds tens of millions
Chinese anti-satellite test in 2007 created 2,500 new fragments
A domino effect could create havoc for future space launches
These existing facilities might include France's GRAVES (The French acronym means large-scale system adapted for space monitoring) radar system, which can survey objects in low-Earth orbit up to distances of 2,000km, the Zimmerwald optical telescope observatory in Switzerland, and the Esa Space Debris Telescope in Tenerife, Spain.
These preliminary services should provide users with access to a catalogue detailing the orbits of functioning spacecraft and debris.
They should also alert satellite operators to potential collisions between their spacecraft and other objects in orbit.
In the event that a possible collision is identified, users could request a more detailed analysis of the objects' trajectories using a high power radar such as the Tracking and Imaging Radar (TIRA) in Wachtberg, Germany.
The fully fledged system is expected to provide many additional services. Richard Tremayne-Smith, a former head of space environment at the British National Space Centre (BNSC), told BBC News: "Space surveillance is one thing, but to get to space situational awareness you need more than either optical sensors for high altitudes and radar for low altitudes.
RAF Fylingdales in Yorkshire is part of the US surveillance network
"You need details from satellite operators. Otherwise, you can only guess when they might carry out station-keeping with a satellite, or when they might do an impulse burn to change the orbit it is in."
Satellite operators overwhelmingly recognise the need for better data on the melee of objects whizzing over our heads.
The US already makes available data from its Space Surveillance Network on the internet. But this US Air Force data, known as two-line elements, is of relatively low quality, with satellite positions only accurate to within 20-30km (distances which are covered in 3-4 seconds at typical low Earth orbit velocities of 7.5 km/s).
Additional uncertainties are introduced when satellite orbits are extrapolated days or weeks ahead. This is because spacecraft are perturbed by drag, solar radiation pressure and the Earth's gravity field.
The more inaccurate the initial data on a satellite's position, the more inaccurate these predictions will be.
In addition, for a satellite constellation in low-Earth orbit, two-line element data might throw up hundreds of potential collision alerts every day. Many satellite operators simply lack the financial resources to perform detailed analyses on each potential collision.
The US Air Force maintains a second, more precise database of information on the same orbital objects. But these more accurate data are deemed far too sensitive to share publicly - for fear the data could give away clues about the capabilities of US sensors.
However, keeping close tabs on all the junk up there is beyond even the resources of the US military. High accuracy surveillance is reserved only for a handful of high-value assets such as the space shuttle, the space station and multi-billion-dollar spy satellites.
In March, the ISS crew had to move into the Russian Soyuz capsule
In the longer-term, computer modelling work has identified a worrying effect called a "collision cascade", a kind of domino effect where collisions create more debris, which generates further collisions, creating even more debris.
"This is one of the reasons why the satellite operators and the community are quite deliberately trying to get their spacecraft down within 25 years from the end of the mission," says Stuart Eves, head of business development for military systems at Surrey Satellite Technology Limited (SSTL).
NEAR-EARTH OBJECTS (NEOS)
NEOs are comets and asteroids which enter Earth's neighbourhood
About 1,050 NEOs are classed as potentially hazardous to Earth
An estimated 81% of NEOs bigger than 1km are catalogued
Nasa wants to log objects as small as 140m (460ft) in diameter
Tunguska object exploded with energy of 1,000 atomic bombs
Asteroid Apophis has a 1 in 45,000 chance of striking Earth in 2036
"If you just leave things up there, the statistics show they will start banging into each other. Eventually, you reach the point where you can't sensibly launch satellites into the orbits you want because they'll get pounded to pieces."
Esa might be able to rely on existing facilities for its precursor services, but some sensors are not capable of providing the very high accuracy data required for the fully fledged SSA system.
Beyond 2011, officials acknowledge the need to build new state-of-the-art facilities to achieve high performance, including the ability to track objects down to 10cm in size.
But for true space situational awareness, it will also be necessary to track objects more frequently. This means monitoring the locations of satellites multiple times each day.
The Saphhire satellite system, which is being built by MDA for Canada's Department of National Defence will carry an optical telescope for tracking satellites in high orbits, especially geosynchronous orbit (GEO).
"The advantage of a space-based sensor is that it is above the clouds and is unaffected by the day/night cycle, so its tracking can be far more frequent," says Stuart Eves, whose company SSTL is providing the Sapphire spacecraft bus and other elements for MDA.
The Teide Observatory on Tenerife may be part of a European system
The requirements decided under the European preparatory programme will be submitted for approval by space ministers when next they meet in 2011. Esa is looking at a ten-year timeline for development of the full system.
But how the system should be governed in future and by whom is still to be resolved. Esa has been holding discussions with the European Union about its involvement - specifically whether the EU might assume a greater role in managing and funding SSA beyond 2011.
Esa has also been talking to Nasa and the US Department of Defense (DoD) about the potential for making the European and American networks interoperable, or compatible.
"In short, we are trying to put together, with the Americans, requirements for the European system and future upgrades of the US system to have a better, more reliable source of information for all of us," says Gaele Winters.
