Mercury is the second smallest planet in our Solar System; only Pluto is smaller. It's a dry, rocky and barren world with extreme temperatures. Like the moon - which is similar in size to Mercury, its surface is heavily marked with craters. As the planet nearest the Sun, Mercury has the fastest orbit, taking just 88 days to make its journey around the star. This is why the Romans named the planet after their quick-footed messenger to the Gods, Mercury.
Despite its fast orbit, Mercury spins relatively slowly on its axis, taking 59 days to make one full revolution (the Earth takes just 24 hours). This means that two thirds of the way through its orbit, Mercury has only completed one revolution on its axis. This effect, combined with Mercury's very thin atmosphere, produces dramatic temperature ranges on the planet's surface. The side that faces the Sun has plenty of time to heat up (to more than 400 °C), while the side in shadow grows very cold (to -183 °C).
Mercury's closeness to the Sun has made it a difficult planet to study and we can only detect faint markings on its surface. The only time we have studied the planet in any detail was when the Mariner 10 probe flew by in the early 1970s. However the European Space Agency is launching a mission, BepiColombo,in 2013 to discover more about Mercury.
