November 3, 1973. The Atlas / Centaur launch vehicle launched Mariner 10 to fly by Mercury. This was the last spacecraft in the Mariner program, since Mariner 11 and Mariner 12 were allocated to the Voyager program and redesignated Voyager 1 and Voyager 2. Passing Venus "Mariner-10" was the second space probe (after Luna-3) to use gravity assist, lowering its perihelion with the help of Venus to approach the orbit of Mercury. The spacecraft flew past Mercury three times. The first Mercury encounter took place on 29 March 1974, at a range of 703 kilometers, on 21 September 1974, at a more distant range of 48,069 kilometers and on 16 March 1975, at a range of 327 kilometers. During the flight the spacecraft was only able to map 40–45% of Mercury's surface, taking over 2,800 photos. It revealed a more or less Moon-like surface. Mariner 10 also discovered Mercury’s magnetic field. Its radiometer readings suggested that Mercury has a nighttime temperature of −183 °C and maximum daytime temperatures of 187 °C. It was discovered that Mercury has a tenuous atmosphere consisting primarily of helium. This mission was the first and last visit to Mercury in over 30 years