Interplanetary Projectiles: Asteroids and Comets

Abstract

Besides the Sun, the nine planets, and their moons, the solar system contains countless smaller bodies. These asteroids and comets are not very big by planetary standards. Even the largest one, the asteroid Ceres, is so small that it would take dozens of Ceres-sized bodies to make up just one body the size of our Moon. But by ordinary human standards, these small planets are very big. Ceres is nearly the size of Texas, and thousands of asteroids are larger across than the City of San Francisco. A typical icy comet body is even smaller than San Francisco—some as little as Golden Gate Park—although a comet’s wispy tail of gas and dust may stretch for tens of millions of miles through the inner solar system. Such a tail streaming close to Earth can be a stunning sight in the nighttime sky, which is why so many comets have been recorded throughout history, despite their comparatively small sizes. Actually, astronomers now believe that comets are vastly more abundant than asteroids, numbering literally in the trillions. But nearly all comets are invisible: They wander far beyond the orbit of Pluto, and only dimly reflect the glow of a faraway Sun. The comets we actually see are rare stragglers, deflected in toward the inner solar system.