  Asteroid 2016 HO3, a small asteroid (<60 m) in super fast rotation state (~28
min), and is the target of China's Tianwen-2 asteroid sample-return mission. In
this work, we investigate its structural stability using an advanced
soft-sphere-discrete-element-model code, DEMBody, which is integrated with
bonded-aggregate models to simulate highly irregular boulders. The asteroid
body is numerically constructed by tens of thousands particles, and then is
slowly spin up until structural failure. Rubble piles with different
morphologies, grain size distributions and structures are investigated. We find
a 2016 HO3 shaped granular asteroid would undergo tensile failure at higher
cohesive strengths as opposed to shear failure in lower strengths, regardless
of its shape and constituent grain size ratio. Such a failure mode transition
is consistent with the priority between the Maximum Tensile Stress criterion
criterion and the Drucker-Prager criterion. Therefore, previous works that
solely considered the Drucker-Prager failure criterion have underestimated the
minimal cohesion strength required for fast-rotating asteroids. We predict that
the high spin rate of asteroid 2016 HO3 requires a surface cohesion over ~1 Pa
and a bulk cohesion over ~10-20 Pa. Through comparing these strength conditions
with the latest data from asteroid missions, we suggest a higher likelihood of
a monolithic structure over a typical rubble pile structure. However, the
possibility of the latter cannot be completely ruled out. In addition, the
asteroid's surface could retain a loose regolith layer globally or only near
its poles, which could be the target for sampling of Tianwen-2 mission.
