Dawn Melzer, age 15, of Casper, Wyo., for her question:
WHICH IS THE SMALLEST PLANET?
Mercury is the smallest planet in our solar system. It is also the closest to the sun. It has a diameter of about 3,100 miles, about two fifths the earth's diameter.
Because the size of Mercury is so small and it is so near the brightly shining sun, the planet is often hard to see from the earth without a telescope. At certain times of the year, Mercury can be seen low in the western sky just after sunset. At other times, it can be seen low in the eastern sky just before sunrise.
As Mercury moves around the sun, it rotates on its axis, an imaginary line that runs through its center. The planet rotates once very 59 earth days rotation slower than that of any other planet except Venus. A day on Mercury lasts about 180 earth days because Mercury rotates so slowly. The earth rotates once a day.
Until 1965, astronomers believed that Mercury rotated once every 88 earth days, the same time the planet takes to go around the sun. If Mercury did this, the sun would seem to stand still in Mercury's sky. One side of the planet would always face the sun and the other side would always be dark.
In 1965, astronomers bounced radar beams off Mercury. The signals returning from one side of the planet differed from those from the other side. Using these beams, the astronomers measured the movement of the opposite sides and found that Mercury rotates once in about 59 days.
Mercury travels around the sun in an elliptical or oval shaped orbit. It is about 29 million miles from the sun at its closest point and more than 43 million miles at the farthest point. Mercury moves around the sun faster than any other planet because it is the closest to the sun. Mercury travels about 30 miles per second and goes around the sun once every 88 earth days. The earth goes around the sun once very 365 days, or one year
Because Mercury moves around the sun faster than any other planet, the ancient Romans named the planet in honor of the swift messenger of their gods.
Mercury is dry, extremely hot and almost airless. The sun's hot rays are about seven times as strong on Mercury as they are on earth. The sun also appears about two and a half times as large in Mercury's sky as in the earth's.
The temperature on Mercury is about 625 degrees Fahrenheit during the day and lower than 80 degrees at night.
The Mariner 10 spacecraft, which passed by Mercury twice in 1974 and once in 1975, sent back pictures of a moonlike, crater pocked surface. The space probe was about 130,000 miles from Mercury.
Although Mercury may have a moonlike surface, its interior appears to resemble that of the earth. Many scientists think the interiors of both planets consist largely of iron and other heavy elements.