Sky watch: Vesta asteroid shines with the brightest

In the void between Mars and Jupiter lie millions - perhaps billions - of rocks known as asteroids. Most range in size from tiny pebbles to chunks the size of mountains. The largest of these, known as Ceres, is a mere 600 miles across, yet contains nearly half the mass of all asteroids combined. It was also the first such body to be found - on New Year's Day 1801.

The fourth to be discovered was Vesta. With a diameter of only 326 miles - about the size of Arizona - Vesta appears to even the largest telescopes as a speck of light the size of a dime seen from five miles away.

Astronomers have learned that Vesta reflects 25 percent of all sunlight falling onto its surface, and this accounts for its relatively great brightness (the moon, by comparison, reflects only 12 percent). As a result, Vesta holds the distinction of being the brightest of all asteroids, occasionally appearing to binoculars or even the unaided eye from a dark rural site. Right now is one of those times - a time known as Vesta's "opposition."

Opposition, as regular readers of this column might recall, is a time when a solar system body appears opposite in the sky from the sun. It's during this time that it also lies closest to Earth and shines its brightest. Vesta reaches its opposition this year on Feb. 18.

To find Vesta, dedicated stargazers will need to brave the cold evening temperatures to scan the eastern sky this coming week. After dark, Leo, the lion - marked by a backward question mark of stars that forms the head of the lion - lies just above the eastern horizon. His hind end is formed by a triangle of stars now appearing lower in the sky.

In the middle of the arc that forms the backward question mark lies a medium bright star named Algeiba and, if you aim binoculars toward it, you should see a star pattern similar to that in the accompanying illustration. Vesta will appear just above Algeiba and, of course, much fainter.

It appears just like every other star in the sky so to be sure it's Vesta, make a sketch of the stars you see and return to it night after night. Within a week or two, you should notice that one of these "stars" seems to be moving through this stellar pattern.

This, of course, is no star; it's Vesta.