Mercury is a small, rocky moon-like planet. Up until recently has been the second smallest planet in the Solar System, however, since Pluto has been demoted from a planet to a ‘dwarf planet’ Mercury is now the smallest planet in the Solar System. Mercury has an equatorial radius of 2,439.7 km (1,516.0 miles), it is so small, in fact, that there are two moons in the Solar System that are bigger than Mercury.
Mercury is the closest planet to the Sun, its average distance from Sun is a mere 57,909,175 km (35,983,095 miles). It is so close to the sun that its orbit never takes it far enough away for it to be easily visible from Earth. As a result it is only visible on Earth a few times a year and then only for a few days at a time. Despite this fact Mercury has been known since the 3rd millennium BC, the Sumerians believed Mercury was Enki, the God of creation.
Mercury is so close to the Sun that if you were standing on the surface of the planet the Sun would look twice as big as it does here on Earth. It’s close proximity to the Sun has a strong effect on the daytime temperature; Mercury is a whopping 427C during the day. However, it has the widest temperature range of any planet and during the night the temperature falls to as low as -173C. The days on Mercury are considerably longer than our own lasting an incredible 176 days, Mercury’s years are shorter than its days, lasting only 88 days, which means Mercury orbits the Sun faster than it spins on its own axis.
The Greeks had two names for Mercury, when it appeared in the morning sky they named it Apollo, in the evening they named it Hermes. It was the Romans who gave Mercury its name. They named it after Mercury, the Messenger of the Gods because of the speed at which it crossed the night sky.
As Mercury is often hidden by the light from the Sun the best time to see it is at sunrise or sunset when the Sun’s rays are diminishing. Mercury has no natural satellites, it also has no atmosphere. It looks very similar to the moon with a heavily cratered surface. The largest feature on Mercury is the Caloris Basin, a 1300 km in diameter crater which was probably caused by an impact from an object larger than 100 km in diameter.
In 1973 Mariner 10 flew by the planet three times; on all three flybys the craft only saw the same hemisphere of Mercury, so up until very recently half of the planet’s surface has never been seen up close. The MESSENGER spacecraft is the most recent craft to map Mercury. Launched on the 3rd August 2004 the craft did two flybys of the planet in 2008, first in January then again in October it mapping 30% of Mercury’s surface, it again passed by in March 2009, then it will enter a year-long science orbit of Mercury in 2011.
