The planet Mercury is very difficult to study from the Earth because it is always so close to the Sun. Even at elongation, it is never more than 28 degrees from the Sun in our sky. It is the second smallest planet (it was believed to be the smallest until the discovery that Pluto is actually much smaller than originally thought), and also the fastest in its orbit since it is the innermost planet. In fact, the name Mercury derives from its speed in moving around its orbit.

We began to learn more about Mercury with radar imaging from the Earth in the 1960s, and obtained most of what we know about the planet from the Mariner 10 space probe was placed into a complicated orbit involving Venus and Mercury and which passed close to Mercury and sent back information three times in the period 1974-1976 (image source).