The Origin and Evolution of the Solar System

30 The structure of the Solar SystemTable 1.10. Characteristics of some important asteroids. The ones given are: , thesemi-major axis; , the eccentricity; , the inclination; and , the diameter (U = unknown).Year of Name discovery (AU) ( Æ ) (km)Ceres 1801 2.75 0.079 10.6 1003Pallas 1802 2.77 0.237 34.9 608Juno 1804 2.67 0.257 13.0 250Vesta 1807 2.58 0.089 7.1 538Hygeia 1849 3.15 0.100 3.8 450Undina 1867 3.20 0.072 9.9 250Eros 1898 1.46 0.223 10.8 UHildago 1920 5.81 0.657 42.5 15Apollo 1932 1.47 0.566 6.4 UIcarus 1949 1.08 0.827 22.9 2Chiron 1977 13.50 0.378 6.9 Usynchronization is found for all the satellites in the Solar System. Another effectis to cause a precession of the spin axis of the Earth. The spin of the Earth doesnot contribute to tides but it does create an equatorial bulge distorting it fromspherical symmetry. The equatorial radius of the Earth is about 22 km greaterthan the polar radius. The spinning Earth acts like a gyroscope and the Moonexerts a torque on it due to the differential pull on the near and far regions of theequatorial bulge. It is this torque which gives a spin-axis precession period ofabout 26 000 years.1.5 AsteroidsIt was seen in table 1.2 that to fill a gap in Bode’s law it was necessary to introducea body between Mars and Jupiter. The small body Ceres, discovered in1801, filled the gap admirably but the pattern was made more complicated by thediscovery of many other bodies, all smaller than Ceres, in the same region of theSolar System. These bodies, called asteroids, raise many questions as to theirorigin but here we shall just consider their characteristics.1.5.1 Characteristics of the major asteroidsIn table 1.10 the data for a number of asteroids, chosen because they span theperiod from the first discovery to recent times and because they illustrate differentorbital characteristics, are shown.The sizes of asteroids are best measured by stellar occultation. By timingan occultation and knowing the orbit of the asteroid it is possible to obtain quite