Little was known about Mercury until the space probe Mariner 10 photographed the planet in 1975. Mariner 10 first approached the planet Venus in February 1974, and then used that planet’s gravitational field to slingshot itself in the direction of Mercury. The journey from Venus to Mercury took seven weeks. On its first flyby of Mercury, Mariner 10 came within 470 miles (750 kilometers) of the planet, and photographed about 40 percent of its surface. The probe then continued into orbit around the Sun, flying past Mercury twice more in the next year before running out of fuel.