A team of amateur astronomers has spotted a previously unknown asteroid about 18 million miles from Earth.

The team spotted the asteroid, which is known as 2011 SF108, in September using a telescope in the Canary Islands. The asteroid is close enough to be classified as a near-Earth object, the class of space rock that could pose a threat.

The amateurs were using time allocated to them by the European Space Agency's Space Situation Awareness program when they made the discovery.

"As volunteer work, it is very rewarding," said Detlef Koschny, head of near-Earth object activity for SSA. "When you do spot something, you contribute to Europe's efforts in defending against asteroid hazards."

The Teide Observatory Tenerife Asteroid Survey team, a collection of 20 amateur astronomers, used the 1-meter telescope at the European Space Agency's Optical Ground Station on Tenerife to see the near-Earth object.

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"Images are distributed to the entire team for review, and any one of them could be the discoverer of a new asteroid," Koschny said. "This time, the luck of the draw fell to Rainer Kracht."

Kracht, a retired schoolteacher from Elmshorn, Germany, is therefore the official discover of 2011 SF108. It is the 46th asteroid he has discovered.

So far, 8,000 near-Earth objects have been discovered and mapped in an effort to determine which ones might hit the Earth in the future.

Scientists say 2011 SF108 does not pose much of a risk to Earth, but further observations could help determine its orbit and assess how dangerous it could be, researchers said.

It is not clear how big the asteroid is as ESA have not yet released the details.