Killer Asteroids

-is one lurking in our neighborhood?

By Tim Hunter

"…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 with it"

David Morrison

Introduction – Asteroids, Meteoroids, and Impacts on the Earth

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).

Thousands of asteroids are known and thousands of small asteroids are being discovered every year. Most of them lie in the Main Belt between the orbits of Mars and Jupiter, but some of them are in stable resonant orbits associated with Mars, Jupiter, Saturn, Uranus, and Neptune. Their origin is unknown, but the current thinking favors their being left over material from the formation of the Solar System that never coalesced into a larger body (Moore, 2002).

The Earth is constantly bombarded with debris (meteoroids) from space. This debris represents bits of rock and metal left over from the formation of the Solar System as well as varying-sized chunks of asteroid fragments produced and scattered in our direction by countless asteroid collisions mostly in the Main Belt. As comets swing by the Sun, they leave debris in their wake – tiny rocks and ices – which the Earth frequently encounters. The amount of material being swept up by the Earth is estimated at approximately 50,000 metric tons a year (MadSci Network). This is a lot of material, though its contribution to the overall volume and mass of the Earth is trivial compared to the large size of the Earth.

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 year.

The Moon and other Solar System bodies are heavily cratered from massive bombardment of asteroids and comets in their past. Such heavy bombardment is rare today, but a modest sized asteroid or comet impact is still possible, and it could wreck havoc on the Earth, even ending civilization as we know it (Cook, 2004; Morrison).

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.

A 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 average individual.

Impacts by smaller objects are much more common than those of large objects. Smaller objects could produce destructive tsunamis, directly strike a large city, or explode over a large populated area killing millions of people. Fortunately, the Earth’s atmosphere provides an increasingly protective role as objects become smaller. Most meteoroids fragment into small relatively harmless pieces as they travel through the atmosphere at high speeds. Even a very solid mainly metallic asteroid fragment probably needs to be 100 meters in diameter for it to survive passage through the atmosphere intact (Cooke, 2004).

Satellites operating for the United States Department of Defense have been monitoring the Earth for nuclear weapons and missile launches for several decades. Their sensors in visible and infrared light can detect flashes of light from meteoroid atmospheric impacts. Five kiloton equivalent (Hiroshima atomic bomb size) impact events occur in the Earth’s atmosphere every year, but we are saved from harm by the Earth’s atmospheric blanket.

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 (Moore, 2002).

Many other extinction events have been attributed to impacts, but the evidence is less convincing than for the extinctions at the end of the Cretaceous. However, it is well accepted the Chesapeake Bay was produced by an impact 35-36 million years ago, and there is evidence suggesting a role for comet or asteroid impacts in the changes of the Earth in the late Eocene (Tagle, 2004). The largest extinction in the Earth’s history took place at the end of the Permian Era approximately 250 million years ago. More than 90% of marine taxa became extinct. The cause for this extinction is vigorously debated, but some claim it was caused by a large impact off the coast of Northwestern Australia (Becker, 2004). The dramatic impact of Comet Shoemaker Levy 9 with Jupiter in July 1994 also provided us with a visual display of what a relatively small object traveling at high speed can produce by its collision with a much larger body.

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