Introduction The Inner Solar System
The best known part of the Solar System is, naturally enough, that part of it nearest to the Earth. Mercury, Venus, the Earth and Mars are known as the TERRESTRIAL PLANETS, but only because of their position in the inner Solar System, not because they are just like the Earth. The Moon, by comparison with its primary, the Earth, is a very massive satellite which is often, with justification considered to be a planet in its own right. Mars has two tiny satellites to make a total of seven known bodies in the inner Solar System. Other bodies – asteroids, meteorites and comets — also enter this region of space; they are considered in Occasion¬ally, claims have been made that there is a planet (Vulcan) inside the orbit of Mercury. Each time, the observations have been shown to be spurious and we can be sure that there are no unknown planets of any significant size in the inner Solar System.
The advent of spacecraft observations has increased our know¬ledge of the inner planets so much that planetary astronomy is effectively only 10 to 15 years old. Before 1965, no features smaller than several hundred kilometers across could be seen on Mars whereas several per cent of the surface has now been observed with a resolution of 100m. Much the same is true for the other inner planets. The resultant new interest has also had an effect on studies of the Earth, which can bow benefit from comparison with the other planets.