Exploration of the Planets: An Invited Discourse Presented before the Sixteenth General Assembly of the International Astronomical Union Grenoble, France, August, 1976

Abstract

Until recently the entire astronomical endeavor was restricted to passive observations from the surface of the Earth. Natural experiments -- such as stellar occultations, eclipsing binaries, or supernova explosions -- might fortuitously arrange themselves, and systematic inventories were performed of information carried by reflected or emitted electromagnetic radiation. But it is only in the last twenty years that our species has been able to carry out in situ experiments on astronomical objects.