In Space

Text A.It is one of nine planets revolving in nearly circular (elliptical) orbits around our star, the sun. This planet is the third planet out from the sun and the fifth largest planet in our solar system, having a diameter of about 7,918 miles. It completes one orbit around the sun in about 3651/4 days, the length of time that gives us our unit of time called a year.

Text B.Theyvary in size, composition, and orbit. Mercury, with a diameter of 3,112 miles, is the planet nearest to the sun. It orbits the sun in just three earth months. Jupiter, about ten times the diameter of Earth (88,000 miles), is the largest planet and fifth in distance from the sun, taking about 113/4 earth years to orbit the sun. Pluto, the most distant planet, takes about 2473/4 earth years to orbit around the sun. The inner planets have densities, and probably compositions similar to Earth’s; outer planets are gaseous, liquid, or frozen hydrogen and other gases.

Text C.It is an average-sized star which makes up about 99 percent of the mass of the solar system. Its size may be illustrated by visualizing it as a marble. At this scale, the earth would be the size of a small grain of sand one yard away. Pluto would be a rather smaller grain 40 yards away.

Text D.These thingsrevolve around seven planets. Including the earth’s moon, there are 61 satellites altogether; Mars has 2, Jupiter 16, Saturn 18, Uranus 15, Neptune 8, and Pluto 1.

Text E.These objects are among the oldest members of the solar system. They orbit the sun in extremely long, elliptical orbits. As comets approach the sun, their tails begin to glow from friction with the solar wind.

Text F. It is the earth’s natural satellite, it has about 1/4 the diameter, 1/81 the weight, and 3/5 the density of our planet. The moon completes one orbit around the earth every 271/3 days. It takes about the same length of time, 291/2 days, to rotate on its own axis; hence, the same side, with an 18% variation, always faces us. The moon’s surface, cratered by meteorite impact, consists of dark areas (maria) which are separated by lighter mountainous areas (terrae). Terrae are parts of the original crust, formed about 4.5 billion years ago; maria are basins, excavated by meteorite falls, filled by basaltic lavas formed from 3 to 4 billion years ago.

Text G.These objects are the so-called minor planets, are rocky, airless, barren, irregularly shaped objects that range from less than a mile to about 480 miles in diameter. Most of the asteroids that have been charted travel in elongated orbits between Mars and Jupiter. The great width of this zone suggests that the asteroids may be remnants of a disintegrated planet formerly having occupied this space.

Text H. These objects, loosely called shooting stars, are smaller than asteroids, some being the size of grains of dust. Millions daily race into the earth’s atmosphere, where friction heats them to incandescence. Most meteors disintegrate to dust, but fragments of larger meteors sometimes reach the earth’s surface as “meteorites.” About 30 elements, closely matching those of the earth, have been identified in meteorites.