Our present scientific knowledge of the four nearer planets, Mercury, Venus, Mars, and Jupiter, has been obtained from ground-based experiments as well as from planetary missions. We find the planets to be quite different, and each one gives opportunities for studying the basic questions of planetary exploration: How did the solar system come into being, and does life of any kind exist within it?
It seems that at least a ten year program of automated planetary spacecraft will be needed, carrying an increasing degree of complexity in the scientific experiments. The exobiological experiments require that we maintain planetary quarantine, and this constraint has to be allowed for. The possibility of manned approaches to Mars, though far in the future, also affects the strategy of a program of scientific exploration.