If you’re the type to worry, don’t click here —
especially if the type of worrying you do is about things that sound
really, really scary and that you have absolutely no control over.
Clicking on that site you really shouldn’t click on will take you to a
page on NASA’s Near-Earth
Object Program website, which lists every comet or asteroid of any size
at all that will be passing through Earth’s orbital neighborhood in the
upcoming days, months and years. The fact is, there are a whole lot
more of them than you likely know: from Feb. 5 to May 5 of this year, no
fewer than 77 space rocks that could, in theory have Earth’s name on
them, will be whizzing by. On March 20 alone, when you may have been
planning to celebrate the first day of spring, there will be seven.
Errant space rocks, of course, can do a lot of damage. Our moon is
believed to have been created when a Mars-size planetesimal sideswiped
the Earth 4.5 billion years ago. The dinosaurs were all but certainly
wiped out by a direct hit by a far smaller projectile 65 million years
ago. As recently as June 30, 1908, an asteroid measuring up to 330 ft.
(100 m) across famously exploded in the skies over the Tunguska region
in Siberia, unleashing a blast with the equivalent of 30 megatons of TNT
and destroying 770 sq. mi. (2,000 sq km) of forest.
What has a lot of cosmic worriers glancing skyward this month is the
announcement that on Feb. 15, a 148-ft.-long (45 m) asteroid known as
2012 DA 14 will pass just 17,200 miles (27,7000 km) above the Earth. And
if 17,200 miles sounds like a lot, consider that it’s only
one-thirteenth of the distance to the moon and actually below
the 22,000 miles (35,800 km) altitude at which some of our satellites
orbit. That leaves awfully little margin for error in NASA’s cosmic
calculations. So there’s plenty of reason to worry, yes? Well, no,
actually. But making that call for any one object — knowing which space
bullets are likely to hit us and which ones we’re likely to dodge — can
be a complicated business.
Near-Earth objects (NEOs) are very particular things. They’re bodies
that at some point in their orbit through the solar system dip within
1.3 AUs (astronomical units) from the sun.
A single AU is the sun-Earth distance, or 93 million miles (150 million
km), so 1.3 AU would be 121 million miles. There are untold thousands
of NEOs at large — 9,668 of which have so far been counted and cataloged
by NASA and other space agencies and observatories around the world.
Most are comparatively small, but 861 have a diameter of at least 1 km
(.62 miles), which could pack quite a wallop.
Still, just being within 1.3 AUs is only part of the danger equation.
Some NEOs circle the sun either entirely within Earth’s orbit or
entirely outside it, and while they may pass close by, they pose little
hazard, since a collision is likely only when the orbits of two bodies
cross. Others do cross Earth’s orbit with varying degrees of proximity.
Of all the known objects, 1,376 are classified Potentially Hazardous
Asteroids, based both on their size and on an approach distance that
brings them within .05 AU, or 4.65 million miles (7.5 million km), of
us.
The amount of damage an asteroid could do if it clobbered us is
determined largely by its mass. Anything that’s up to 40 m (131 ft.)
across or, as NASA puts it on its website, “smaller than a modest office
building,” would be incinerated by the atmosphere before it hit the
ground. At most, its remains would produce a blast equivalent to three
megatons — very bad news for anyone in the vicinity but not the kind of
Earth-cracking disaster they make bad action movies out of. Asteroids
from 40 m to 1 km could do “tremendous damage on a local scale,” NASA
says. A hit by a 2-km or larger asteroid would cause a million-megaton
blast and “produce severe environmental damage on a global scale. The
probable consequence would be an ‘impact winter’ with loss of crops
worldwide and subsequent starvation and disease.”
The overall risk any one asteroid poses is calculated on what’s known
as the Torino Scale, a grid with the probability of impact — from
effectively zero to effectively 100% — on its x axis and the size of the object on the y.
After objects are given a Torino score, they are ranked on a five-color
chart — think of the department of Homeland Security’s now defunct
terrorism-threat level — going from white (no hazard) through yellow
(“meeting attention of astronomers”) to red (certain collision, capable
of causing at least regional devastation of a kind seen only once every
10,000 to 100,000 years). And what is the Torino rating for 2012 DA 14,
which will whiz by on Feb. 15? A reassuring white — or no hazard at all.
(MORE: The Great, Wet Cosmic Rock)
None of this means that the bright red rocks aren’t out there, and
the world’s space agencies have a lot of smart minds working on ways to
deflect or destroy them. Advanced tracking and trajectory-modeling
computers allow us to know sometimes centuries in advance just where any
object will be at any time. And NASA has already proved itself adept at
rendezvousing with asteroids: the Dawn probe is in the midst of a
multiyear asteroid pas de deux, having orbited Vesta — the second
largest object in our solar system’s asteroid belt — from 2011 to 2012,
then peeled off to visit Ceres, the largest, with an arrival date set
for 2015. In 2001, the NEAR Shoemaker spacecraft actually touched down
on the asteroid Eros.
There will be no such attention paid to 2012 DA 14 when it passes,
though stargazers with telescopes in Australia, Eastern Europe, Asia and
especially Indonesia will be able to see it, moving south to north at
an apparent speed of 1 degree per minute. The little sky show belies the
dangers 2012 DA 14’s bigger, deadlier cousins pose, something that
keeps NASA scientists lying awake at night — partly so that you don’t
have to.
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Rakatalenta - Sabtu, 09 Februari 2013
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