Ikaros -- which aside from being a somewhat clumsy acronym is also an allusion to the Greek mythological character who flew too close to the sun -- marks the first time a space craft will enter deep space towed by solar sails, which provide a fuel-free means to explore the solar system provided the craft is near enough the sun to catch a particle powered breeze. Several prototype space sails have been unfurled in orbit by various agencies including NASA, but as far as long-term propulsion is concerned the Ikaros mission is solar sailing's coming out party.