[meteorite-list] The Threat from Outer Space

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 23 Jul 2007 12:52:00 -0700 (PDT)
Message-ID: <200707231952.MAA05072_at_zagami.jpl.nasa.gov>

http://www.economist.com/world/international/displaystory.cfm?story_id=9533468

The threat from outer space
Economist.com
July 23, 2007

The ultimate environmental catastrophe

ONE of the main weaknesses of the environmental movement has been its
unfortunate predilection for using doom-laden language and catastrophic
superlatives to describe problems that are serious but not immediately
disastrous. But one calamity that truly deserves such a description is
almost never talked about. There are tens of millions of asteroids in
the solar system, and several thousand move in orbits that take them
close to Earth. Sooner or later, one of them is going to hit it.

Several have done so in the past. Earth's active surface and
enthusiastic weather conspire to scrub the tell-tale impact craters from
the planet's surface relatively quickly, but the pockmarked surface of
the moon - where such scars endure for much longer - testifies to the amount
of rubble floating in the solar system. Earth's thick atmosphere makes
it better protected than the moon: asteroids smaller than about 35
metres (115 feet) across will burn up before hitting its surface.
Nevertheless, plenty of craters exist. The Earth Impact Database in
Canada lists more than 170.

Fortunately, such impacts are relatively rare, at least on human
timescales. Statisticians calculate that the risk to lives and property
posed by meteorite strikes are roughly comparable with those posed by
earthquakes.

Although the chance of an impact may be small in any given year, the
consequences could be enormous. The effect of an impact depends on an
object's size and speed. A meteorite a few metres wide could level a
city. The largest (a kilometre or more in diameter) could wreak
ecological havoc across the entire globe. David Morrison, a NASA
scientist, argued at a recent conference that a large meteorite strike
is the only known disaster (except perhaps global nuclear war) that
could put civilisation at risk.

Nasa Armageddon

Examples give a more visceral illustration than statistics. The
Chicxulub crater, buried beneath modern Mexico, is 65m years old and
180km (112 miles) across. Some think that the ten-kilometre meteorite
that created it threw so much dust into the atmosphere that it blotted
out the sun and led to the extinction of the dinosaurs. In 1908 a
comparatively tiny piece of space-borne rock, 30-50 metres across,
exploded above Tunguska, a remote part of Siberia. The blast - hundreds of
times more powerful than the atom bomb dropped on Hiroshima 37 years
later - felled 80m trees over 2,150 square kilometres. Only blind luck
ensured that it took place in a relatively unpopulated part of the
world. Astronomers are currently trying to work out whether a 270-metre
asteroid named 99942 Apophis will hit Earth in 2036 (probably not, but
it would be nice to be sure).

Happily for humanity, technology has advanced to the point where it is
possible, in principle, to avoid such a collision. In 1998 NASA agreed
to try to find and catalogue, by 2008, 90% of those asteroids bigger
than 1km in diameter that might pose a threat to Earth. Any deemed
dangerous would have to be pushed into a safer orbit. One obvious way to
do this is with nuclear weapons, a method that has the pleasing symmetry
of using one potential catastrophe to avert another. But scientists
counsel caution. A nuclear blast could simply split one large asteroid
into several smaller ones, some of which could still be on a collision
course.

Other plans have been suggested. One is to use a high-speed spaceship
simply to ram the asteroid out of the way; another is to land a craft on
the rock's surface and use its engines to manoeuvre the asteroid to
safety. A subtler method is to park a spaceship nearby and use its tiny
gravity to pull the asteroid gradually off course. For now, all such
suggestions are theoretical, although the European Space Agency is
planning a mission, named Don Quijote, to test the ramming tactic in 2011.

These schemes offer consolation, but any effort to deflect an asteroid
requires plenty of advance warning, and that may not always be
available. NASA has so far catalogued only the very largest,
"civilisation-killing" asteroids. Plenty of smaller ones remain
undiscovered, and they could inflict considerable damage. In 2002 a
mid-sized asteroid (50-120 metres across) missed Earth by
121,000km - one-third of the distance to the moon. Astronomers discovered
it three days after the event. Comets, which originate from the outer
reaches of the solar system, are faster moving and harder to track than
asteroids, but carry just as much potential for catastrophe.

But perhaps the biggest problem is humanity's indifference. Currently
only America is spending any money on detection, and even there,
politicians have other priorities. Much of the work is done by Cornell
University's Arecibo radar in Puerto Rico, which is facing federal
funding cuts. The telescope costs roughly $1m a year to operate. As an
insurance policy for civilisation, the price looks cheap.
Received on Mon 23 Jul 2007 03:52:00 PM PDT