[meteorite-list] Planet X

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:55:45 2004
Message-ID: <200201130225.SAA16846_at_zagami.jpl.nasa.gov>

http://www.theage.com.au/news/state/2002/01/13/FFXSQH4ACWC.html

Planet X
By PAUL DAVIES
The Age (Australia)
13 January 2002

After the initial shock, mankind reacted with a determination and a unity
that no earlier age could have shown. Such a disaster, it was realised,
might not occur again for a thousand years - but it might occur tomorrow ...
So began Project Spaceguard.

Thus wrote the famous futurist Arthur C. Clarke in his 1973 science fiction
novel Rendezvous With Rama. Clarke was describing the aftermath of the worst
foreseeable ecological catastrophe - impact by an asteroid packing enough
punch to wipe out an entire country.

A few days ago, our planet narrowly escaped just such a fate when a
300-metre-wide chunk of rock hurtled past us on its way around the sun. It
was but one among thousands of rogue asteroids that constantly menace life
on Earth as they flash by. Alarmingly, scientists have very little idea
where most of these objects are, and sooner or later one of them will slam
directly into us, causing massive destruction.

Computer simulations suggest that a one-kilometre rock hitting Earth at 30
kilometres per second would explode with the power of a million-megaton
bomb, blasting a crater 20 kilometres across. Billions of tonnes of rock
would spew forth, some of it crashing down in molten lumps around the globe,
igniting widespread fires.

The blast and seismic shock might devastate millions of square kilometres.
If the object plunged into the ocean, tsunamis looming higher than
skyscrapers would pulverise coastal cities. Vast quantities of dust would
blanket the planet, blotting out the sun for weeks, causing crops to fail on
a huge scale.

Worldwide starvation and economic collapse would follow. The final death
toll from the resulting mayhem is estimated to be upward of one billion.

Admittedly, a cosmic impact of this severity is extremely rare; there is
roughly one chance in a million it will happen in the next month or two. But
a one-in-a-million chance of a billion deaths implies an expectation of
death of a thousand people per month, greater than many familiar disasters
like plane crashes.

Translated into personal terms, you are more likely to be killed by an
asteroid than, say, a bolt of lightning.

When it comes to risk assessment, humans are remarkably irrational. Many
people shrug aside a million-to-one risk. At the same time, the uncontrolled
descent of the Mir space station, in which the threat to human life was
minuscule, was greeted by near hysteria in some quarters.

That asteroids, comets and large meteorites do collide with Earth from time
to time is evident from even a casual survey. At Wolfe Creek in Western
Australia you can see a one kilometre-wide 300,000-year-old impact crater. A
much larger feature, Lake Acraman in South Australia, is the remnant of a
600 million-year-old crater 100 kilometres across.

A collection of 100-metre holes at Henbury, near Alice Springs, has become a
well-known tourist attraction. In all, a couple of dozen impact sites have
been identified in Australia, and many more in other parts of the world.

The solar system is swarming with rocks, most of them rubble remaining from
the formation of the solar system 4.5 billion years ago. In the far past,
the bombardment of the planets by this debris was intense. A glance at the
moon reveals a surface pockmarked with ancient impact craters, some of them
enormous. On Earth, these surface scars have been mostly obliterated by
geological processes, but our planet will certainly have taken its share of
punishment over the aeons.

Although the fierce early bombardment abated about 3.8 billion years ago, it
never entirely ceased. Throughout geological history there have been several
episodes of sudden, massive species extinctions, which many scientists now
attribute to colossal impacts. The most severe occurred 250 million years
ago, when 90 per cent of species, including the famous trilobites (extinct
animals), suddenly disappeared, probably from the combined effects of the
impact and the extensive volcanic eruptions it triggered. It also seems
likely that the dinosaurs met their end 65 million years ago in similar
fashion, when a 16-kilometre-wide object struck what is now the Yucatan
Peninsula in Mexico, excavating a crater over 200 kilometres across.

Today, almost all the large asteroids are safely confined to a belt between
Mars and Jupiter. However, smaller bodies cross our orbit all the time,
threatening catastrophe. Part of the problem in quantifying that threat is
that we have only a rough idea how many near-Earth objects are out there. In
astronomical terms a one-kilometre rock is pretty small and inconspicuous.
Most of them could hit us with only a few seconds warning.

Even much smaller rocks still have awesome destructive power, and they are
far more numerous. In 1908, 2000 square kilometres of forest were flattened
when an object the size of an office block came out of the blue and exploded
over the remote Tunguska River area of Siberia. This sort of event can be
expected every few decades. Indeed, a similar explosion is believed to have
taken place in the South American jungle in the 1930s.

