Before MESSENGER, only Mariner 10 had visited it, and it may contain a liquid iron core that produces the second-strongest magnetic field within the asteroid belt.  
BepiColombo is scheduled to visit it in 2019.  MESSENGER is currently making flybys around this planet, which has a weak atmosphere that contains sodium and potassium.  Before MESSENGER, only Mariner 10 had visited it, and it may contain a liquid iron core that produces the second-strongest magnetic field within the asteroid belt.  A theory that said a planet named Vulcan lied within its orbit was discredited after general relativity was used to explain its precession at perihelion.  Its day and year are locked into a 2:3 ratio, and the largest crater on it is Caloris Basin.  For 10 points, name this smallest planet, the closest to the sun.
BepiColombo is scheduled to visit it in 2019.  MESSENGER is currently making flybys around this planet, which has a weak atmosphere that contains sodium and potassium.  
{Mercury}