Next to Pluto, Mercury is the smallest planet in the solar system: with a mass just over one 20th that of Earth’s, it is actually smaller than two solar system moons, Ganymede of Jupiter and Titan of Saturn. Because of the planet’s small size as seen from Earth, astronomers could not discern much of its detail with Earth-based telescopes. They were also hindered by the fact that Mercury appears close to the Sun (never more than 28 degrees away). Only with the advent of unmanned planetary exploration did detailed images such as the one seen here, taken from the Mariner 10 spacecraft, become possible. 
    The Mariner 10 expedition was launched in 1973 and reached