Most asteroids can be found in the main asteroid belt that lies between Mars and Jupiter. Scientists have always wondered why these rocks sometimes change their orbits and aim for a possible collision with Earth. A new study suggests that the sun?s energy may be responsible for sending the objects hurtling down on us, in a process called the Yarkowsky Effect, named for a Russian engineer who discovered it a hundred years ago.

The Y effect explains that different parts of the sun emit radiation in different amounts. Although scientists have known about the Y effect for a century, they didn?t realize it was responsible for pulling asteroids out of their orbits. A tiny difference in radiation is enough to kick a rock out of the asteroid belt and onto a path into the inner solar system where Earth is located.

The smaller the asteroid, the larger the Y effect. Even a small asteroid could do significant damage, depending on such factors as its angle of approach, the impact speed, the density of the asteroid, and whether it struck water, land, or a populated area.

Astronomers are trying to estimate the size and orbits of the 900 largest asteroids that could possibly collide with earth. About 40 per cent of these have been located and none are predicted to hit the Earth within the next few hundred years. However, this means that the location of the remaining 60 per cent is unknown. They?re hard to spot because they appear as small, faint glowing objects and are located in parts of the sky that are hard for astronomers to survey.

Once all the asteroids are located, astronomers can calculate the odds of them hitting the Earth. “People probably don?t need to stay awake worrying about asteroids hitting the Earth, but the probability isn?t zero,” said author William Bottke, of Cornell University. “It?s only prudent to find as many of these asteroids [as possible] just to make sure we?re not in trouble.”

These 900 asteroids have all left the asteroid belt, probably due to the Y effect. The largest of them is 40 miles across. A 6 to 10 mile wide asteroid was big enough to destroy the dinosaurs, as well as 75 per cent of all living species on Earth, 65 million years ago.

In 1998, astronomer James Scotti discovered a mile-wide asteroid that was at first predicted to pass within 30,000 miles of earth. Later, the prediction was revised to a more comfortable 600,000 miles.

One asteroid that has caught the attention of astronomers recently is Eros, a peanut-shaped rock 21 miles long, 8 miles wide, and 8 miles thick. About the size of Manhattan, it tumbles through the solar system, flipping from top to bottom every 5 hours. It orbits the sun in a path that brings it as close as 12 million miles to Earth at times. Right now it is 150 million miles away from us, beyond Mars.

The NEAR-Shoemaker space probe, which weighs 1,775 pounds and is the size of an automobile, has been orbiting Eros for about a year, and is now beginning to run out of fuel. Scientists have decided to try landing the space probe on Eros, which will be a first. NEAR-Shoemaker carries instruments that can analyze the asteroid?s chemical components and magnetic field and has an array of cameras to map its surface.

“This wasn?t in the original plan, but it?s been in my mind for a long time,” said Robert Farquhar, mission chief at the Johns Hopkins University Applied Physics Laboratory, where scientists built the space probe and are directing the mission for NASA. “We?re out of time, out of money and out of fuel. This is the first time anyone has tried to land on a small body. It would be a nice way to end it.”

On October 26, the spacecraft swooped as low as 3 miles from the surface of Eros, sending back provocative photos of the asteroid. It will be traveling about a yard a second when it touches down. “That?s about jogging speed, but if you hit a brick wall when you?re jogging, it hurts,” says Farquhar. “It?s risky, but the mission is ending anyway. We?ll get a lot of bonus science.”

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