The lesser objects of the solar system

So far in our studies, no notice has been taken of the smaller bodies in the solar system, i.e. Pluto, the asteroids and the satellites of Mars and the major planets, for their properties are but poorly known on the whole and it is not very rewarding to apply to them the type of analysis that was applied in the foregoing parts of this book to the greater objects. Yet, in considering the solar system as a whole, their existence and such information as we have of them cannot be ignored.

Pluto, the outermost known planet, is in a highly eccentric orbit highly inclined to the ecliptic, and, in consequence, although it comes on occasion within the orbit of Neptune, it never approaches Neptune closely, and detailed studies have shown the outer solar system to be stable. Pluto is a very small object as seen from the Earth. Its diameter is estimated from the brightness and supposed reflectivity. The latter has recently been redetermined from infra-red spectroscopy and the diameter of Pluto is consequently now estimated to lie between 2800 and 3300 km (Cruickshank, Pilcher and Morrison, 1976). The mass of Pluto was originally estimated from the perturbations of the orbits of Uranus and Neptune, but a satellite has now been detected (Christy and Harrington, 1978) with a period of 6.4 d, from which the mass of Pluto is estimated to be about 0.002 times that of the Earth (Meadows, 1980).