From laser beams and wooden satellites to galactic tow-truck services, start-ups in Japan are trying to imagine ways to deal with a growing environmental problem: space debris.

Junk like used satellites, parts of rockets and wreckage from collisions has been piling up since the space age began, with the problem accelerating in recent decades.

"We're entering an era when many satellites will be launched one after another. Space will become more and more crowded," said Miki Ito, general manager at Astroscale, a company dedicated to "space sustainability".

"There are simulations suggesting space won't be usable if we go on like this," she told AFP. "So we must improve the celestial environment before it's too late."

The European Space Agency (ESA) estimates that around one million pieces of debris larger than a centimeter—big enough to "disable a spacecraft"—are in Earth's orbit.