The End of the World as We Know It

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Apocalyptic
thought has a tradition that dates to the Persian prophet Zoroaster
in the 14th century BC. Recently, anxiety has grown over the prediction
of the end of the world in the Mayan calendar.

It’s true that
the Mayan odometer will hit zeros on 21 December 2012, as it reaches
the end of a 394-year cycle called a baktun. But this baktun is
part of a larger 8,000-year cycle called a pictun, and there’s no
evidence that anything astronomically untoward will happen as the
current baktun slides into the next. However, that hasn’t stopped
the feverish speculating that sells books and cinema tickets.

What kind of
catastrophe would it take to end the world? Astronomical intruders
provide a potentially serious threat. Impacts can be caused by stray
rubble from the Asteroid Belt and the rocky snowballs that travel
in highly elliptical orbits in the comet cloud. There are many fewer
large bits of debris than small bits, so the interval between large
impacts is much longer than the interval between small impacts.

That’s good
news. Every century or so, a 10-meter meteor slams into the Earth
with the force of a small nuclear device. Tunguska was the site
of the last, in 1908, and it was pure luck that that meteor landed
in the uninhabited wilderness of Siberia. Every few thousand years,
Earth can pass through unusually thick parts of the debris trail
of comets, turning the familiar light show of a meteor shower into
a deadly firestorm. Roughly every 100,000 years, a projectile hundreds
of meters across unleashes power equal to the world’s nuclear arsenals.
The result is devastation over an area the size of England, global
tidal waves (if the impact is in the ocean), and enough dust flung
into the atmosphere to dim the Sun and kill off vegetation. That
could ruin your day.

Then there’s
the "Big One". About every 100 million years, a rock the
size of a small asteroid slams into the Earth, causing global earthquakes,
kilometre-high tidal waves, and immediately killing all large land
animals. Creatures in the sea soon follow, as trillions of tons
of vaporised rock cause drastic cooling and the destruction of the
food chain based on photosynthesis. There’s good evidence that this
happened 65 million years ago and our tiny mammal ancestors were
the beneficiaries as the giant lizards were extinguished.

A hundred million
years sounds like a safe buffer, but the next one could happen at
any time. But you can take it off your worry list – astronomers
have it covered. A network of ground-based telescopes scans the
skies for bits of rogue rubble larger than a few hundred meters.
That’s ample time to dust off the nuclear arsenals for an interception
mission if we had to. Unfortunately, the Dr Strangelove approach
creates lethal shrapnel travelling in the same direction as the
original object; a smarter strategy is to send a spacecraft alongside
it and gently "tug" it with gravity onto a slightly different
trajectory.

When massive
stars exhaust their nuclear fuel, the result is a titanic explosion
called a supernova. The dying star brightens to rival an entire
galaxy and emits high-energy particles that can destroy the ozone
layer of a planet like Earth if it occurs within 30 light years.
The demise of large North American mammals 41,000 years ago has
been linked to a supernova, and several other mini-extinctions may
be tied to the cataclysm of stellar death.

A supernova
is a small squib compared to a hypernova. In this dramatic and rare
event, the violent collapse of a very massive star ejects jets of
gas and high-energy particles at close to the speed of light, and
for a few moments the star outshines the entire universe in gamma
rays.

If a hypernova
went off within 1,000 light years, and Earth was within the narrow
cone of high- energy radiation, we’d experience an immediate global
conflagration. It’s brutal luck if a hyper nova ever goes off with
its beam aimed at us.

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the rest of the article

June
22, 2010

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