Mercury
Compared to the other planets, we know relatively little about the smallest planet in our Solar System
Although we’ve been observing Mercury from Earth for thousands of years, its close proximity to the Sun - about 58 million kilometres on average - has made it difficult for astronomers to learn much about the planet. The Hubble Space Telescope is unable to observe the tiny planet because turning that close towards the Sun would damage the telescope’s instruments. Most of what we know came from the Mariner 10 space probe’s flybys in 1974 and 1975 and from NASA’s MESSENGER, which orbited Mercury between 2011 and 2015. Europe’s BepiColombo spacecraft will gather more data on the planet when it arrives in 2025.
With the naked eye Mercury can only be seen at dawn or dusk - depending on the time of year. Mercury can also be seen as a small black spot moving across the Sun at intervals of seven, 13 and 33 years. This is known as a transit of Mercury, and occurs when the planet passes directly between Earth and the Sun.