How Venus redirected Mariner 10 to Mercury

A gravity assist uses a planet’s motion to redirect a spacecraft; Mariner 10 used its encounter with Venus to change course toward Mercury, making it the first mission to explore both planets and the first to use a gravity assist to change its flight path^[1]. On 5 February 1974, it passed Venus at a closest range of 5,768 kilometers, returning 4,165 photographs and scientific data before heading for Mercury^[2]. Its first Mercury flyby, on 29 March 1974, brought it within 703 kilometers: images showed a cratered, Moon-like surface, instruments detected a weak magnetic field, and an occultation experiment indicated no atmosphere or ionosphere^[3]. After looping around the Sun and making course corrections, Mariner 10 returned on 21 September 1974, passing at 48,069 kilometers and adding images of the southern polar region^[4]. Those repeated passes extended the observations beyond the first encounter, though the closest and final flyby, on 16 March 1975 at 327 kilometers, had a sharp limit: a failed tape recorder and restricted data-reception rate meant that only the central quarter of each of 300 high-resolution images was received^[5].