text: 1566 Icarus is an Apollo asteroid ( a sub-class of near-Earth asteroid ) that at perihelion comes closer to the Sun than Mercury , i.e. it is a Mercury-crossing asteroid .
simplified: 1566 Icarus is an Apollo asteroid ( a sub-class of near-Earth asteroid ) whose strange characteristic is that at perihelion ( is closest approach to the Sun ) it is closer to the Sun than Mercury ; it is said to be a Mercury-crosser asteroid .