(From "NASA, The First 25 Years, 1958-1983," NASA, U.S. Government Printing Office, Washington, D.C., 1983, p. 97.)
First successful interplanetary probe, a flyby of Venus; data led to an accurate determination of the planet's mass, and measured high temperatures.
Mars flyby; shroud failed to jettison and communications were lost with the spacecraft.
November 28, 1964
Mars flyby; reached the planet July 14, 1965; photographed a heavily cratered, moonlike surface; found that the Martian atmosphere is thin, with less than 1 percent the pressure of Earth's atmosphere, and is composed largely of carbon dioxide.
Venus flyby; found weak magnetic field and very dense atmosphere.
Mariners 6 and 7
Launched February 25 and March 27,1969.
Mars flybys; closest approach to planet achieved on July 31 and August 5, 1969.
Findings: Nitrogen is virtually absent from the atmosphere, solid carbon dioxide ("dry ice") occurs in the clouds and near the polar caps, and the dust particles in the Martian atmosphere probably consist of silicate materials derived from the planetary surface.
Mars orbiter mission; launch vehicle failed.
May 30, 1971
First Mars-orbiting spacecraft, November 1971; transmitted 7,400 pictures of 100 percent of Martian surface as well as the planet's small moons, Phobos and Deimos.
Results: (1) Mars is a two-part world, with an ancient cratered surface in the Southern Hemisphere and a geologically younger surface, with volcanoes, canyons, and dry river channels, in the Northern Hemisphere. (2) It has a huge canyon, some enormous volcanoes, and sinuous channels which appear to be former river beds. (3) Landforms resembling lava flows occur in flat regions. (4) Layered deposits in the Martian polar regions suggest glacial periods in past times. (5) Solar ultraviolet light is not absorbed by the atmosphere and reaches the surface of Mars. (6) Periodic global dust storms were observed. (7) Phobos and Deimos are very dark and have irregular shapes and cratered surfaces.
November 3, 1973 Venus (February 5, 1974)/Mercury (March 29, 1974) flyby; the trajectory around the Sun swung it back for a second encounter with Mercury in September 1974 and for a third in March 1975.
Data from Venus: (1) No significant magnetic field; (2) a notable disturbance in the solar wind is produced as it flows past Venus; (3) it is closer to a perfect sphere than Earth is; (4) ultraviolet images of the atmosphere revealed streamline and circulation patterns, including Y- and C-shaped structures; (5) the upper atmosphere rotates much more rapidly (once in about 4 Earth days) than Venus itself; (6) hydrogen and helium were detected in the atmosphere.
Data from Mercury: (1) Photographs showed desolate landscapes, remarkably similar to the Moon's with huge craters; long narrow valleys; a feature unique to Mercury, long scarps, or cliffs; flat plains; and a huge circular impact basin (Mare Caloris) about 13,000 km (810 mi) in diameter; (2) it's closer to a perfect sphere than Earth is; (3) it's not only the smallest planet, but the densest with a metal-rich core; (4) a tenuous atmosphere includes exotic gases such as argon, neon, and helium; (5) its magnetic field is about a tenth as strong as Earth's; and (6) the temperatures are extreme, from 425°C (770°F) to -183°C (-297.4°F).