  Hayabusa2 is the Japanese Asteroid Return Mission and targeted the
carbonaceous asteroid Ryugu, conducted by the Japan Aerospace Exploration
Agency (JAXA). The goal of this mission was to conduct proximity operations
including remote sensing observations, material sampling, and a Small Carry-On
Impact experiment, as well as sample analyses. As of September 2020, the
spacecraft is on the way back to Earth with samples from Ryugu with no critical
issues after the successful departure in November 2019. Here, we propose an
extended mission in which the spacecraft will rendezvous with a small asteroid
with ~30 m - ~40 m in diameter that is rotating at a spin period of ~10 min
after an additional ~10-year cruise phase. We introduce that two scenarios are
suitable for the extended mission. In the first scenario, the spacecraft will
perform swing-by maneuvers at Venus once and Earth twice to arrive at asteroid
2001 AV43. In the second scenario, it will perform swing-by maneuvers at Earth
twice to reach asteroid 1998 KY26. In both scenarios, the mission will continue
until the early 2030s. JAXA recently released the decision that the spacecraft
will rendezvous with 1998 KY26. This paper focuses on our scientific
assessments of the two scenarios but leaves the decision process to go to 1998
KY26 for future reports. Rendezvous operations will be planned to detail the
physical properties and surrounding environments of the target, one of the
smallest elements of small planetary bodies. By achieving the planned
operations, the mission will provide critical hints on the violent histories of
collisions and accumulations of small bodies in the solar system. Furthermore,
the established scientific knowledge and techniques will advance key
technologies for planetary defense.
