Diameter: 3,029 miles (4,875km).

Mass: 0.055 Earth masses

Rotation period: 58.6 days.

Inclination of equator to orbit: 0.5 degrees.

Mean orbital speed: 30 Miles per second (48 km/s)

Mean distance from the sun: 36 million miles (58 million km)

THE STRUCTURE OF MERCURY

Mercury never strays less than 28 degrees from the sun, which makes it difficult for the planet to be seen against the suns glare. Being just 36 million miles from the sun, this allows Mercury to  live up to its fleet footed namesake, its orbital period is 88 days , the solar systems shortest. Contrasting with its year is the length of Mercury's day. If you measure a day by how long it takes a planet to rotate, then a day on Mercury is equivalent to slightly less that 59 earth days, or two thirds of its year. However if you define a day by how long it takes for the sun to return to the same point in the sky, then a day is two mercury years, or 176 earth days.  These facts, and the high eccentricity of Mercury's orbit, would produce very strange effects for an observer on the surface of Mercury. At some longitudes the observer would see the sun rise and then gradually increase in apparent size as it slowly moved towards the zenith. At that point the sun would stop, briefly reverse course, and stop again  before resuming its path toward the horizon and decreasing in apparent size. All the while the stars would be moving three times faster across the sky.

Mercury has only been visited by one space craft, Mariner 10. It flew by three times in 1974 and 1975, and it was only able to map 45% of the surface. However NASA launched Messenger in 2004, which is due to orbit Mercury from 2011.

It was long thought that as the mass of this planet is just one eighteenth that of the earths, it is much too small to retain an atmosphere. However there is actually a thin atmosphere consisting of atoms blasted off its surface by the solar wind. Because Mercury is so hot these atoms quickly escape into space, and so unlike our own atmosphere which is stable, Mercury's atmosphere is being constantly replenished. With the sun too close, and lacking the protection from extremes that an atmosphere would give, Mercury has a very uncomfortable climate. The planets day side reaches 800F by noon, with the heat dissipating into space by night, when temperatures drop dramatically to around –280F. This is the widest temperature range of any of the planets.

In 1974 Mariner 10 saw a world that could almost double as the moon. The entire planet is pock–marked with impact craters ranging in size from the smallest detectable (Approx half a mile across) to the giant 'bullseye' of the Caloris basin, some 830 miles wide. Also detected have been ridges, highlands and flooded basins. With regards to its composition, despite the fact that Mercury is only 1.4 times larger than the moon, it is 1.7 times denser, which indicates that the composition of these two bodies is very different. Scientists believe that Mercury has a unique internal structure , like a thick rind on a piece of fruit, a silicate crust only 435 miles thick covers a huge metallic, iron–nickel core  making up 60% of the planets total mass and filling three quarters of its radius. The large core may have formed that way, or it may be the result of Mercury losing some of its upper layers in a massive collision.