Here are the two main missions:

* Mariner 10: This NASA spacecraft flew by Mercury three times in 1974 and 1975, mapping about 45% of the planet's surface.

* MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging): This NASA spacecraft orbited Mercury from 2011 to 2015, mapping the entire planet and providing a wealth of data about its composition, geology, and magnetic field.

Additionally, the BepiColombo mission, a joint project by the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), is currently en route to Mercury. It is expected to arrive in 2025 and will spend at least one Earth year studying the planet.

So, while the number of missions to Mercury is limited compared to other planets, several spacecraft have successfully visited and studied this intriguing world.