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Star Spangled Banter:
09/01/2008
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Mercury
Mercury may be the black sheep in the sun’s family, at least as far as our perception is concerned. So far, the only space mission to reach Mercury was NASA’s Mariner 10, which linked Mercury and Venus in two-planet flybys in 1974 and 1975. Since then, it seemed as though scientists had lost interest in that hot, rocky planet, for there have been no further attempts to send rockets to Mercury until the launch of Messenger, in 2004, scheduled to arrive later this year. The interest is probably there, but space expeditions are frightfully expensive, and discoveries in the outer planets and their satellites have overshadowed curiosity about Mercury. Conditions more favorable to the development of life are being explored in the planets outside the earth’s orbit, and human priorities seem to center around exploring where there might be clues about the possibilities of life elsewhere. Let’s start off with a few factoids. Mercury is the smallest body in the solar system, with the exception of the asteroids, comets, planetary satellites, and Pluto (that bastard in the family tree, no longer officially classified as a planet). Even Ganymede, one of Jupiter’s satellites, is larger than Mercury, which is approximately 3,000 miles in diameter, and moves around the sun at a mean distance of 36 million miles, completing its orbit in 116 days. It rotates on its axis only three times during two of its revolutions around the sun, resulting in a very long “day” on that planet. Because of its very slow rotation, Mercury’s temperature varies between extremes of 800°K on the sun-facing side to only about 100°K on its dark side. The surface of Mercury has been mapped by radar, and has been shown to be heavily cratered. Mercury also has a very strong magnetic field, and this, coupled with its exceptionally high density, has led to the conclusion that the tiny planet has a large core of iron, the largest in proportion to its size of any planet in the solar system. Although Mercury is one of the brightest objects in the sky, a surprisingly large number of people, including astronomers, have never seen it. The reason for this is simple. Mercury is one of the “inner planets,” which means that it lies between the earth and the sun, and is the planet closest to the sun. It never appears farther than 28° from the sun. As a result, Mercury can never be seen in our night sky, where it would be a sight impossible to miss. Its brightness, however, is overshadowed by the twilight sky in which it is always found. Like the moon and Venus, Mercury also shows phases. When it is between the earth and the sun, much of what we see is the dark side of the planet, hence “New Mercury.” Mercury is “full” when it is on the opposite side of the sun, where observation is very difficult or impossible. Like Venus, Mercury appears brightest when it is a thin sliver, nearest the “new Mercury” position. This seems counter-intuitive because one would expect it to be brightest during its full phase. However, when Mercury (and Venus) is full, it is also at its farthest distance from the earth, and hence appears much smaller. This is not the case with the moon, which orbits the earth, and its distance varies relatively little. Mercury and Venus are the only planets in our solar system with no satellites. In late 2008 or 2009, NASA’s Messenger spacecraft will begin to orbit Mercury to observe the physical conditions on its surface and near environment, and will offer the planet some companionship, its first in a long and lonely history. Messenger will sweep a highly elliptical orbit, varying in distance from between more than 9,400 miles and 125 miles. Whatever it finds is likely to include some surprises, so please stay tuned! © 2008, by Nathan B. Miron, Ph.D. Email: nathanmiron@kenwoodpress.com |
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