Mariner 10 was the seventh successful launch in the Mariner spacecraft series, and the first to use the gravitational pull of one planet (Venus) to reach another (Mercury). Instruments on board the spacecraft were designed to measure the atmospheric, surface, and physical characteristics of Mercury and Venus. Experiments included television photography, magnetic field, plasma, infrared radiometry, ultraviolet spectroscopy, and radio science detectors. An experimental X-band, high-frequency transmitter was flown for the first time on the spacecraft.
On November 3, 1973, Mariner 10 was placed in a parking orbit after launch for approximately 25 minutes, then placed in orbit around the Sun en route to Venus. The orbit direction around the Sun was opposite to that of the Earth. Mariner 10 flew past Venus on February 5, 1974, at a distance of 4,200 kilometers (2,610 miles). More than 4,000 photos of Venus revealed a nearly round planet enveloped in smooth cloud layers. Venus exhibited a slow rotational period of 243 days and had only 0.05 percent of Earth's magnetic field. The planet's atmosphere was composed mostly of carbon dioxide.
After the Venus flyby, Mariner's trajectory was bent in toward the Sun to accelerate and fling it out of Venus's gravitational field and onward to Mercury. Mariner 10 reached Mercury on March 29, 1974, passing over the planet at 705 kilometers (438 miles) above the surface. A second encounter with Mercury occurred on September 21, 1974, at an altitude of about 47,000 kilometers (29,200 miles). The sunlit side of the planet and the south polar region were photographed. A third and last Mercury encounter, at an altitude of 327 kilometers (203 miles), occurred on March 16, 1975. About 300 additional photographs were obtained along with magnetic field measurements. Photographs of the planet reveal an intensely cratered, Moon-like surface and a faint atmosphere of mostly helium, resulting from solar wind bombardment. Engineering tests were continued until March 24, 1975, when the supply of attitude-control gas was depleted and the mission was terminated.
Here the Mariner 10 spacecraft is being assembled prior to its November 1973 launch. During its two-year mission, the spacecraft transmitted more than 12,000 images of Mercury and Venus. The mission ended in March 1975. Mariner 10 is still orbiting the sun, even though its electronic systems have probably been destroyed by solar radiation. (Courtesy NASA/JPL)
A nighttime lift-off of the Atlas Centaur launch vehicle propelled Mariner 10 on its historic mission to Venus and Mercury. (Courtesy NASA/JPL)
This model of Mariner 10 shows the spacecraft as it appeared during flight. The Mariner 10 mission required more course corrections than any previous mission and was the first spacecraft to use the gravitational pull of one planet to help it reach another planet. This craft was also the first to use the solar wind as a means of locomotion; when the probe's thruster fuel ran low, scientists used the solar panels as sails to make course corrections. (Courtesy NASA/JPL)
Mariner 10, shown in this artist's rendering, was the last in a series of Mariner missions designed to survey other planets in the solar system. Launched in November 1973, this mission provided new information about Mercury and Venus in the Mariner program's first dual-planet mission. (Courtesy NASA/JPL)