Venera 4 left Baikonur Cosmodrome in the central Soviet Union
early in the morning of 12 June 1967. The first two stages of
its three-stage Molniya-M launch vehicle placed the
1106-kilogram automated spacecraft into a 173-by-212-kilometer
parking orbit about the Earth, then the launcher’s third stage
boosted Venera 4 out of orbit onto a fast path Sunward toward
the cloudy planet Venus. Two days later, after launch on a
Atlas-Agena D rocket from the Eastern Test Range-12 launch pad
at Cape Kennedy, Florida, 244.8-kilogram Mariner 5 followed
Venera 4 toward Venus. Mariner 5 had been built as the backup
for Mariner IV, which flew successfully past Mars in July 1965.
Hardware modifications for its new mission included a reflective
solar shield, smaller solar panels, and deletion of the visual-
spectrum TV system in favor of instruments better suited to
exploring Venus’s hidden surface. When Mariner 5 and Venera 4
left Earth, the nature of Venus’s surface was only beginning to
be understood. Though the Mariner II V