National Geographic : 1970 Aug
National Geographic, August 1970 (Crocus and Begonia), cities (Yalta and Chi cago), and women (Sheba, Dulcinea, and Mar lene-yes, Marlene Dietrich). An easy assumption would be that the asteroids are the debris of a planet that ex ploded. But the reverse may well be true. Dr. Tom Gehrels, a long-time student of these objects at the Lunar and Planetary Laboratory of the University of Arizona, regards them as building stones of the solar system. "The asteroids are probably part of the original record of the nebula, or dust cloud, from which we believe the sun and planets condensed some five billion years ago," he says. "Asteroids are dust that has been com pacted-accretion products-or fragments from collisions of such bodies." While most of these small bodies stay close to the asteroid belt, a few have eccentric orbits The voice of Jupiter: Spasmodically and cryp tically, restless Jupiter bursts forth with powerful radio noises-emissions once thought peculiar to the stars and galaxies. Apparently associated with the planet's magnetic field, the radiation is affected by the position of the satellite Io. Here at a University of Texas radio telescope near Marfa, Jupiter symbols mark a Jovian radio storm. Other squiggles are noise and interference. that invade the inner solar system, and some fly so close to our own planet that they might be called "Earth-grazers." Icarus, for example, swings out 183 million miles from the sun, but every 409 days, at perihelion, it comes within 17 million miles of the sun's raging heat. At such close encounter its half-mile-thick bulk may become red hot. Appropriately, it bears the name of the lad in Greek mythology who came to grief when he tried to fly with wax-bound wings and ap proached too close to the sun. In June 1968, Icarus missed Earth by only 4,000,000 miles. Another space rock, Hermes, only 1,000 feet across, came within half a million miles of us in 1937, barely twice the distance to the moon (painting, page 155). And just a year ago this month, astrono mers took aim at a most unusual Earth-grazer, Geographos, which came within 5.6 million miles of their telescopes. Discovered in 1951 during the National Geographic Society Palomar Observatory Sky Survey, this plan etoid was named in honor of the Society. Its catalog number is 1620, which leads one astronomer to refer to it as "Plymouth Rock." When Geographos's light was measured with a photometer, it showed a peculiar rhyth mic fluctuation in brightness. At periodic intervals, it seemed to be six times as bright as at other times. No other asteroid shows such extreme variation. From this and other light measurements, Dr. Gehrels has recently determined that Geographos is a cigar-shaped body about 2'/2 miles long and half a mile wide. It tumbles end over end, so that part of the time we see reflection from its long side and part of the time only from an end. This explains the wide fluctuation in brightness (page 175). UGE CRATERS on the moon testify to what happens when the orbits of these flying mountains bring them into collision with other celestial bodies. Scientists generally accept the idea that some meteoroids are pieces from the asteroid belt, although others are the debris of comets. Earth's atmosphere burns up the small meteoroids. And no one need worry about a large one striking Earth. Calculations suggest that it happens no more than once in 50,000 years in North America, even for a rock only 100 feet across, the size of the one that blasted out Arizona's Meteor Crater.