Astronomers may now be straining their scientific vision to catch a glimpse of the coming collision on Jupiter, but nearly a century ago they could have had a front row seat at another cosmic crash right here on Earth. In 1908, a large comet or asteroid appears to have exploded over Siberia, felling trees over nearly 800 square miles, igniting fires and sending so much debris into the atmosphere that Europeans could read newspapers at midnight by the light of the glowing sky.
The Siberian crash is but one of many cosmic bombardments that have showered the Earth for eons. Most of the celestial wayfarers that collide with Earth are so small they burn up high in the atmosphere -- sometimes appearing as meteors -- before ever hitting the ground.
On the rare occasions that Earth is hit by larger heavenly bodies, the results can be cataclysmic. The most infamous collision is thought to have occurred some 65 million years ago; scientists believe that a huge asteroid struck Earth near the Yucatan Peninsula, darkening the skies with dust and leading to the extinction of the dinosaurs.
While celestial bodies big enough to wreak major damage hit Earth only every 10 million to 40 million years, it is not always easy for astronomers to predict whether a big one is headed our way. Four years ago, for instance, an unforeseen asteroid swept within 450,000 miles of the Earth, which is a close call in astronomical terms. In 1968, the half-mile-wide asteroid Icarus also passed uncomfortably close by.
Thanks to the coming collision on Jupiter, astronomers should soon know more about the effects of cometary bombardments. For instance, if tiny pieces of the comet drill deep into the Jovian atmosphere, says Mordecai-Mark Mac Low of the University of Chicago, astronomers will know that "we have to worry about even the little ones."
