Asteroid's trajectory could pose danger for Earth in 2036
Pastor Dale Morgan
Asteroid's trajectory could pose danger for Earth in 2036*
By Wayne Harris Myrick
On Friday, April 13, 2029, the asteroid Apophis will pass quite close to
Earth. And, no, NASA did not make a mistake in calculating its impact
chance and have that mistake corrected by a 13-year-old German student.
That e-mail you may have read stating such is a new urban myth (see
http://tinyurl.com/5feuvy). But depending upon the exact path the
asteroid takes in 2029, it could potentially strike Earth during its
subsequent pass by our planet in 2036. NASA's best estimate at the
moment is that the 2036 pass will miss by 30,500,000 miles.
The last significant impact by an asteroid or comet on Earth occurred
100 years ago, over the forests over the Tunguska River area of southern
Siberia in Russia. On June 30, 1908, at 7:17 local time that morning, a
nomadic Evenki herdsman sat outside his hut watching his reindeer graze
and the birds flit about noisily . He jerked his head up at a sudden
flash of light behind him. He spotted a dazzling fireball crossing the
sky, followed by a flare brighter than the sun. As he turned, the blast
of a shock wave knocked him unconscious. He awakened later to find his
entire herd of reindeer lay dead and wondering what hit him.
At the time of the Tunguska event, Russia was in the throes of a
revolution, and no scientists reached the remote site for 19 years. When
scientists finally arrived, they found a region of flattened and charred
trees for 30 miles around. Eyewitnesses recalled a huge fireball
traveling north-northeast with a flame that "split the sky in two.”
Something had entered Earth's atmosphere from space. Despite intense
surveys of the region, no crater or meteorites were ever found. It took
several decades and powerful computers before scientists' understanding
of such collisions allowed them to explain the Tunguska event.
The asteroid that created Meteor Crater in Arizona was composed almost
entirely of iron and nickel. Large chunks of it survived to reach the
ground as meteorites, but none were found at Tunguska. Asteroids may be
composed of metal, stone or a mixture of the two. Iron meteorites often
survive to reach the ground. Stony meteorites are more common but more
fragile. Unless they are very large, their passage through our
atmosphere destroys them.
Asteroids race through the air at speeds up to 150,000 mph. At such
speeds, air can't move out of the way, and tremendous pressure builds up
in front of the object. And air can't get in behind the asteroid because
of its speed, so a vacuum forms on the back side. The pressure
differential shatters all but the densest and largest asteroids.
Nickel-iron meteorites often shatter into pieces and strike the ground
as a swarm of smaller meteorites, but stony meteorites may be completely
vaporized.
The Tunguska object exploded five miles above the surface with such
energy that only microscopic dust survived. With nothing to strike
Earth, no crater formed. When the stony asteroid entered Earth's
atmosphere, it was perhaps several hundred feet across. But it entered
at a shallow angle and most of it was vaporized away in its passage
through the air. By time it exploded, it was only 70 feet across. Had it
come straight in, more of the object might have survived until it
exploded, with more devastating results.
Twice a day, the ocean waves along all coasts crawl up and down the
shoreline. These tides result from the tug of the moon's gravity. It
causes the oceans all around the world to rise upward when the moon is
overhead and to lift up when the gravity is decreased because the moon
is on the opposite side of Earth (high tides), and to move away when the
moon is on the horizon, either rising or setting (low tides).
This tidal sloshing also creates a tiny bit of friction on our planet.
It's extremely small — imagine the frictional pull you might get when a
string rubs against you as you jog — but over the 4.5 billion years that
the moon has been orbiting Earth, it adds up. In ever so tiny bits, the
friction slows our planet's daily rotation; the days are growing longer
by minuscule increments.
We can even see this in the study of tree rings on the oldest trees, a
thousand years or more old. We can see this in the geological record,
too, studying the striations of ancient beaches cause by the tidal ebb
and flow. Take it back to the beginning, and our day was less than 18
hours long instead of the 24 hours we now enjoy. And it will continue to
grow.
For consistency, scientists define a "day” as exactly 86,400 seconds.
Our current day length, as determined by the rotation of Earth compared
to the stars, is actually a bit longer than that, 86,400.002 seconds.
Over time, that difference adds up to one full second, and in order to
keep time coordinated with the rotation of the planet, we add a "leap
second,” not to be confused with a leap day which corrects for Earth's
orbit around the sun. At Earth's rotation rate now, we have to add a
leap second in seven out of every 10 years, but by the middle of this
century, we'll be adding one a year. If we keep the exact same system of
timekeeping, we will eventually have to add several per day, but that
won't occur for many centuries.
The very first leap second was added to our clocks in 1972. Since then,
our clocks have been adjusted ahead by 23 seconds. By the order of Earth
Orientation Center of the International Earth Rotation and Reference
Systems Service in Paris leap seconds are added, when necessary, only on
either June 30 or Dec. 31.
But the problem occurs when your computer doesn't know about an added
leap second. I'm generally happy if my watch is correct to within a few
minutes, but if your clock determines the accuracy of, say, the Global
Positioning System network, one second off will cause a huge difference
in the reported position. GPS isn't the only computer-controlled system
that demands exact time marking. There is an ongoing debate on whether
it would be better to add leap seconds as we do now and have to
continually update various clocks and computers or to define a more
precise way as to when to add leap seconds or to just ignore them
altogether and add, say, a leap minute every 50 years, something that
computer administrators can more easily handle.