Image caption Many concepts are now being developed to grab hold of dead satellites in orbit

There is now so much debris in orbit that the space environment is close to a cascade of collisions that would make space extremely hazardous, a major international meeting has concluded.

Its summary position stated there was an "urgent need" to start pulling redundant objects out of the sky.

Scientists estimate there are nearly 30,000 items circling the Earth larger than 10cm in size.

Some are whole satellites and rocket bodies, but many are just fragments.

These have resulted from explosions in fuel tanks and batteries, and from the high-velocity impacts between objects.

Upwards of 10cm is trackable with radar, but there are tens of thousands more pieces that are smaller and move unseen.

Only active removal of five to 10 large objects per year can reverse the debris growth Prof Heiner Klinkrad, Esa Space Debris Office

And it is the prospect of an increase in the frequency of catastrophic collisions among all this material that now worries the experts.

"There is a consensus among debris researchers that the present orbit debris-environment is at the rim of becoming unstable within a few decades, a phenomenon that is commonly known as the Kessler Syndrome, and that only active removal of five to 10 large objects per year can reverse the debris growth," Prof Heiner Klinkrad, the head of the European Space Agency's (Esa) Space Debris Office told reporters.

Prof Klinkrad was the chairman for the 6th European Conference on Space Debris in Darmstadt, Germany.

Insufficient compliance

The meeting was presented with a study earlier in the week that suggested the population of objects in low-Earth orbits (LEO) - the important altitudes used by imaging spacecraft to health-check the planet - would likely rise steadily over the next 200 years even under the most optimistic of scenarios.

The research highlighted the need for better adherence to best-practice guidelines.

Image caption DEOS is a demonstration mission being pursued by the German space agency

These "rules" call on space operators in LEO to make sure their equipment naturally falls out of the sky within 25 years of the end of a mission.

But compliance with the guidelines is far from perfect, and the panel said active removal was now the urgent topic on the agenda.

Quite how much time there was to act before conditions became intolerable was not yet clear, said Christophe Bonnal from the French space agency (Cnes).

"We say we want to 'stabilise' the environment. Does that mean we are satisfied with today's situation? Could we live with a situation that is two times worse than today, or do we need to decrease [the debris population]? These are questions which are ongoing at international level," he told BBC News.

Active removal would see new spacecraft launched specifically to take other, redundant satellites out of orbit. And the Darmstadt meeting was presented with an array of concepts that included the use of nets, harpoons, tentacles, ion thrusters and lasers.

The conference summary panel told the media it was vital that pilot programmes were implemented to advance these technologies.

Commercial barrier

A few have been approved. The German Space Agency (DLR) is developing a project called DEOS that would demonstrate the robotic capture of a tumbling object in space.

"In this mission, what we want to show is that it is technically possible to safely approach a satellite, which we launch together with our main satellite, to capture it by means of a robotic arm and to perform a number of services like repairing or maintenance operations," explained DLR's Dr Manuel Metz.

"Many of the technologies which are currently being developed for DEOS would be useful for potential future international active debris-removal missions."

The experts also stated that the international community needed to sort through the myriad legal issues that would currently frustrate attempts to clean up space.

At the moment, international law permits only the launching nation or agency to touch an object in orbit, something that would prevent, for example, commercial debris removal activities.

"My dream is that a new agency like the International Telecommunications Union will be proposed at UN level to coordinate all this activity," said Dr Claudio Portelli from the Italian space agency (Asi).

Costly mission

Esa was hosting this week's meeting. It has two old satellites in orbit that are likely to become targets for a future de-orbiting exercise.

ERS-1 and Envisat both suffered major failures that left them drifting uncontrolled through LEO.

Image caption At over eight tonnes, Envisat is considered a high priority for active removal

The duo can be tracked but nothing can be done to move them off a potential collision course, should one arise.

Envisat in particular is considered a high priority for removal because of its great size - over eight tonnes.

However, de-orbiting this dead satellite would probably be very expensive. And the robotic spacecraft sent up to bring Envisat down would itself be very large.

Prof Klinkrad explained: "If you want to have a controlled de-orbit - and this is what you should have for Envisat because large portions are going to survive to ground impact - then you should have a highly energetic chemical propulsion system, and to reliably de-orbit Envisat from its altitude you'd need, I'd say, about 6% of its mass in terms of fuel.

"With everything included, you are talking about a two-tonne-type spacecraft [to do the de-orbiting]," he told BBC News.

To date, there have only been a handful of major collisions in orbit involving the largest objects.

Perhaps the best known was the 2009 impact between the defunct Russian Cosmos 2251 spacecraft and the American Iridium 33 satellite. The collision produced over 1,500 trackable fragments, many of which continue to pose a threat to operational missions.

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos