We’re about to enter a comet ‘swarm’

Every year, Earth passes through the tail of the comet Encke - twice. This isn't unusual. We regularly cross cometary trails of ice and dust. But this one may be different.

It could have been the source of the dramatic 1908 Tunguska airburst which flattened hundreds of square kilometres of Russian forest. The 45m wide comet fragment struck on June 30, at the height of the Beta Taurid shower.

Was it, in fact, a part of that shower?

Astronomer and science advocate Dr Phil Plait says it is possible. Calculations of its trajectory indicate the Tunguska object came from the same patch of sky.

But comet trails are usually just scatterings of dust and debris, rarely bigger than a pebble.

So what makes that belonging to comet Encke different?

Dr Plait says there is an idea being examined by astronomers that the gravitational influence of Jupiter may be having a particular effect on its trail.

Usually, comet debris simply slowly drift away.

But in Encke's case, Jupiter could be rolling its offcasts into a tighter, more compact clump - the Taurid Swarm.

Earth crosses the path of this swarm in June (Beta Taurid shower) and October (Taurid shower), every year. They're called Taurid as the meteors appear to flash out of the constellation Taurus.

Comet Ison, left, is seen above the more distant and fainter comet Encke in November 2013, as the pair approached the Sun.  Picture: NASA
Comet Ison, left, is seen above the more distant and fainter comet Encke in November 2013, as the pair approached the Sun. Picture: NASA

And the Taurid Swarm hypothesis postulates the presence of much larger cometary chunks hurtling along the flow.

There is speculation a large impact from the Taurid Swarm fragment occured some 13,000 years ago in the northern hemisphere, contributing to the disappearance of the Clovis culture in Northern America.

But a recently discovered crater in Greenland is yet to be accurately dated or identified. More extreme extrapolations attempt to link comet Encke to the demise of the fictional Atlantis of Plato's imagination.

"While this is all unproven, it's somewhat worrisome. If the Tunguska impactor was part of the Beta Taurids, and they reoccur every year, is it possible another Tunguska-sized chunk of comet is out there with our name on it?" Dr Plait writes.

"I'll remind you that space is big and the Earth small, so the odds of such an impact are really low - we expect to see them on the order of once every thousand years or so. It would be nice, however, to survey the Beta Taurids and see what's actually there."

And attempt to do just that will be made between July 5 and 11, and again between July 21 and August 10 this year.

It has been calculated that Earth will be passing through the centre of the swarm this time. Combined with favourable phases of the moon, professional-grade telescopes should be able to spot objects of up to 100 meters or so barrelling along among the dust.

Even the big ones will simply be too faint for amateur telescopes or binoculars. And the show will be coming from the direction of the sun - meaning the meteors will peak during daytime.

"The idea that Tunguska might be just one of many such objects in the Taurid swarm is circumstantial, but critical to test," Dr Plait says.