By Maria Sheahan
FRANKFURT, April 25 (Reuters) - Space junk such as debris from rockets must be removed from the Earth's orbit to avoid crashes that could cost satellite operators millions of euros and knock out mobile and GPS networks, the European Space Agency said.
At the current density of debris, there will be an in-orbit collision about every five years, however research presented at a conference hosted by ESA in Germany showed that an increase in such junk made more collisions likely in the future.
Five to 10 large objects need to be collected from space a year to help cut down on smashes and stem the risk of fragments being sprayed into space that could cause more damage, it said.
Scientists estimate there are about 29,000 objects larger than 10 cm (4 inches) orbiting Earth at average speeds of 25,000 kph (15,500 mph) - about 40 times faster than airplanes travel.
At that speed, even small pieces of fast-travelling debris can damage or destroy spacecraft and satellites - which could cost billions of dollars to replace and disrupt mobile phone communication or satellite navigation.
"Within a few decades, there are going to be collisions among large objects that will create fragments that can do further damage," Heiner Klinkrad, the head of ESA's Space Debris Office, told Reuters.
"The only way to keep this from happening is to go up there and remove them," he said. "The longer you wait, the more difficult and far more expensive it is going to be."
Space debris includes any man-made litter left in space - parts of rocket launchers, inactive satellites and broken parts from past collisions.
Space agencies around the world are cooperating on space debris research, and ESA's Clean Space initiative, launched in 2012, aims to develop the technology to safely capture and remove space debris.
Researchers are looking at several different methods for removing space debris from orbit, Klinkrad said, ranging from the use of propulsion packages, conductive tethers or lasers, to nets and harpoons.
But any decision to go ahead with a mission, as well as funds to pay for it, would need to come ESA's 20 member states, which include France, Germany, Italy and Britain.
Demand for the removal of objects from orbit could eventually offer opportunities for private companies, Klinkrad said, though many issues, including legal ones, surrounding space debris would need to be settled first. ($1 = 0.7695 euros) (Editing by Alison Williams)