Mercury, the Messenger of the gods is the planet closest to the sun. It's a pretty small planet with a diameter of 4880 km so it's about ¹/₃ the size of the Earth.  Mercury has a big Iron core which makes up 75% of the whole planet's diameter. This core is surrounded by a rocky mantle and crust about 600km thick. It's actual surface is quite cratered and, like the moon, it's covered in fine soil. It also has an elliptical orbit which, although not as elliptical as Pluto's, is worth noting since it means the closest it gets to the Sun is 46 million km while the furthest it ever gets is 70 million km.  Mercury is the only solid planet other than Earth known to have a magnetic field. It doesn't have much of an atmosphere because it's so close to the Sun, and in size it's about intermediate between the Earth and its Moon.  At the moment, the only spacecraft to have visited Mercury is Mariner 10, which was only 45.5 cm high and 1.2m wide.  When it visited though, it took over 8000 photographs.

Since Mercury is so much closer to the Sun than we are, in the Mercurian sky the Sun appears twice as big as here on Earth.  If you want to see Mercury from Earth, it's faintly visible appearing for a short while just before sunrise or just after sunset. The biggest basin on Mercury, the Caloris Basin was formed when an asteroid 100 km wide travelling at 500,000 km/hour crashed into Mercury.  The impact was so great that it sent an almost instant shockwave throughout the planet, creating the hilly, lineated terrain it has now.

Mercury was first spotted in 1610 by the great Italian astronomer Galileo Galilei.   It wasn't until 1639, however, that another Italian astronomer, Giovanni Zupus discovered that it circled the Sun.  Then in 1889 Giovanni Schiaparelli made the first map of Mercury's surface features.  Much later, in 1968, Surveyor 7 took the first spacecraft picture of Mercury from the Moon and in 1974-75 the Mariner 10 space probe paid Mercury a visit.