The Asteroids (or “Minor Planets”)

Abstract

Observation of planetary orbits, the radii of which increase in a geometric progression (the Titius-Bode law, see Sect. 1.1.2), shows a “gap” between Mars and Jupiter. On 1st January 1801, the Sicilian, Giuseppe Piazzi (1746–1826) discovered Ceres, a small planet about a thousand kilometres in diameter, orbiting at about 2.8 AU. The orbit of Ceres corresponded exactly to that of the “missing planet” predicted by the Titius-Bode law. But the mass of Ceres was far smaller than that of Mercury, the smallest of the planets. It was soon discovered that Ceres was not the only body orbiting between Mars and Jupiter: three other asteroids (or “minor planets”) were discovered at the beginning of the last century: Pallas, Juno and Vesta. Further discoveries were made as observational techniques improved, and several thousand asteroids have now been catalogued (Tables 1.4 and 6.1), the total mass of which hardly amounts to twice that of Ceres. We may expect observations by the Hipparcos satellite to lead to a considerable increase in the number of asteroids studied.