Sept. 21, 1974: Mariner 10 flies by Mercury

Today in the history of astronomy, Mariner 10 has its second encounter with Mercury. | Published: September 21, 2026 Mariner 10 made three flybys of Mercury, which is shown here in an enhanced-color image from NASA’s Messenger probe nearly four decades later. Credit: NASA / JHU Applied Physics Lab / Carnegie Inst. Washington On Sept. 21, 1974, the Mariner 10 spacecraft made its second flyby of Mercury, passing by the surface at a distance of just under 30,000 miles (48,000 kilometers). Launched Nov. 3, 1973, Mariner 10 was the first spacecraft sent to Mercury, as well as the first mission designed to visit two planets. (It also visited Venus.) The probe’s goals at Mercury included studying the planet’s physical characteristics and atmosphere, and collecting photos and data. Across its three flybys, Mariner 10 took over 2,700 photos, ultimately mapping nearly half of Mercury’s surface. The second pass in September of 1974 was specifically focused on the south pole. Elisa Neckar is senior production editor of Astronomy magazine, and the editor of the Today in the History of Astronomy feature on Astronomy.com.