Mercury

Mercury is the closest planet to the Sun in our Solar System. The first close-up images of Mercury were taken in 1974 and 1975 by the Mariner 10 space probe. It took another 30 years before the follow-up mission came with Messenger, and this continues today. Images of Mercury show a moon-like landscape, with an ancient surface littered with craters. Mercury was and is vulnerably exposed to external impacting, because its proximity to the Sun means it lacks a protective atmosphere, as this is simply blown away by the solar wind. The lack of atmosphere also means that temperature fluctuations on Mercury are extreme. After sunset, the temperature drops very quickly to below -173 degrees, while on the day side it is around 427 degrees.

Compared to the other planets, Mercury's orbit is highly eccentric, whereas the other planets move in almost circular orbits. Its orbital plane is also more inclined when compared to the other planets, as the latter all move in approximately the same plane. Apart from Venus, Mercury is the only planet in the Solar System without a moon. There are even discussions as to whether Mercury itself was originally a moon of Venus, but was thrown out of its orbit by the forces of the nearby Sun. This could also explain why Mercury's orbit is so different.