  We present a unique and significant polarimetric result regarding the
near-Earth asteroid (152679) 1998 KU$_\mathrm{2}$ , which has a very low
geometric albedo. From our observations, we find that the linear polarization
degrees of 1998 KU$_\mathrm{2}$ are 44.6 $\pm$ 0.5\% in the R$_\mathrm{C}$ band
and 44.0 $\pm$ 0.6\% in the V band at a solar phase angle of 81.0\degr. These
values are the highest of any known airless body in the solar system (i.e.,
high-polarization comets, asteroids, and planetary satellites) at similar phase
angles. This polarimetric observation is not only the first for primitive
asteroids at large phase angles, but also for low-albedo (< 0.1) airless
bodies.
  Based on spectroscopic similarities and polarimetric measurements of
materials that have been sorted by size in previous studies, we conjecture that
1998 KU$_\mathrm{2}$ has a highly microporous regolith structure comprising
nano-sized carbon grains on the surface.
