Mariner 10 was the first (and only) spacecraft sent to the planet Mercury, the first mission to explore two planets (Mercury and Venus) during a single mission, the first to use gravity-assist to change its flight path, the first to return to its target after an initial encounter, and the first to use the solar wind as a major means of spacecraft orientation during flight.

The primary goal of Mariner 10 was to study the atmosphere (if any), surface, and physical characteristics of Mercury. Soon after leaving Earth orbit, the spacecraft returned photos of both Earth and the Moon as it sped to its first destination, Venus. During the coast, there were numerous technical problems, including malfunctions in the high-gain antenna and the attitude-control system. After midcourse corrections on 13 November 1973 and 21 January 1974, Mariner 10 approached Venus on 5 February 1974 and returned a total of 4,165 photos of the planet and collected important scientific data during its encounter. The closest flyby range was 5,768 kilometers at 17:01 UT.

Assisted by Venusian gravity, the spacecraft now headed to the innermost planet, which it reached after another midcourse correction on 16 March 1974. As Mariner 10 approached Mercury, its photos began to show a very Moon-like surface with craters, ridges, and chaotic terrain. The spacecraft’s magnetometers revealed a weak magnetic field. Radiometer readings suggested nighttime temperatures of –183°C and maximum daytime temperatures of 187°C. The closest encounter was at 20:47 UT on 29 March 1974 at a range of 703 kilometers.

Having looped around the Sun, Mariner 10 flew by Mercury once more on 21 September 1974 at a more distant range of 48,069 kilometers. The spacecraft used solar pressure on its solar panels and high-gain antenna for attitude control. A third and final encounter, the closest to Mercury, took place on 16 March 1975 at a range of 327 kilometers. Contact with the spacecraft was terminated on 24 March 1975.

Source: NASA