Space debris is a growing problem, with disused satellites, rocket remains and millions of smaller pieces of junk all capable of damaging spacecraft and even destroying them.

"We will increasingly be confronted with collisions in the future," says Holger Krag, who heads the Space Debris Office at the European Space Agency.

"The critical altitude lies between 800km and 1000km from the earth's surface. Congestion is already extreme there," Krag says.

Space junk has been created mainly by more than 250 breakups and explosions. There are about 18,000 fragments large enough to be tracked by detection systems. However, even smaller pieces can be dangerous. Estimates say there are more than 750,000 objects of between 1cm and 10cm in diameter.


With potential collision speeds of up to 40,000km/h, these pieces can exert the kind of force given off by a hand grenade.

At the Seventh European Conference on Space Debris in Darmstadt between April 18 and 21, 400 participants will seek solutions.

These include engineers, scientists, managers, industrial concerns, universities and decision-makers from countries prominently involved in space travel.

The issue will only become more urgent. The number of rockets and satellites being launched into space is set to increase, as traditional state-run space agencies are joined by commercial companies.

"Mega constellations of this kind is one of the priority issues at our conference on space debris," says Manuel Metz of the German Aerospace Centre.

"No one had this kind of space-travel boom on their radar just four years ago," Metz says, pointing to plans for commercial missions from as early as next year.

Krag is also concerned about projects planned by the likes of Samsung and Google, some with a large number of satellites.

"We're talking about several thousand per mission," he says, pointing out that, throughout the history of space travel, "around 7000 satellites have been launched".

This is known as the "Kessler syndrome". Posited by Nasa scientist Donald Kessler in 1978, the idea refers to an unpredictable chain reaction of collisions caused by a high density of objects in low earth orbit.

Several approaches have been suggested, including drawing pieces into the atmosphere and having them burn out over the Pacific.

Having control over the satellites is key to disposal, Krag says, pointing out that hauling in a lost satellite with a grapple arm is "a huge challenge" and still well in the future.

But the chance of being hit by a piece of space debris is small, even though theoretically possible. "The probability is tiny," Krag says.