National Geographic : 1970 Aug
Voyage to the Planets KODACHROMEBYNATIONALGEOGRAPHICPHOTOGRAPHER EMORYKRISTOF@ N.G.S . Veteran viewer of the solar system, Dr. Gerard P. Kuiper has discovered two satellites of outer planets: Miranda, a moon of Uranus, in 1948, and Neptune's Nereid in 1949. Director of the Lunar and Planetary Laboratory of the University of Arizona in Tucson, he stands beside a balloon riding telescope that records ultraviolet radiation. ercury Catapulted by Venus's gravitational field, our spacecraft bends its flight path by some 40 degrees and races on toward Mercury, the solar system's innermost and smallest planet. On March 30, 1974, we reach this second goal. Mercury has only about a third the di ameter of Earth. We approachit sofast and it looms so swiftly that we almost feel vertigo. Now our cameras and instruments race to record information. After years of prepara tion and 51/2 months in flight, we have only two hours to gather all the close-up infor mation on Mercury we will get in this decade. Earth lies 93 million miles behind, still a very bright point of light. The scientists there, at the Jet Propulsion Laboratory in Pasadena, California, wait tensely for the informationfrom our instruments and tape recorders; the signals take more than eight minutes to reach Earth. The sun, now only 43 million miles away, appears more than twice as large as when seen from Earth; the solar radiation bom barding us is five times as intense as that striking Earth'satmosphere. If our spacecraft were truly designed for manned flight, it would require much more radiationshielding and temperature control. The surface of Mercury filling our view is a rare sight, never clearly seen from Earth. Now we can see it with perfect clarity; no atmospheric effects block the vista. We are only about 600 milesfrom the surface, and our eyes can distinguishobjects as small as 1,700 feet across. Everywhere we see evidence that this rocky cinder has been crateredby comets and asteroids, and it is not hard to imagine that it was once scorched by tremendous heat. Dr. Gerard Kuiper (left), Director of the Lunar and Planetary Laboratory of the Uni versity of Arizona, explains Mercury's heated past this way: "Early in solar-system history, I believe, the sun blazed for a short time, maybe ten thousand years, with a luminosity as much as thirty times greater than that of today. Mercury was probably twice as mas sive then as it is now, but the sun evaporated away half its substance. The lighter, more volatile elements escaped, leaving a heavy planet that is probably about 30 percent sili cates, or rock, and 70 percent metals. It is 51/2 times as dense as water." Even today Mercury bathes constantly in ferocious heat. When the planet is at aphelion, the farthermost point from the sun in its eccentric orbit, the flow of solar energy is five times as great as that reaching the vicinity of Earth. When Mercury comes into perihelion, its closest approach, the searing radiation is ten times as great. Temperatures reach 6500 F. on the equator, though they probably drop during the long night to -3000 F.