Those "green UFOs" that caused a stir in Australia four years ago? Researchers say they definitely weren't alien spaceships (not like they were going to say anything different), but they still aren't sure what they actually were. The three green fireballs were spotted by more than 100 people in the sky over Queensland, Australia on May 16th, 2006. The potential abductees said the lights were brighter than the moon, but not as bright as the sun. A single farmer claims to have seen one of the green balls bouncing down the side of a mountain after hitting the earth. Stephen Hughes, a researcher at Queensland University of Technology in Brisbane, has just published a paper on the phenomenon in the Proceedings of the Royal Society A. He explained to LiveScience that the main fireballs were most likely caused by a meteor breaking up and burning in earth's atmosphere:

In fact, a commercial airline pilot who landed in New Zealand that day reported seeing a meteor breaking up into fragments, which turned green as the bits descended in the direction of Australia. The timing of the fireballs suggests they might have been debris from Comet 73P/Schwassmann–Wachmann 3.

Hughes believes that the strange color of the meteor shards might be due to electrically charged oxygen molecules around the debris, similar to how electrically charged particles in the upper atmosphere produce the northern lights. But the meteor theory doesn't explain the farmer's sighting of the light-ball rolling down the hillside. A foot-wide perfectly spherical meteorite wouldn't be slowly meandering down a hillside after impacting the earth. To explain the bouncing ball, Hughes told Live Science

that he has a second hypothesis--ball lightning

, which he thinks could have been caused by the meteor, though it is usually thought to be caused by storms:

"A transient electrical link between the ionosphere and ground, created by meteors or some other means, could help to solve the mystery of many UFO sightings," Hughes told LiveScience. "Since such balls would be very insubstantial they would be able to move and change direction very fast as has often been observed."

Ball lightning is itself poorly understood and controversial: One recent study suggested that the phenomenon might just be a hallucination. But other researchers seem to buy Hughes' explanation, according to New Scientist:

"It is certainly plausible," says John Lattanzio, an astrophysist at Monash University in Victoria, Australia. But he adds: "It's almost impossible to prove anything with such an ephemeral event as this."

Of course, Hughes told New Scientist

that no one else reported seeing the rolling ball that the farmer described:

Hughes ... set up an online survey to find out more. More than 100 people, scattered over a 600-kilometre-long strip along Australia's east coast, reported seeing a bright fireball like the first green ball that Vernon saw, but no else saw the bouncing ball.

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