Prompted by Clarke's apocalyptic scenario, scientists have proposed Project
Spaceguard to deal with the near-Earth objects threat. It consists of two
distinct stages. First is a systematic search program designed to find as
many near-Earth objects as possible, and compute their orbits. The
observations, to be carried out over many years, should determine when the
next impact will occur. The second stage is to develop the technology needed
to deflect the approaching object from its threatening path.

What can be done about an asteroid coming our way? Blasting it with nuclear
warheads is not a sensible option, for it would serve only to turn a bullet
into buckshot. A better strategy is to alter the orbit. Moving a
billion-tonne rock isn't easy, and nobody is quite sure how to do it, but
one idea is to detonate nuclear bombs in a controlled manner near the
surface, vaporising a layer of material to propel the rock slightly to one
side. Another proposal is to attach a giant sail and let the solar wind blow
the asteroid off course.

Unfortunately, not much is known about the make-up of asteroids. A year ago
the US space agency NASA landed a space probe on Eros, an irregular-shaped
rock 35 kilometres long. Mission scientists concluded that Eros is indeed a
solid object likely to hold together if nudged. But the same probe had
earlier visited a bigger asteroid named Mathilde, which looked more like a
collection of boulders held together only by its own feeble gravity.

Whatever deflection strategy is adopted, the sooner it is done the better. A
tiny change in orbit decades ahead of projected impact will be more
effective than a big push when time is running out. Hence the need for
efficient early warning, which only a comprehensive search program can
provide.

Last year, the British Government pledged £35 million ($A96 million) to
planetary defence. Now the United Nations is establishing a taskforce to
review the danger.

Most of the bigger objects could be found and tracked using a global network
of cheap telescopes equipped with some fancy electronics and data analysis.
Already the United States has several research groups dedicated to finding
near-Earth objects this way. Expanding the search over 10 or 20 years and
involving countries such as Australia would deal with the major threat, at
relatively modest cost.

But what to do about the smaller, more frequent, yet still potentially
deadly impacts?

A rock the size of the one that just missed us last week could devastate an
area as large as Tasmania. American scientists have called for a more
ambitious detection system known as the Large-Aperture Synoptic Survey
Telescope, a 6.5-metre instrument costing about $US150 million ($A287
million) and capable of finding 90 per cent of rocks down to 300 metres
across. With such an early warning system, evacuation measures could be
taken in the impact zone. Even deploying all this technology, there will
remain a class of hazardous objects that we can do very little about. These
are comets - loose aggregations of rock and ice that mostly inhabit a realm
far beyond the planets. From time to time one gets diverted into the inner
solar system on an elongated trajectory, whence it sprouts the distinctive
tail.

As long ago as the seventeenth century, Edmund Halley warned of the peril of
a comet striking a planet. The awesome consequences were dramatically
evident in July 1994, when the core of comet Shoemaker-Levy 6 broke into
fragments and ploughed into Jupiter, creating a series of titanic fireballs
so big they could be seen through backyard telescopes.

Typically a new comet is spotted only months before it sweeps by. If one
were to have our number on it, there would be far too little time to get out
there and deflect it, even if a way could be found to move such a fragile
object without breaking it into pieces.

Some critics of near-Earth objects research argue that it is a waste of
money tracking down dangerous asteroids when killer comets will still go
undetected, but that's rather like refusing to check the steering on your
car because the brakes might fail. Though we are certainly in the firing
line from comets, our astronomical location is actually rather favourable.
In fact, we have Jupiter to thank for keeping Earth relatively free of
impacts. Its immense gravitational field deflects many incoming comets and
can even fling them right out of the solar system. In effect, this giant
planet acts as a cosmic vacuum cleaner, sucking up or tossing out dangerous
debris before it can penetrate the habitable zone.

In the last few years, dozens of planets have been discovered around other
stars, but so far the configuration of the solar system looks to be unusual,
even unique. If there are other Earths out there, chances are they will be
pounded mercilessly for billions of years, preventing life from establishing
more than a toe-hold.

It would be the supreme irony if on the rare planet that permitted life to
evolve as far as intelligence, the resulting beings were still too stupid to
protect their species from cosmic catastrophe.

Paul Davies is a physicist in the Australian Centre for Astrobiology at
Macquarie University.
His latest book is How to Build a Time Machine (Penguin).
Received on Sat 12 Jan 2002 09:25:43 PM PST