Mercury Probe Searches for Vulcanoids, Spies Venus

That's the fundamental problem with vulcanoids, a hypothetical band of asteroids orbiting between the sun and the closest planet in, Mercury.

In fact, for years that was the problem with studying Mercury, since looking at the tiny planet through a backyard telescope is like trying to make out the patterns on a moth's wing as it sits on a football stadium floodlight.

Bigger telescopes on the ground or in Earth orbit can see the planet, but in doing so, glare from the sun would damage the instruments' sensitive lenses.

Even Hubble, capable of peering into the far reaches of the universe, can't safely look too hard at the innermost planet.

To really see details on Mercury, you need a spacecraft that gets close enough to keep the sun's glare out of the frame.

Mariner 10 gave humans our first good look at Mercury during a series of flybys in 1974 and 1975. But that mission was able to take pictures of just half the planet—we had to wait until January 2008 to see the other side!

Our first glimpse of Mercury's "hidden" face came via the MESSENGER mission, a spacecraft now swirling around Mercury in a gravitational dance that will eventually see the probe settle into orbit in 2011.

Along the way, MESSENGER has been taking scads of pictures, and one of its targets has been the stretch of space inside Mercury's orbit where small, faint vulcanoids could be hiding.

The concept of vulcanoids arose from research done in the late 1800s, when astronomers trying to use the classical rules of celestial mechanics to chart Mercury's orbit kept finding things wrong with their calculations.

French mathematician Urbain Jean Joseph le Verrier took the challenge to heart, and in 1860 he announced that discrepancies in Mercury's orbit were due to an unseen planet, which he named Vulcan.

Le Verrier's theory was eventually disproven thanks to Einstein's revolutionary theory of relativity—when you include the sun's gravitational field in the mix, Mercury's orbit works out just fine, thanks, no extra planet required.

But the concept of something being between Mercury and the sun has lived long and prospered, and a number of missions (some using fighter jets!) have kept the search alive over the years.

MESSENGER has been making its vulcanoid searches when its orbit brings it closest to the sun. The craft has taken a host of snapshots in June 2008, February 2009, and most recently in January 2010. So far, nada.

But on January 16 MESSENGER did get an eyeful of neighboring Venus, the brightest dot in this polka dotted field of view.

Of course, Venus is so wildly overexposed in this picture that it looks like someone shot a hole in the sky.

But that highlights just how hard MESSENGER has to stare to even hope to catch a glimpse of a vulcanoid—if any are out there at all.