填空题

Note: When more than one answer is required, these may be given in any order. Some choices may be required more than once.
A=Saturn B=Venus C=Mercury
Which planets ...
· has been explored by 15 spacecraft 21. ______
· has a system of rings and more than I satellites 22. ______
· has a weak magnetic field 23. ______
· has the lowest density 24. ______
· is named after the Roman goddess of beauty 25. ______
· was believed to have a sinister influence upon people 26. ______
· was thought by ancient people as two separate stars 27. ______
· is completely covered with opaque clouds 28. ______
· are closer to the Sun than the Earth 29. ______
30. ______
Saturn For beauty and interest alike, there are few objects in the starry heavens to compare with Saturn. This magnificent planet, with the system of rings that encircles it, provides an unforgettable spectacle when it is viewed through a powerful telescope. The Saturnian system includes not only the planet and its rings, but also 11 or more satellites, or moons.
To the ancients Saturn appeared to be the most insignificant of the heavenly bodies that were supposed to circle the earth (the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn), as distinguished from the fixed stars. The glorious rings that surround the planet were invisible before the invention of the telescope in the first decade of the seventeenth century. Otherwise this magnificent crown might have saved Saturn from the sinister reputation that it once bore. Ancient astrologers maintained that it had a sinister influence upon people.
Saturn is far from the center of the solar system. The mean distance of Saturn from the sun is 1,428,000,000 kilometers, or about 9.5 times the distance of the earth from the sun. The density of Saturn is very low, much lower than that of any other planet. In fact it is only about three-quarters that of water. Because of this fact some astronomers hold that Saturn is far from having reached the solid condition.
Venus The beautiful white planet whose orbit lies between those of Mercury and of the Earth is called Venus after the Roman goddess of beauty. The planet is similar to our earth in size and mass. Its diameter is about 12,100 kilometers; the earth’s is 12,725 kilometers. Its mass is a little more than four-fifths that of the earth. Its density is about nine-tenths that of our planet.
Venus revolves around the sun once every 225 days in an orbit that is very nearly circular. As the planet revolves, it rotates about its axis once every 243.1 earth days, from east to west instead of in the west-to-east direction of most other celestial bodies. The planet is tilted only slightly with respect to the plane of its orbit.
As it proceeds along its orbit, Venus is sometimes on the far side of the sun from the earth, or at superior conjunction. At other times Venus is between the sun and the earth, at inferior conjunction. At superior conjunction it is quite far form earth. But at inferior conjunction it is only about 41,840,000 kilometers away -closer than any other planet. These variations in distance result in notable differences in the apparent size of the planet as viewed from the earth, at inferior conjunction, the apparent diameter is six times greater than at superior conjunction.
Venus has been explored by 15 spacecraft of which five were from the United States and ten were from the soviet Union. Some of these were orbiters, some were landers, and some were both. The planet is completely covered with opaque clouds, which make an almost perfect reflecting layer.
Mercury Mercury is the nearest of the planets to the sun. It is the smallest of all and also, at certain intervals, one of the brightest. In spite of that fact, it is generally not easy to see with the naked eye. For one thing, it appears in the heavens only during the hours of twilight and dawn, when even very bright stars do not appear at their best. Besides, it is often obscured by haze near the horizon. The great Polish astronomer Nicholas Copernicus once lamented the fact that he had not been able to see Mercury at all in his many years of observation of the heavens. Perhaps this was due to the nature of the district where he lived -the low and misty region of eastern Prussia where the Vistula flows into the Baltic.
Mercury makes such a small circuit around the sun that it is always comparatively near that body. It never rises in the morning or sets in the evening much before or after the sun. Because of its appearance sometimes in the east and sometimes in the west, some ancient peoples including the Egyptians, Hindus, and Greeks, thought of it as two separate heavenly bodies -a morning star and an evening star. The Greeks called the morning star "Apollon" after the god of the sun, and the evening star "Hermes," the name of the swift messenger of the gods, because the planet’s apparent motion among the stars was so swift. It is said that the Greek philosopher Pythagoras, who lived in the sixth century B.C., was the first to recognize that the morning star and evening star were one and the same heavenly body. That fact was well known to Roman astronomers. Hermes was worshiped by the Romans under the name of Mercury.
Scientists were surprised to discover that Mercury has a very thin atmosphere consisting of helium. It is so thin that the word "atmosphere" gives the wrong impression, but no such gas envelope has been expected at all. Another surprise was that Mercury has a weak magnetic field. Whether this field is produced by the planet itself or produced in some way by the solar wind -the stream of particles flowing out from the sun -is not yet certain. But at any rate, the interior of Mercury is probably earthlike in composition, with an iron core and a less dense outer crust.