Three Space Rocks Will Pass Very Close To Earth Today


The Sunday Nation


17 July 2005


July 4th 2005 was an exciting day in the United States of America: they celebrated the 229th anniversary of their independence from Britain and on the same day their space probe, Deep Impact Collided with Comet Tempel 1. This crash was not a freak accident Ė it was planned. The probe was launched in January this year and put on a collision course with the comet.

A comet is a muddy ice ball (or icy mud ball!) in space. Usually, comets travel in highly elongated, elliptical paths that take them from the outer fringes of the solar system to the inner core only a few millions of kilometres from the sun. But Tempel 1 is a bit unusual in that its orbit lies between the planets Jupiter and Mars. (Visit to see a simulation of the orbit.)

As a result, it takes only five and half years to revolve around the sun. This is a very short time for a comet since the average is about 200 years. When Tempel 1 is close to Jupiter, the cometís orbit is greatly affected by the gravitational pull of the giant planet. Consequently, the Tempel 1ís period of revolution is quite erratic, varying by as much as 20 years.

The Deep Impact mission has caused concern amongst some people. They fear that the comet will collide with the Earth. These worries probably arise from memories of a Hollywood movie by the same name in which a comet struck the Earth. But there is no need to worry: the closest that Tempel 1 can get to our planet is 70 million kilometres!

However, for the Martians (if they exist) may not be so lucky. Current orbital predictions indicate that in mid January 2022, the comet Tempel 1 will pass at only 5.6 million km from Mars. By astronomy standards, thatís a close shave Ė and considering the erratic nature of the cometís orbit, it may be hard to tell it will miss or hit.

Now that we are reassured about Tempel 1, are there any other objects that can strike our planet? The answer is yes! There are currently 710 Potentially Hazardous Asteroids (PHA) known to astronomers. In fact three such objects will come within 25 million km of Earth today (July 17, 2005). And at about 10:00am on March 22, the year 2016, a comet will pass at less than 5 million km from us!

Luckily, none of the 710 known PHAs are on a collision course with Earth. However, astronomers estimate that a comet collides with our planet once every 32 million years, and a large asteroid once every 500,000 years. Since the last known comet collision occurred 65 million years ago, does this mean that we should expect another one any time now? Only God knows.

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