Mercury is the closest planet to the Sun, at an average distance of 58 million km, and it is a rocky world like Earth. Its surface is mountainous and heavily cratered, like the Moon, and the two bodies are of similar sizes.
With a diameter of 4,880 km, Mercury is even smaller than the largest moons orbiting Jupiter and Saturn – Ganymede and Titan.
Little was known about Mercury before the first spaceprobes visited, photographing a world covered in craters. But it had been noted that Mercury is extremely dense for its size. This was appreciated as long ago as 1841 when German astronomer Johann Franz Encke observed the planet’s pull on a passing comet.
Evidence showed that Mercury has a very large and partly liquid iron core that makes up more than 60 per cent of its volume and around three-quarters of the the planet’s diameter.
When NASA’s Messenger spacecraft flew by in early 2008, however, we discovered that Mercury is unlike the Moon because it has an active interior volcanically. It has no atmosphere to speak of, just a thin layer of atoms, called an exosphere, blasted from the surface by sunlight and meteoroid impacts.
Lying so close to the Sun, Mercury experiences a huge range in temperature. The side facing the Sun reaching a fiery 430 degrees C, but the night side plunges to -180 C. It is thought there could be ice within the walls of some craters where the Sun never shines.
Mercury rotates once every 59 Earth days and it takes 88 days to orbit the Sun. It has the most eccentric orbit of any planets, with its distance from the Sun ranging from 46 million to 70 million km. You can see Mercury for yourself if you know just when to look. Here is how to find it.
The Mesenger flyby in January 2008 was the first visit by a spaceprobe in more than 32 years. Previous visitor was NASA’s Mariner 10 which flew past in 1974 and 1975, taking more than 2,700 photos and transforming our knowledge of the planet.
Mariner photographed less than half of Mercury’s surface but revealed its craters and a pattern of ridges believed to have formed when the planet originally cooled and shrank like a dried orange.
Messenger eventually went into orbit around Mercury, in March 2011, pictured more of its landscape, including a mysterious spider formation, formed by more than 100 raised, narrow troughs that radiate from a smaller crater inside the 1,300 km wide Caloris Basin. It resembles a spider in its web. Other large impact sites were also found.
Mercury’s surface is darker than the Moon and covered with graphite. Ridges soar to heights of 1.5 km or more. Messenger spent seven years mapping Mercury, finding water ice at the shadowed regions of the poles, and features that confirmed the planet was shrinking.
Messenger finally crashed into the planet’s surface on April 30, 2015, after its propellant ran out.
Another mission, BepiColombo, is due to launch in October, 2018, carrying two probes – one European and one Japanese – on a 7.5 year journey to go into orbit around Mercury.
Because Mercury lies between Earth and the Sun, it sometimes passes directly in front of it, an event known as a transit of Mercury. These can only occur in May or November, when the plane of Mercury’s orbit intersects with the plane of Earth’s orbit around the Sun.
Related: How to observe Mercury for yourself