Mercury in the Evening

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A planet that's getting a little extra love this year is putting in a decent appearance in the evening sky. It's quite low in the west-northwest as darkness begins to fall, and sets soon after.

The planet is Mercury, the closest planet to the Sun. It's a small, dense, battered chunk of rock that looks a lot like the Moon, with a surface that's scarred by countless impact craters.

The largest feature on Mercury is a big "œbull's-eye" known as the Caloris Basin. It consists of several rings of hills around a fairly smooth center. It probably formed when a boulder as big as a state slammed into Mercury billions of years ago. The impact created the rings, and lava bubbled up from below the surface to pave the center.

Caloris Basin was discovered by Mariner 10, a spacecraft that flew past Mercury in the 1970s. Because of the lighting, though, it couldn't see the whole basin. But a spacecraft called Messenger got a better view early this year, when it became the first craft to visit Mercury since Mariner 10. It found that the Caloris Basin is about 1800 miles across -- 200 miles bigger than earlier estimates.

Messenger will pass by Mercury again later this year, then enter orbit around the planet in 2011.

Mercury is quite low in the sky in early evening. It looks like a moderately bright star. Binoculars will help you find both Mercury and the little Pleiades star cluster just to its lower right.

Script by Damond Benningfield, Copyright 2008

For more skywatching tips, astronomy news, and much more, read StarDate magazine.