Mariner 10 was an unmanned spacecraft designed to give scientists their first view of Mercury and to further study Venus. To date it is the only mission ever to perform up close imaging of the planet Mercury and the first to visit two planets. It was launched on November 3, 1973. During its two-year mission, it transmitted over 12,000 images of these two mysterious planets. The probe discovered a previously unknown magnetic field surrounding Mercury and mapped out almost half of the planet's surface. Mariner 10 was turned off on March 24, 1975 after its fuel had completely run out. It has been orbiting the Sun ever since even though its electrical system has most likely been severely damaged by solar radiation.
The Mariner spacecraft was designed to follow up on the Pioneer series space craft and to conduct more indepth studies of the inner planets. It was powered by four solar panels which gathered sunlight and converted it to electrical power. Scientists kept up communications with Mariner by continuous radio transmissions from Earth. The Mariner spacecraft used its on-board radiometers, spectrometers, and magnetometers to measure the temperatures, atmospheric pressures, and chemical compositions of the planets it passed. The first close-up photos of Venus and Mercury were relayed back to scientists on Earth by using television cameras mounted on the probe.
NASA successfully accomplished a number of different maneuvers during the Mariner 10 mission. First, NASA discovered that it was possible to make giant corrections in the course of the spacecraft from Earth. This was also the first mission where NASA used the gravitational force of Venus to accelerate the craft toward Mercury for its first rendezvous. This technique was used by NASA in almost all of the subsequent space exploration missions. When the fuel level of the thrusters on Mariner 10 became low, scientists creatively used the probe's solar panels as sails, harnessing solar wind and propelling it toward Mercury for its second flyby. It made its third and final pass by Mercury in March of 1975 before heading back into solar orbit. Pictures relayed back to Earth revealed Mercury as having a heavily cratered, moon-like surface and an extremely thin atmosphere. Mariner 10 discovered rotating clouds over the surface of Venus and found that the extremely thin atmosphere of Mercury was mostly composed of helium.