Mercury is the closest planet to the sun and it is also the smallest planet in the solar system, now that Pluto has been demoted. Until now, Mercury has been the only major body in the inner solar system that has not been orbited by human spacecraft. As of Thursday, NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging probe, aka MESSENGER, (what an acronym!) successfully entered orbit around the planet, after two flybys of Venus, 3 of Mercury and one of Earth itself. There, it will map the surface and study the environment around Mercury. There are a number of mysteries surrounding the planet, including why it has such a large iron core. There are also a number of questions about strange and bizarre terrain found opposite the planet from a huge 1,550 km-wide impact basin.

Mercury’s next visitors will arrive as part of a joint European Space Agency and Japanese Space Agency project, BepiColombo, that is expected to reach the planet in 2019.

Wikipedia: Mercury