The European network must be able to function independently of that used by America, he says, adding: "There is a certain level of independence Europe wants to have in its ability to protect its own space assets. But
it also offers the opportunity to get more out of two systems."
Good to talk
Insiders say officials are determined to avoid a repeat of the dispute with America over Europe's Galileo satellite navigation system, due to be operational by 2013. Fuelled by US concerns that the European network could be used by hostile nations in attacks on American targets, the row was only resolved after lengthy negotiations.
Sources say that, in principle at least, there is a willingness on both sides to work together on SSA, though some in Europe remain wary of US intentions.
There were tensions between the US and Europe over Galileo
"Any endeavour by Europe to enhance space situational awareness will only increase our ability to conduct safe and responsible operations in space," a DoD official told the magazine Space News earlier this year.
"The United States supports international cooperation in SSA."
Britain, meanwhile, has joined the preparatory programme, contributing one million euros at the Esa ministerial meeting last November. But participation presents particular questions for the UK because of its "special relationship" with America.
The UK already has some privileged access to American data and some national users might not want to see the status quo shaken up. Nevertheless, some commentators recognise a need for Britain to have more control over the space surveillance data it gets.
A policy directing how the data will be used is also important for Europe's system, given the sensitivities over sharing information from military sensors.
Richard Tremayne-Smith comments: "I personally believe one thing that would have to be done is to have data anonymity. People will be quite glad to put things in as long as you can't trace things back and work out the power and capability of the sensor.
Many space weather phenomena relate to the activity of the Sun
Solar storms may occur a few times a week or a few times a day
Caused by bubbles of plasma erupting from Sun's atmosphere
Bubbles pack a punch equivalent to about 100 hurricanes
Also known as coronal mass ejections, or CMEs
Affect functioning of technology in space and on the ground
Solar activity monitored by Stereo and Soho satellites
"If you want to get as many people as possible to provide data, you want to give them a warm feeling that it's not going to be misused, or allowed to drift off to people who might use it for things they didn't want."
One way to address the issue, he said, might be to place a software application between the sensitive data and end users. Information provided by participating countries would be used to train this interface. Users would then obtain the answers they require without ever seeing details of the satellite positions.
Space debris was once the principal concern of space surveillance, but a different threat comes from near-Earth objects - the primordial rocks left over from the formation of the Solar System.
It has been difficult to find funding for facilities directed exclusively towards Neo discovery. But Mr Tremayne-Smith said existing ground and space telescopes used for astronomy or military activities are eminently suitable for logging asteroids and comets.
"The third big peak in discovery of near-Earth objects below the 1km size was with the US GEODSS system used for looking at satellites. The US military allowed those satellites to be used for NEO detection while they weren't being otherwise utilised," he explains.
Space weather is the third component of Esa's SSA programme. The bulk of this discipline is concerned with solar activity. Radiation from flares and coronal mass ejections (CMEs) on the Sun may launch X-rays and high energy particles towards Earth.
These phenomena can interfere with the operation of space systems, especially those beyond the protective screen of the Van Allen radiation belts surrounding Earth.
The UK MoD's Skynet 5C satellite carries sophisticated anti-jamming technology
But officials acknowledge the potential threat from deliberate attempts to interfere with satellites.
In September 2005, a London-based radio station called Sowt al-Amel (Voice of Hope), began beaming into Libya via satellite with the aim of promoting political reform in the North African country.
Within minutes of Sowt al-Amel's first broadcast, a high-powered signal of garbled noise was unleashed on the satellite uplink, drowning out the dissident station. But in jamming the signal, several other broadcasters, among them CNN and BBC World, were also blocked out.
The US carried out on-orbit inspection of a missile warning satellite
Esa is a civilian agency and is not mandated to deal with security matters. But Gaele Winters explains: "If you develop the system, it could contain additional capabilities.
He adds: "The first priority is to build ground-based infrastructure. But it is not to be excluded that later in the programme, space-based monitoring could take place.
"If there is a problem with our satellite, we would like to have the ability to analyse precisely what is wrong with it. One way of doing that is from the ground using telescopes. Another is to have a satellite in orbit which could approach the damaged satellite and carry out a close inspection to see what is happening."
In January, it was reported that the US DoD had commanded two covert inspection satellites to examine a failed US Air Force missile warning satellite in geosynchronous orbit.
Mr Winters says several organisations, including the European Defence Agency, are studying the SSA proposals and may come up with security-related requirements to be added to those being compiled by Esa.
He comments: "It could well be that in the future, the European Union will take a bigger role in managing and funding the development of this system because of these aspects that are outside the scope of Esa, but are inside the mandate of the EU."
Space situational awareness is also regarded as an important step towards the holy grail of space traffic management. A system analogous to that which currently governs the movements of aircraft is still some way off.
But Richard Tremayne-Smith thinks Europe should consider this objective in the design of its SSA programme.
He explains: "This is just a personal view, but if Europe does something, it should have a very ambitious target in the longer term that leapfrogs other capabilities and goes to what is really needed to do the job, rather than just getting to the current state of the art."