For centuries before spacecraft transformed our under- standing
of the solar system, almost all the planets were believed, with
varying degrees of conviction, to be inhabited. This belief conformed
to the Copernican world view, in which Earth is not unique among the
planets. When the Space Age opened in 1957, the evidence for
indigenous life was strongest for Mars - almost conclusive, on the
face of it - and from the beginning the search for life on Mars became
a major goal of the United States' Space Program. The result was the
historic series of spacecraft that began with MARINER 4, in 1965, and
ended with VIKINGS 1 & 2, in 1976.
The dawning of the Space Age is a material event, occurring in
the latter part of the 20th Century, and as the description that
follows is a result of a developed mental means of communication, it