THE SOUTHWORTH PLANETARIUM 207-780-4249 www.usm.maine.edu/planet 70 Falmouth Street Portland, Maine 04103 43.6667° N, 70.2667° W Founded January 1970 "Earth with art is Eh" -seen on a bumper sticker on the way to work this morning THE DAILY ASTRONOMER Thursday, February 4, 2016 The Wrong View of the Asteroid Belt The Universe is stiff with belts. And, as is true with the belts once finds in clothing stores, they appear in many varieties. However, unlike sartorial belts that assume one function, celestial belts differ both in character, composition and function. Today, we explore the most famous of these belts: the one that separates the inner solar system of rocky worlds from the outer region of the gas giants: ASTEROID BELT The structure that 99.9% of all people, including ourselves, envision incorrectly. Because we watched the Millenium Falcon navigate its way unscathed through an asteroid belt or because, in the days of our youth, we and the Pharaoh's other tomb builders played "Asteroids" on long afternoons, we believe that asteroid belts are crammed elbow to pelvis with flying rocks. The reality, as far as the solar system's asteroid belt is concerned, is far less perilous. Were we to meander about the solar system, we would find this belt between the orbits of Mars and Jupiter and if we teleported ourselves there, the first thought we'd entertain would be, "Where are those confounded asteroids? All I see is darkness!" We'll explain: The asteroid belt, named the "main belt," by astronomers, consists of thousands of asteroids revolving around the Sun. Unlike that pesky and erroneous vision that won't go away, the asteroid are not just randomly tossed anywhere. They move within a disc between 2.1 AU and 3.3 AU. (An AU is an "astronomical unit," defined as Earth's average distance from the Sun or approximately 93 million miles.) Within this disk one would regions devoid of asteroids, referred to as "Kirkwood's Gaps." Jupiter, the giant planet far beyond the belt's outer edge, literally created those gaps within areas of -don't hang up- "orbital resonance." Objects within the resonant regions would have orbital periods equal to an integer fraction of Jupiter's orbit. For instance, we see a Kirkwood Gap at 2.5 AU because an asteroid at this distance would complete exactly three orbits during the time Jupiter required to complete one. We'd find another gap at 3.28 AU as any asteroid at that distance would have an orbital period exactly one half that of Jupiter's. Any body with these gaps would develop unstable orbits and would move to another part of the belt. Within the "populated" regions, one would have hundreds, perhaps a few thousand asteroids: those we've discovered and those not yet found. While this number might make one think the asteroid belt is truly a swarm of renegade mountains, it is, instead, rather diffuse. Recall that this belt encircles the Sun at a distance of more than 180 million miles: we recall from geometry that one can make a rather large circle or ellipse with a radius (or semi-major axis) that is so long. The circumference is almost one billion miles: about equal to the distance between the Earth and Saturn. The combined mass of all asteroids within the main belt is less than that of the Moon! By planetary standards, the Moon is quite small Imagine demolishing the Moon into an array of minuscule pieces and then distributing these pieces around a billion mile loop: one would have a diffuse belt indeed. That is splendid news for those would want to manuever a spacecraft through the belt toward the outer solar system or beyond. NASA has deployed a few ships, such as the Voyager and Pioneer probes, through the asteroid belt without much trouble. There's hardly anything there except empty space. Then again, one could make that remark about the solar system, itself.