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The planet Mercury is the closest to the Sun and is now the smallest planet in our Solar System. Temperatures on Mercury range from 700 Kelvin on the sunlight side to 90 Kelvin on the night side. Mercury orbits the Sun once every 88 days and rotates on its axis once every 58 days. Mercury’s orbit is highly elliptical, bringing it closer to the Sun up to 46 million kilometers and as far from the Sun as 70 million kilometers. Since it is so close to the Sun, Mercury can only be seen from Earth during morning or evening twilight.
We can observe Mercury from Earth using both optical telescopes and radio telescopes, but much of what we know about Mercury is the result of three flybys made by the Mariner 10 spacecraft during the 1970s. Mariner 10 only photographed 40-45 percent of Mercury’s surface and the rest has never been seen up close. Photographs sent by Mariner 10 revealed a crater-filled, rocky surface similar to Earth’s 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 as massive thanks to its relatively high density that is second only to Earth. This high density is the result of Mercury’s internal structure, which has a relatively large iron core that can be fully or partially molten. The large iron core also generates a weak magnetic field about 1 percent as strong as Earth. 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.
Until 1962 it was believed 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 in the same way that one side of the Earth’s Moons always faces the Earth. . Doppler radar observation carried out in 1965 showed that this is not the case. Mercury actually rotates on its axis three times during the course of two of its orbits around the Sun. This has some pretty strange effects, especially when combined with Mercury’s highly elliptical orbit. If you were standing on Mercury, you would see the Sun rise and then grow in size. Then the Sun would stop on its journey across the sky and reverse its course. After tracking backwards, the Sun would stop again and resume its original course. The Sun would then appear to shrink in size and fall below the horizon.
Mercury’s orbit gets even weirder as a result of being so close to the Sun where the Sun’s gravitational field is incredibly strong. As Mercury orbits the Sun, the point where Mercury begins a new orbit moves slightly. This is called perihelion precession and cannot be explained using Newtonian physics. For a time it was hypothesized that another planet, which was even called Vulcan, was exerting its gravitational pull on Mercury and this was causing the precession of perihelion. This was shown to be false and the existence of Vulcan was discarded when Albert Einstein’s Theory of General Relativity provided a better explanation.
At present, Mercury is the least studied of the planets, but that will change soon. On August 3, 2005, NASA launched a new mission to Mercury called MESSENGER, which stands for Mercury Surface Space Environment geochemistry and range. The MESSENGER spacecraft will make three flybys of Mercury in January 2008, October 2008 and September 2009. The MESSENGER spacecraft will go into orbit around Mercury in March 2011. Japan and the European Space Agency are also planning a joint mission. Mercury called BepiColombo, which will arrive at Mercury in 2019. These spacecraft will use a variety of scientific instruments to tell us more about all aspects of the planet Mercury.