Margin of Error
 Astronomers at McDonald Observatory and elsewhere are keeping a close eye on asteroids that could someday collide with Earth. These are the "Near-Earth" asteroids, and include Eros, which is shown in this 2001 image from the NEAR-Shoemaker mission. These chunks of rock and metal can be up to several miles in diameter. Their orbits cross or come close to Earth's orbit, so they could someday slam into Earth. Such a collision could have catastrophic effects on a local, regional, or global level. Finding and tracking these asteroids will help astronomers determine if and when any of them may menace Earth. [NASA/JHU-APL]
Such stories have become common in recent years. Astronomers have discovered about 3,000 asteroids with orbits that cross or come close to Earth's orbit. Most don't actually get very close to Earth, but a few will pass within a couple of million miles or less.
The scares come about because it takes a while to pin down an asteroid's orbit. When astronomers discover an asteroid, its location is passed along to other astronomers. They can then track the object on later nights.
Astronomers can calculate a rough orbit with as little as two or three sightings. Such an early calculation has a large margin of error, though. For an asteroid that's going to pass close to Earth, the margin of error can be so big that it looks like the asteroid might actually hit our planet. With more observations, though, astronomers can calculate the orbit much more precisely. In every case so far, that's been enough to rule out the possibility of collision.
We'll have more about keeping tabs on these asteroids tomorrow.
Script by Damond Benningfield, Copyright 2005
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