Long before Star Trek's Mr. Spock (inset), many astronomers during the 19th and early 20th centuries thought a planet named Vulcan circled the sun inside the orbit of Mercury (shown transiting the sun, main image) and tugged on the latter, accounting for peculiarities in Mercury's motion. Well, Vulcan doesn't exist—Albert Einstein's general theory of relativity explains Mercury's orbit—but asteroids known as vulcanoids could, circling so close to the sun that we can't see them in its glare. If vulcanoids are real, they're only 7% to 21% as far from the sun as Earth is: closer than that and they evaporate in the sun's heat, farther than that and Mercury kicks them away. Now, in the March issue of Icarus, astronomers report results of a search using STEREO, two NASA spacecraft that revolve around the sun near Earth's orbit. The scientists found no new objects, ruling out the existence of any vulcanoids larger than about 6 kilometers, which is less than half the mean diameter of the little martian moon Deimos. But the search would have missed smaller vulcanoids, so an even deeper exploration might be, as Spock would say, the logical thing to do.
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