Mercury is the planet closest to the Sun, surrounding it once every 88 earthly days. Its brightness varies between -2.0 and 5.5 in apparent magnitude but is not easy to see because the greatest angular separation (the longest elongation) from the Sun is only 28.3 °, meaning it can be seen just after sunset. Between 1974 and 1975, Mercury was studied with the Mariner 10 probe, which mapped only 40-45% of the planet's surface. Starting in 2011, the MESSENGER space probe is orbiting the planet to study chemical composition, geology and magnetic field.
