Mercury might once have orbited the sun in a “synchronous” rotation, before being hit by a large asteroid that flicked the rocky planet into a strange orbit.
A synchronous orbit is one in which a body (Mercury) orbits another body (the sun) in the same time it takes the first body to complete one rotation on its axis. In this way, the same face of the orbiting body is always facing the body being orbited. (This is the case with the moon’s orbit of earth.
New research suggests Mercury’s encounter with a large asteroid altered its orbit, putting it into the “3/2” spin it currently enjoys. That is, Mercury rotates three times on its axis for every two orbits it completes of the sun.
Read more at Institut de Physique du Globe