Table of Contents

Orbital Mechanics

Near Earth Asteroids

Watch Out for Falling Rocks

Asteroids - What is Their Impact?

Through both documented history and the Earth's geological record itself, it is clear that our planet has been the target for massive asteroid bombardment since the beginning of time.

One of the youngest and most famous meteor craters is the Barringer Meteor crater in Arizona. It is located just off Interstate 40, close to the New Mexico border.

While most meteorite material breaks up upon impact with the Earth's atmosphere, asteroids as large as 4 m in diameter, and with an explosive yield of about 16 kilotons of TNT, hit the earth at a frequency of about 1 every year.

Larger impacts, such as the 150m, 844,000 kiloton asteroid that hit northern Arizona 50 thousand years ago, (see left), or the 10 km asteroid that hit the Yucatan 65 million years ago, cause such widespread devastation that life itself is terminated.

The Earth has had enough hits and near-misses to warrant continued identification and tracking of these potentially life threatening projectiles.

Well (and Not So Well) Behaved Girls and Boys

In the late 1700's, after the discovery of the seventh planet, Uranus, by British astronomer Sir William Herschel, it was concluded by the Prussian philosopher Hegel that since the inherent perfection of the number seven had been realized, astronomy had reached its end and there could be no additional planets.

In 1801, however, Giuseppe Piazzi discovered what he believed to be an eighth planet, orbiting the Sun between Mars and Jupiter. The planet was very small, only about 900 kilometers in diameter, and Piazzi named it Ceres.

Over the next few years three additional planets were discovered in similar orbits close by. After careful consideration, German Astronomer Heinrich Olbers suggested that perhaps the objects he and his fellow astronomers were discovering in the night skies were not small planets at all, but rather large fragments of planets which had broken up and were orbiting the Sun. Olbers concluded that if this was the case then astronomers could expect to find more fragments as they continued to study the skies.

As telescopes improved over time, Oblers hypothesis was substantiated, and by 1890 over 300 'asteroids' were discovered, all orbiting the Sun, and bracketed by the orbits of Mars and Jupiter. Upon each discovery these well-behaved asteroids were given female names.

In 1898, however, an asteroid was discovered which actually crossed the orbit of Mars and sometimes swung within 20 million kilometers of Earth. Due to its errant behaviour, and to distinguish it from its better-behaved female counterparts, which continued to orbit in an orderly fashion around the Sun, this large asteroid was given the male name Eros. Over time additional male planet-crossing asteroids were discovered. Some of these poorly behaved asteroids crossed not only the orbit of Mars, but that of the Earth as well; and in 1932 the asteroid Apollo crossed our orbit and passed within 10 million kilometers of our Earth. In 1936, the asteroid Adonis passed within 2 million kilometers, and in 1989 a small, (800 meter), unnamed asteroid passed within only 720,000 km of Earth, having swept through a point in space that the Earth had occupied less than six hours before!

Of the estimated 2000 "male" or "near-Earth" asteroids larger than 1 km out there in space today, only about 200 are known to scientists. Over 5000 "female" asteroids, including every one larger than 10 km, hundreds larger than 100 km, and even one 900 km, are known to exist in the main belt between Mars and Jupiter.

Female asteroids are much further away, and therefore harder to see than their Earth-crossing male counterparts; however, it is estimated that there are well over 2 million Main Belt "female" asteroids larger than 1 km.

The risks associated with asteroid collisions are enormous. The largest mass extinction of animals on the planet Earth was the result of an asteroid collision with the Earth 65 million years ago. The impact left a 180 km diameter crater (known as the Chicxulub Crater), on the Yucatan peninsula in Mexico.

Monitoring the Orbits of Asteroids

Manicouagan Crater

Try to imagine the blast that came from this impact crater. The crater is almost 100 km in diameter.

The blast shock would have expanded to a distance of several thousand kilometres and created a pressure wave strong enough to circle the planet several times.

The Manicouagan crater is slightly more than 200 million years old. It is located in the province of Quebec about 200 km south of Labrador City.

Could a similar impact occur today? Luckily large impacts are exceedingly rare, but not impossible. That is why astronomers have set up a program to scan the skies for asteroids which might have orbital paths which would lead them to collide with the Earth.

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