Mercury is the planet nearest to the Sun. It orbits our star on a comparatively highly elliptic trajectory and has a rare ratio between the length of day and year - it rotates three times during two Mercury years. This leads to extremes in temperature (90 to 700 Kelvin) between dayside and nightside. Sometimes an observer on Mercury would even be able to see the Sun reverse its course over the sky for some weeks. Mercury has no moons. For a planet Mercury is quite small - even some moons of Jupiter are larger - however, it is a significantly dense planet which suggests a large iron core. It also possesses a weak global magnetic field. Mercury has no atmosphere although outgassed particles do form a thin exosphere which is exposed to the strong solar wind near the Sun. Moon and Mercury have a lot in common - among other things both show us a crater-pocked landscape. Only about half of Mercury's surface has been photographed so far, with a giant impact crater being one of its most significant features. Until now only one space probe, Mariner 10, has done measurements around Mercury during three flybys.
The planned ESA mission BepiColombo to Mercury will explore the planet in detail and over a longer period of time in the coming decade - with Austrian participation.
Further information on Mercury is found at The Nine Planets.