Which spacecraft was launched to study Mercury?

The spacecraft launched to study Mercury was MESSENGER. This mission, which stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, specifically aimed to gather detailed information about the planet's surface, geological history, and environment. Launched in 2004, MESSENGER was significant as it provided the first orbital study of Mercury, allowing scientists to collect extensive data during its mission. The spacecraft revealed much about Mercury's composition, magnetic field, and even its exosphere.

Voyager is known for its mission to the outer planets and interstellar space and did not focus on Mercury. The Mars Rover, as its name suggests, is specifically designed to explore Mars rather than Mercury or other planets. Hubble, while an important space telescope, is primarily used to observe a variety of astronomical phenomena but is not specifically aimed at studying planets in the same capacity as MESSENGER. Thus, MESSENGER stands out as the only spacecraft among the options listed that was dedicated to the study of Mercury.