  We present time-resolved visible spectrophotometry of minimoon 2020 CD$_3$,
the second asteroid known to become temporarily captured by the Earth-Moon
system's gravitational field. The spectrophotometry was taken with Keck I/LRIS
between wavelengths 434 nm and 912 nm in $B$, $g$, $V$, $R$, $I$ and RG850
filters as it was leaving the Earth-Moon system on 2020 March 23 UTC. The
spectrophotometry of 2020 CD$_3$ most closely resembles the spectra of V-type
asteroids and some Lunar rock samples with a reddish slope of ~18$\%$/100 nm
between 434 nm and 761 nm corresponding to colors of $g$-$r$ = 0.62$\pm$0.08,
$r$-$i$ = 0.21 $\pm$ 0.06 and an absorption band at ~900 nm corresponding to
$i$-$z$ = -0.54$\pm$0.10. Combining our measured 31.9$\pm$0.1 absolute
magnitude with an albedo of 0.35 typical for V-type asteroids, we determine
2020 CD$_3$'s diameter to be ~0.9$\pm$0.1 m making it the first minimoon and
one of the smallest asteroids to be spectrally studied. We use our time-series
photometry to detect periodic lightcurve variations with a $<$10$^{-4}$ false
alarm probability corresponding to a lightcurve period of ~573 s and a
lightcurve amplitude of ~1 mag implying 2020 CD$_3$ possesses a $b/a$ axial
ratio of ~2.5. In addition, we extend the observational arc of 2020 CD$_3$ to
37 days between 2020 February 15 UTC and 2020 March 23 UTC. From the improved
orbital solution for 2020 CD$_3$, we estimate its likely duration of its
capture to be ~2 y, and we measure the non-gravitation perturbation on its
orbit due to radiation pressure with an area-to-mass ratio of
6.9$\pm$2.4$\times$10$^{-4}$ m$^2$/kg implying a density of 2.3$\pm$0.8
g/cm$^3$, broadly compatible with the densities of other meter-scale asteroids
and Lunar rock. We searched for pre-discovery detections of 2020 CD$_3$ in the
ZTF archive as far back as 2018 October, but were unable to locate any positive
detections.
