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The New Solar System
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11328 |
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Section : |
NATURAL SCIENCE
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| Issue
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5 / 1986 |
5,717 Words |
| Author
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Charles Sheffield
Charles Sheffield is the author of several science fiction
novels as well as numerous articles and essays on physics and
space science. |
The solar system is usually looked on as a near permanent, slowly changing entity, much the same now as it was a hundred million years ago. From one point of view, however, there have been enormous and fundamental changes, in the past few decades. Those changes have been in our state of knowledge--our ideas as to what the solar system is like. Not a single planet seems the same today as it did forty years ago.
What are these changes in the solar system "family" which consists of all the planets, moons, and other bodies that are gravitationally bound to our Sun? Let us begin at the center which, since the time of Copernicus, means the Sun, and move from there outward.
Mercury
The planet closest to the Sun has been known since ancient times. Forty years ago it was considered to be a hot, airless ball, moving around the Sun in a rather elongated ellipse every 88 days. It was believed to present the same face to the Sun all the time, so that one side would be fiercely hot, the other chillingly cold.
New information came with the U.S. Mariner 10 spacecraft, which during in 1974-1975 sent back pictures from three close encounters. At first sight the surface of mercury is just like the Moon--cratered, barren, and airless. The spacecraft also discovered a magnetic field--about 1 percent the strength of Earth's. This, together with the planet's high density, suggests a substantial iron core, probably 1,500 kilometers in diameter (the whole planet is only 4,500 kilometers in diameter.)
Mercury's rotation period was a surprise. The idea that it was totally locked and presented the same face to the Sun all the time was wrong. If that were the case, the rotation period of Mercury would be 88 days, the same length as its year. Mercury actually goes through one complete turn on its axis in 58.6 Earth-days, which is exactly two-thirds of its year. However, since a day is the time interval required for a given location on a planet to rotate from pointing exactly at the Sun to again pointing exactly at it, a day on Mercury lasts two of its years. The planet is fiercely hot all over, not just on one side as was previously thought.
Mercury has probably changed little in appearance in the past three billion years. One of its differences from the Moon is that it is wrinkled on its surface, probably as a result of more cooling and contraction
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