General

The minor planets – asteroids or planetoids as they are often called – are a swarm of many thousands of small solid bodies most of which revolve around the sun in orbits between those of Mars and Jupiter (fig. 8.1). F. C. Watson estimated that on the basis of the sizes of the larger minor planets and the brightness of the smaller bodies, they altogether would make up a body about 620 miles (1000 km) in diameter which is only about four times the diameter of Juno, one of the larger asteroids, and the total combined mass would be about 1/4000 of the Earth's. Orbital revolution periods of the main belt asteroids range from 3.5 to 6.0 years. The most frequent period is somewhat less than half of Jupiter's.

Of 1568 minor planets whose orbits had been determined by 1953, 97% had mean distances from the sun ranging from 195 to 288 million miles (313.8 to 463.4 million km). Their orbital eccentricities average at 0.15 which is greater than that of the larger planets with the exception of Mercury and Pluto. Some individual asteroids have eccentricities 3–4 times the average. The longerperiod asteroids with large orbits except Hidalgo have small eccentricities. Orbital inclinations to the ecliptic of asteroids nos. 1–3000 average 9.4° which is greater than that of the larger planets with the exception of Pluto; a few almost lie in the ecliptic plane and a few others have orbital inclinations exceeding 30°.