  We report new observations of the active asteroid P/2010 A2 taken when it
made its closest approach to the Earth (1.06 au in 2017 January) after its
first discovery in 2010. Despite a crucial role of the rotational period in
clarifying its ejection mechanism, the rotational property of P/2010 A2 has not
yet been studied due to the extreme faintness of this tiny object ($\sim$120 m
in diameter). Taking advantage of the best observing geometry since the
discovery, we succeed in obtaining the rotational light curve of the largest
fragment with Gemini/GMOS-N. We find that (1) the largest fragment has a
double-peaked period of $11.36\pm0.02$ hr spinning much slower than its
critical spin period; (2) the largest fragment is a highly elongated object
($a/b\geqslant 1.94$) with an effective radius of $61.9^{+16.8}_{-9.2}$ m; (3)
the size distribution of the ejecta follows a broken power law (the power
indices of the cumulative size distributions of the dust and fragments are
$2.5\pm0.1$ and $5.2\pm0.1$, respectively); (4) the mass ratio of the largest
fragment to the total ejecta is around 0.8; and (5) the dust cloud morphology
is in agreement with the anisotropic ejection model in Kim et al. (2017). These
new characteristics of the ejecta obtained in this work are favorable to the
impact shattering hypothesis.
