StarDate: September 27, 2009

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The Messenger spacecraft is swooping toward its third encounter with Mercury, the planet closest to the Sun. It's scheduled to fly about 150 miles above the planet's surface tomorrow night.

Back in the 18th and 19th centuries, astronomers thought there might be another planet even closer to the Sun than Mercury is. It was called Vulcan for an ancient god of fire. A few astronomers even thought they'd found it, but none of the findings could ever be confirmed.

Even though Vulcan doesn't exist, those who hunted for it weren't completely off target. It's possible that asteroids may orbit the Sun inside the orbit of Mercury, right where Vulcan was supposed to be. These possible chunks of rock have been dubbed "vulcanoids," and astronomers have conducted several searches for them in the last couple of decades.

But if vulcanoids do exist, they're so small and so close to the Sun that they're hard to spot from Earth.

So Messenger has joined the search. It passes quite close to the Sun on each orbit, giving it a much better angle for scanning the space on either side of the Sun than telescopes on Earth have.

But like the ground-based searches, Messenger has come up empty. It's taken hundreds of pictures, but none of them has captured a vulcanoid in flight. The search will continue, but so far, vulcanoids seem to be going up in smoke.

We'll have more about Messenger's encounter with Mercury tomorrow.

Script by Damond Benningfield, Copyright 2009

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