Ginger Melton, age 13, of Beaumont, Tex., for her question:
WHICH IS THE SMALLEST PLANET?
Mercury is the smallest of our planets. It is also the planet that is nearest to the sun. Mercury's mean distance from the sun is about 36 million miles, compared to 67 million miles for Venus, the second closest planet, and 93 million miles from Earth, the third closest.
Because of Mercury's small size and nearness to the brightly shining sun, the planet is often hard to see without a telescope. At certain times of the year, Mercury can be seen low in the western sky just after sunset and at other times low in the eastern sky just before sunrise.
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 its farthest point.
No other planet moves around the sun faster than Mercury. Traveling at a speed of about 30 miles per second, Mercury goes around the sun once every 88 earth days. The earth goes around the sun once every 365 days, or one year.
As Mercury moves around the sun, it rotates on its axis, an imaginary line that runs through its center. The planet rotates once about every 59 earth days a rotation slower than that of any other planet except Venus.
A day on Mercury would last about 180 earth days because Mercury rotates so slowly. The earth, on the other hand, rotates once a day.
Mercury is extremely hot, dry and almost airless. The suns rays are about seven times as strong on Mercury as they are on the Earth. Also, the sun appears about two and a half times as large in Mercury's sky as in the Earth's.
There are not enough gases in Mercury's atmosphere to reduce the amount of heat and light it receives from the sun. This means that temperature on the planet is about 625 degrees Fahrenheit during the day.
Mercury's surface appears to be much like that of the moon. It reflects about 6 percent of the sunlight it receives, about the same as the moons surface reflects.
The planet has broad, flat plains, steep cliffs and deep craters similar to those on the moon. Astronomers believe the craters were formed by meteors crashing into the planet. Mercury does not have enough atmosphere to slow down approaching meteors and burn them up by friction.
The United States Mariner X became the first spacecraft to reach Mercury. The unmanned spacecraft flew to within 460 miles of Mercury on March 29, 1974. It swept past the planet again on Septemnber 24, 1974, and on March 16, 1975.
During these flights, the spacecraft photographed the surface of Mercury and made scientific measurements of the planet. It also detected Mercury's magnetic field.
Mariner X became the first spacecraft to study two planets. The probe photographed and made scientific measurements of Venus while traveling to Mercury. For latest information on Mercury go to Jet Propulsion site on Science News on Youaskandy main menu or http://sse.jpl.nasa.gov/planets/profile.cfm?Object=Mercury