* Astronomy

Destructive comets, like the one many people say slammed into the Tunguska region of Siberia last century, are much rarer than we think, new research finds.

Australian scientist Dr Paul Francis of the Research School of Astronomy and Astrophysics at Mount Stromlo, estimates small comets that pose a risk to Earth are half as common as others predict.

"These things are pretty rare," says Francis, who will report his findings in the Astrophysical Journal.

"I calculate that small comets, capable of destroying a city, only hit the Earth once every 40 million years or so. Big continent-busting comets, as shown in the movies Armageddon and Deep Impact, are rarer still, only hitting once every 150 million years or so. So I don't loose sleep over it, but you're still more likely to be killed by a comet than to win the jackpot at Lotto" - Dr Paul Francis

But the risk to Earth from larger comets, around 1 kilometre wide, has not changed.
Francis says previous estimates on comet hazards were based on incorrect extrapolations from the comet reports of amateur astronomers.
In 1967 a US astronomer, Edgar Everhart, calculated that the two comets a year being picked by amateur astronomers represented only 3% of the comets actually out there.
Francis' suspicions about the accuracy of this estimate were roused when he used Everhart's calculations to estimate the number of comets that would be picked up by a new telescope being built at Mount Stromlo.
Francis calculated the SkyMapper telescope would find 10,000 "long-period" comets a year, which he thought "seemed ridiculous".
These long-period comets originate from the Oort Cloud, believed to be a vast cloud of comets orbiting the Sun far beyond Pluto, that are occasionally nudged into an orbit that threatens Earth.
To investigate further he used actual comet data from the US-based Lincoln Near Earth Asteroid Research (LINEAR) Project optical telescope at White Sands in New Mexico.

He found LINEAR was seeing around 17 long-period comets a year.

Yet, by Everhart's calculations LINEAR should have been picking up something like 2000 long-period comets a year.
Using a computer model Francis changed the comet population until predictions agreed with the data.
Francis calculated that overall there were 2 trillion long-period comets in the Oort Cloud, a figure seven times lower than previous estimates using Everhart's calculations.
From that Francis calculated the risk of smaller comets posing a risk to Earth, be they long-period or ones closer to Earth, known as short-period comets, was half what others think.
Previously these small comets were thought to pose a risk to Earth once in about 500 years rather than once in 1000 years.
Although short-period comets are rarer, Francis says their path is easier to predict because they pass by Earth more frequently.
And this means we could get hundreds of years notice before an impact, giving us a chance to defend ourselves.
Long-period comets are "nastier" because they're "totally unpredictable", says Francis.

"If we see one on a collision course we'd have at best one or two years warning, not long enough to do anything"

One of the most famous recent cases of a devastating visit from space is the huge fireball that hit the Tunguska region of Siberia in 1908.
Trees within about a 14 kilometre radius were incinerated and those within a 40 kilometre radius were knocked down.
Many scientists believe the fireball was caused by an exploding comet, around 100 metres wide, says Francis. Others believe a meteor caused the devastation.

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