Mercury, the smallest planet and the closest to the Sun, is only slightly larger than Earth’s Moon. Mercury’s surface is very like the Moon, covered by mountainous regions and large basins full of craters. Its atmosphere is mostly hydrogen and helium. Its surface heats up and cools down very quickly, reaching 427°C (800°F) when it is closest to the Sun and falling to -179°C (-290°F) when it is farthest away.

Mercury travels around the Sun every 88 days in an oval-shaped orbit, taking it as close as 47,000,000km (29,000,000 miles) to the Sun. It travels at almost 50km (31 miles) per second which is faster than any other planet and that is why it is named after the Roman god Mercury, the winged messenger. The first spacecraft to visit and photograph Mercury was Mariner 10, followed several years later by the MESSENGER spacecraft.