The planet Mercury is the closest to the Sun and is now the smallest planet in our Solar System. The temperatures on Mercury range from 700 degrees Kelvin on the sunlight side to 90 degrees Kelvin on the night side. Mercury orbits the Sun once every 88 days and rotates on its axis once every 58 days. The orbit of Mercury is very elliptical and brings it as close to the Sun as 46 million kilometers and as far away from the Sun as 70 million kilometers. Since it is so close to the Sun, Mercury can only be seen from Earth during the early morning or evening twilight.
We can observe Mercury from the Earth using both optical and radio telescopes, but much of what we know about Mercury is the result of three fly bys performed by the Mariner 10 spacecraft during the 1970s. The Mariner 10 only photographed 40 to 45 percent of the surface of Mercury and the rest has never been seen up close. The photographs the Mariner 10 did send back reveled a rocky, cratered surface similar to the Earths own Moon.
Mercury is about 4878 kilometers in diameter and this makes Mercury slightly smaller than the moons Ganymede and Titan. However, Mercury is more than twice a massive thanks to its relatively high density that is second only to the Earths. This high density is the result of Mercurys inner structure which has a relatively large iron core that may be all or partly molten. The large iron core also generates a weak magnetic field about 1 percent as strong as the Earths. Despite being weak this magnetic field allows Mercury to maintain a very thin atmosphere in what is called the magnetosphere. The magnetic field does this by deflecting the solar wind.
Up until 1962 it was belived that Mercury rotated on its axis once each time it orbited the Sun. This would mean that one side of Mercury would always face the Sun the same way one side of Earths Moons always faces the Earth. Doppler radar observation conducted in 1965 showed this is not so. Mercury actually rotates on its axis three times during the course of two of its orbits around the Sun. This has some rather odd effects especially when combined with Mercurys highly elliptical orbit. If you were standing on Mercury you would see the Sun rise and then grow larger in size. The Sun would then stop in its journey across the sky and reverse its course. After back tracking a ways the Sun would then stop again and resume its original course. The Sun would then appear to shrink in size and drop below the horizon.
The orbit of Mercury gets even stranger as a result of being to close to the Sun where the Suns gravitational field is incredibly strong. As Mercury orbits the Sun the point where Mercury starts a new orbit moves slightly. This is called the precession of perihelion and it can not be explained using Newtonian physics. For a while it was hypothesized that another planet, that was even named Vulcan, was exerting its gravitational pull on Mercury and this was causing the precession of perihelion. This was proved false and the existence of Vulcan was dismissed when Albert Einsteins General Theory of Relativity provided a better explanation.
At present, Mercury is the least studied of the planets but that will soon change. On August 3, 2005 NASA launched a new mission to Mercury named MESSENGER which stands for Mercury Surface Space Environment geochemistry and Ranging. The MESSENGER spacecraft will make three fly bys of Mercury in January 2008, October 2008 and September 2009. The MESSENGER spacecraft will then settle into orbit around Mercury in March 2011. Japan and the European Space Agency are also planning a joint mission to Mercury called BepiColombo which will arrive at Mercury in the year 2019. These spacecraft will use a variety of scientific instruments to tell us more about all aspects of the planet Mercury.