A novel concept of the nearly-fuel-free planetary exploration by the sail aerocapture vehicle, which uses the sail not only for the solar sailing but also for an aerodynamic decelerator at the orbit insertion, and its design methodology are presented. Considering a mission to Saturn, the trajectory analysis shows that the corridor of the entry path angle for successful aerocapture is significantly widened and that the aerodynamic heating is much reduced, thanks to the ultra-low ballistic coefficient of the sail-type vehicle. The coupled analyses of the rarefied flow field and the deformation of the sail demonstrate that the solar sail vehicle can survive under the condition of the aerodynamic heating and the stress due to the aerodynamic force. To sustain the disk-like shape of the sail during the atmospheric flight, an inflatable torus attached as a hoop support is recommended as well as a spin motion producing the centrifugal force.