Mariner 10

Mariner 10 was the seventh successful launch in the Mariner series and the first spacecraft to visit Mercury. It was also the first spacecraft to use the gravitational pull of one planet (Venus) to reach another (Mercury), and the first spacecraft mission to visit two planets. The spacecraft flew by Mercury three times in a retrograde heliocentric orbit and returned images and data on the planet. Mariner 10 returned the first-ever close-up images of Venus and Mercury.

The primary scientific objectives of the mission were to measure Mercury’s environment, atmosphere, surface, and body characteristics and to make similar investigations of Venus. Secondary objectives were to perform experiments in the interplanetary medium and to obtain experience with a dual-planet gravity-assist mission.

Mariner 10 was launched by NASA on 3 November 1973, and following a Venus flyby and three Mercury flybys is ceased transmission on 24 March 1975. It’s assumed to still be orbiting the sun, but has not been spotted or tracked since it stopped transmitting.