UFO sightings in Australia were giant fireballs

UFO sightings from Australia has been explained by ball lightning and other atmospheric phenomena, said the country's leading astrophysicist Stephen Hughes.

The scientist has made a detailed study of an unusual event in 2006 when large meteors were observed over Brisbane which caused the public to report numerous UFO sightings.

Their appearance occurred at the same time as a brilliant green object was seen to roll over nearby mountains.

Dr Hughes has put forward a theory linking the object - presumed to be ball lighting - to the fireballs. His idea is that one of the fireballs may have momentarily triggered an electrical connection between the upper atmosphere and the ground, providing energy for the ball lightning to appear above the hills.

He has written up his explanation in a journal of the Royal Society.

"If you put together inexplicable atmospheric phenomena, maybe of an electrical nature, with human psychology and the desire to see something - that could explain a lot of these UFO sightings," he told BBC News.

Dr Hughes, who is a senior lecturer at the Queensland University of Technology, initiated the study after being called in by the local TV station to look over and explain photos of the fireballs captured by members of the public on camera phones.

Fireballs are exceptionally bright meteors and are produced by fragments of space rock larger than the sand-grain-sized particles responsible for shooting stars; but like shooting stars they cross the sky at great speed.

It seems at least three individual fireballs were seen on the night of 16 May 2006.

A survey organised by the university brought forward many more eyewitnesses, including a farmer who recalled seeing a luminous green ball rolling down a slope of the Great Divide, a mountainous ridge about 120km west of Brisbane.

This object described as being about 30cm in diameter appeared to jump over some rocks and follow the path of a metal fence for "some minutes". The farmer said he saw the green object come into view just after a fireball had passed overhead. He thought at first he was witnessing a plane crash and called the police, but a search the following day found no wreckage.

Eyewitnesses were asked to draw what they saw. This is how graphic designer David Sawell recalled a fireball

Ball lightning seems an obvious explanation, says Dr Hughes. These bright, hovering spheres of light are not fully understood. They are known to be associated with thunderstorms, but not always, and there was certainly no electrical storm activity in the vicinity of the Great Divide mountain ridge.

Dr Hughes does not offer a new explanation for the causes of ball lightning, merely how enough energy might have been put into the ground to trigger it.

He proposes that the natural flow of current that exists between the upper-most reaches of the atmosphere, the ionosphere, and the ground was increased by the passage of the meteor that streamed charged particles and other conductive materials in its wake.