1. Characteristics ↑
Mercury is the closest planet to the Sun. It takes about 88 days to orbit the Sun, and only half a day goes by on Mercury during this time. Mercury is the smallest and least massive planet in the Solar System, and its orbit is the most eccentric. This was something that English natural philosopher Isaac Newton's theory of gravitation could not explain but German-Swiss-American physicist Albert Einstein's theory of general relativity could.
Mercury, image taken by the MESSENGER orbiter. Image credit: NASA/Public domain.
Mercury is a rocky planet, similar to the Moon. It has at least as many craters and is covered with dust. It has no satellites of its own, and, like the Moon, it is not massive enough to sustain an atmosphere. Unlike the Moon, however, it has a massive iron core, which generates a magnetic field about 1% as strong as the Earth's[2a].
Radar observations conducted by the Goldstone antenna in California and the Very Large Array in New Mexico, in the early 1990s, suggested that frozen water exists at the bottom of deep craters on Mercury's poles.
Despite the fact that Mercury is bright when viewed from Earth, it's difficult to observe as it's usually hidden in the light of the Sun. It is best viewed early in the morning or at twilight.
The Ancient Greeks gave Mercury two names: Apollo, which was visible at sunrise, and Hermes, which was visible at sunset. Hermes was later named Mercury by the Romans, and was considered the messenger of the gods.
The four inner planets to scale. Image credit: NASA/Public domain.
2. Missions to Mercury ↑
Only two spacecraft have ever been to Mercury. These are NASA's Mariner 10 and MESSENGER spacecraft. Mariner 10 launched in 1973, and mapped almost half of its surface by 1975[2b]. MESSENGER launched in 2004, and orbited Mercury between 2011 and 2015. It mapped the whole surface and found further evidence of frozen water.
The next mission to Mercury, BepiColombo, is a joint mission between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). It is due to launch in 2018, and should arrive in 2024.
Enhanced colour map of Mercury, composed using data taken by MESSENGER. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Public domain.