dipper@utastro.UUCP (12/19/84)
Mercury and Pluto are at the opposite ends of the solar system. More about them -- after this. December 19 Half-Baked Worlds Mercury and Pluto are the smallest of the nine major planets. The two worlds just so happen to orbit the sun at opposite ends of the solar system. Mercury is the planet closest to the sun. It doesn't have a moon of its own -- and it isn't much larger than Earth's moon. Mercury actually resembles Earth's moon -- with lots of craters -- and no atmosphere. Mercury contains a high percentage of iron -- which may make it a possible future site for space mining colonies. It used to be thought that Mercury always keeps the same hemisphere toward the sun, much like one face of our moon always stares toward Earth. But now we know that Mercury's rotation takes two-thirds of the planet's orbit around the sun. So Mercury turns slowly beneath the sun -- like meat roasting over an open fire. At the other end of the solar system from Mercury is Pluto -- the planet that travels farthest from the sun. This icy world is smaller than Earth's moon -- and has a very low density. Pluto has the most eccentric orbit of all the major planets -- and right now it's in the part of its orbit that brings it nearer to the sun than the planet Neptune. Astronomers think that Pluto's relative nearness to the sun may heat up this icy world -- giving it a temporary thin atmosphere of methane gas. Pluto and its large companion moon -- Charon -- whirl around each other out in the far reaches of the solar system -- from where the sun appears merely as an especially bright star. Script by Diana Hadley and Deborah Byrd. (c) Copyright 1983, 1984 McDonald Observatory, University of Texas at Austin