[meteorite-list] Tunguska Event Half As Likely

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Sep 8 13:27:29 2005
Message-ID: <200509081726.j88HQLs21129_at_zagami.jpl.nasa.gov>

http://www.abc.net.au/science/news/stories/s1455279.htm

Tunguska event half as likely
Anna Salleh
Australian Broadcasting Corporation
8 September 2005

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 1000 years or so."

But the risk to Earth from larger comets, around 1 kilometre wide, has
not changed.

Luckily such continent-busting comets, of the size seen in the movie
Deep Impact, are much rarer.

The risk of these large comets posing a risk to Earth is up to one in
every 150 million years, depending on where the comets originate.

Ridiculous estimates of comets

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.

Devastation at Tunguska
Whatever the cause of the fireball at Tunguska, if this type of impact
had occurred in a heavily populated area, the effect would have been
catastrophic (Image: Smithsonian Institution)

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.

Preparing for impact

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."

The Tunguska fireball

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.
Received on Thu 08 Sep 2005 01:26:21 PM PDT