Near-Earth Asteroids
A celestial boulder as big as several city blocks will sneak up on Earth this afternoon. It'll pass about 10 million miles behind the planet. Another one will stage a similar encounter on Thursday. And yet another will pass much closer in July: just two million miles away.
It turns out that our region of the solar system can seem uncomfortably crowded. So far, astronomers have discovered about 3,000 near-Earth asteroids - big chunks of rock and metal that cross or come close to Earth's orbit around the Sun.
Astronomers want to know as much as they can about these objects - how big they are, what they're made of, and what their orbits are like. In part, that's because these objects can provide new insights into the formation and evolution of our solar system.
But there's a more pragmatic reason for studying near-Earth asteroids: About a quarter of them are at least one kilometer in diameter - big enough to cause widespread destruction if they hit our planet. In fact, impacts by large asteroids may have exterminated much of the life on Earth more than once.
Smaller asteroids have caused destruction on a smaller scale. One that was perhaps a hundred yards in diameter exploded above Siberia a century ago, flattening hundreds of square miles of forest.
So far, none of the asteroids that astronomers have discovered and plotted pose an immediate threat. But a few have created some nervous moments. More about that tomorrow.
Script by Damond Benningfield, Copyright 2005
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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