"…although the annual probability of the Earth being struck
by a large asteroid or comet is extremely small, the
consequences of such a collision are so catastrophic that it is
prudent to assess the nature of the threat and prepare to deal
Introduction – Asteroids, Meteoroids, and Impacts on the
Asteroids are small rocky or metallic bodies orbiting the Sun.
They are considerably smaller than the traditional planets and
are often called "minor planets." The largest asteroid is Ceres
with a diameter of 1000 km. While sixteen asteroids have
diameters of 240 km or larger, the vast majority are a kilometer
in size or less (Hamilton).
A meteoroid striking the Earth becomes a meteor or "shooting
star" as it burns up in the atmosphere. Most meteors are from
particles the size of a sand grain, but occasionally larger
objects enter the atmosphere and produce a spectacular display
called a fireball. If the object is large enough, it can survive
its passage through the atmosphere and end up on the ground as a
meteorite. When the Earth encounters a rich lode of concentrated
debris left over from a comet, a meteor shower occurs, and there
are several well known meteor showers visible throughout the
The primary risk to the globe and its major ecosystems is from an object large enough to disturb the Earth’s climate by injecting massive quantities of dust into the stratosphere. A large object striking the Earth at 20 + kilometers per second (a typical meteoroid velocity) has an enormous kinetic energy equal to its mass times its velocity squared. It would vaporize upon contact and excavate a large crater. The object’s vapor and the material excavated from the crater would then be thrown into the atmosphere creating a suffocating mass of dust that rapidly envelopes the globe. This dust could block sunlight for months lowering temperatures around the world causing worldwide crop failures and global starvation. An asteroid or comet mass of several billion tons entering the atmosphere at 10-60 km/sec would be necessary to cause such destruction. This would be the equivalent of a million megaton explosion of TNT.
An asteroid 1-2 kilometers in diameter is large enough to render such havoc, and smaller objects in the order of tens of meters in diameter could easily destroy a large metropolitan area (Morrison). The crater record on the Earth is sparse, because plate tectonics and weathering erase most craters in short geological time spans. It took until the 1960’s for geologists to completely accept that some craters on the Earth were formed by impacts (Barringer). To estimate the Earth impact rate for various sized bodies, a number of different parameters need to be examined. These include counts of meteor craters on the Moon, paleontological evidence of mass extinctions on the Earth, studies of orbits of asteroids and comets, and satellite measurements of explosions in the upper atmosphere from large meteoroids (Cooke, 2004).
A 1 kilometer asteroid probably strikes the Earth every few million years, while a global killer on the order of 5-10 kilometers strikes the Earth every few hundred million years. A 100 meter asteroid may strike the Earth every thousand years (Cooke, 2004). In fact, in 1908, a 60 meter asteroid exploded in the upper atmosphere over a remote area in Eastern Siberia (Tunguska) and flattened trees for thousands of square miles.
On February 15, 2013, a ~ 20 meter asteroid fragment entered the atmosphere over Russia producing a brilliant superbolide meteor over the southern Ural region centered near Chelyabinsk. It entered the atmosphere at 14 km/sec relative to the Earth and was speeded up to 18 km/sec by the Earth's gravity prior to exploding at a height of 23 km with the equivalent of a half-megaton of energy (Harris, 2013). It produced 20-30 times more enegery than was released by the atomic bomb detonation at Hiroshima. About 1500 persons were injured mainly by broken glass and several thousand buildings in six cities across the region were damaged.
high-velocity impact of a 2 kilometer asteroid with the Earth
could kill a billion people. A ten kilometer asteroid impacting
the Earth could extinguish us as a species (Foster, 2005). The
odds of the Earth suffering a catastrophic collision with an
asteroid over the next century is roughly estimated at 1 in 1500
to 1 in 5000 (Schultz, 2001). The probability of being killed by
an impact event is very small on the order of 1 in 10,000 to
1/100,000. Driving and overeating are far bigger risks to the
Past History – Do We Really Have to Worry?
Yes! The Earth has been struck at least once with a large object
that produced sudden global extinctions and altered the dynamics
of life on the Earth. The extinction of the dinosaurs at the end
of the Cretaceous Period was caused by the impact of a 5-10
kilometer asteroid or comet. The 180-300 kilometer Chicxulub
crater from that event is buried in the Gulf of Mexico off the
Yucatan Peninsula. That impact not only eliminated the
dinosaurs, but it devastated many other life forms.
Approximately 60% of all Cretaceous species disappeared suddenly
| Next |