Cometary fragments may have triggered ball-lightning

Cometary fragments may have triggered ball-lightning

(Der Spiegel, December 1st, 2010)
Green balls of fire lit up the night sky over Australia - evoking thoughts of alien visitors to many. Now a scientist has come up with another sensational theory: fragments of a comet may have triggered the heavenly show and sent a ball lightning down the slope of a mountain.

It was an evening which many inhabitants of the Australian federal state of Queensland will remember. On May 16, 2006, three green fireballs lit up the sky over the city of Brisbane. According to eye-witnesses, one of them even rolled down a slope of the Great Dividing Range.

Local television and newspapers reported on the strange phenomenon. Witnesses were interrogated at length. Yet for over four years, it remained unclear, what had been at the origin of the impressive light show. Many an observer mentioned the possibility of visitors from space. Now Stephen Hugues, from the Queensland University of Technology, has come up with a rational explanation - or the beginning of one.

A crumbling meteorite may have been responsible for these sky events, writes Hughes in the British journal "Proceedings of the Royal Society of Astronomy." Moreover: the object supposedly also created ball lightning.

Eyewitnesses have described the light apparition as brighter than the moon and less bright than the sun. Hughes believes that these were parts of comet 73P/Schwassmann-Wachmann 3. The very rapidly flying objects provoked shock-waves in the Earth atmosphere and in so doing, they ionized oxygen - this green light is known from polar lights.

One clue: that same day, the pilot of an airplane en route for New Zealand reported to have observed an meteorite entering the atmosphere. It had fallen apart in the process and moved in the direction of Australia.

A ball lightning rolling down on a slope.

As for the spehere rolling down the mountain - according to eyewitnesses it was some 30 cm in apparent diameter - the scientist believes that it cannot possibly have been a piece of the meteorite. To begin with, round objects of this type are so far unknown on Earth. Second, according to the observers, the sphere rolled downhill very slowly. This would not be expected from a piece of a meteorite. And third, there are no traces of impact on the ground, such as a crater or burned surfaces.

Hughes believes that it may have been a ball lightning, triggered by one of the pieces of debris. It had strongly reduced the electric resistance between the surface of the Earth and the ionosphere - and in so doing, it had made possible a flow of electric current. This may have created the ball lightning.

The so-called Pasamonte Fireball, which was observed in 1933 in the US State of New Mexico, may have had a similar origin, says Hughes. Generally, such phenomena could be responsible for alleged UFO-sightings in many places in the world.

In his work, the researcher proposes several mechanisms, which could have been responsible for the stability of the path of lightning - but he does not privilege definitely any one of them. Hughes points to experiments made at the Max-Planck-Institut für Plasmaphysik (IPP) and the Humboldt University in Berlin. Scientists have been able to generate high-tension discharges of green, glowing plasma clouds over a watersurface. Maybe that these balls, which had a diameter of 10 to 20 cm, were comparable to the phenomenon observed in Australia, Hughes speculates.