Mercury Physical Facts
Mercury is the closest planet to the Sun and the eighth largest. Mercury is smaller in diameter than Ganymede and Titan but more massive.
Mercury has no known satellites.
Mercury's orbit is highly eccentric; at perihelion it is only 46 million-km from the Sun but at aphelion it is 70 million.
Mercury has been visited by only one spacecraft, Mariner 10. It flew by three times in 1974 and 1975. Only 45% of the surface was mapped (and, unfortunately, it is too close to the Sun to be safely imaged by HST).