  We present the first-ever measurements of the surface colors and rotational
properties of a recently discovered distant object in an unusual orbit 2016
ND21. In 2017, this object was close to its perihelion, which allowed us to
observe it with a 2.0-m telescope at the Peak Terskol observatory. Broadband
photometric observations of 2016 ND21 were carried out in October and December
of 2017 in standard BVR filters of the Johnson-Cousins photometric system.
During our observations, we did not detect any cometary activity. We found the
rotational period of 17.53{\pm}0.02 hr, while another a little longer value of
17.65{\pm}0.02 hr is also possible. Assuming an equatorial aspect of
observations, a peak-to-peak amplitude of A = 0.31{\pm}0.05 mag (or even higher
since only one maximum and one minimum were well-measured) corresponds to an
elongated body with an axis ratio a/b ~1.3. The lightcurve behavior indicates a
complex, possibly non-convex, shape of this object. The visible absolute
magnitude is HV =12.4{\pm}0.1 mag, which was estimated by using the linear
phase slope 0.04 mag/deg as the most probable value from our observations.
Assuming a surface albedo in the range of 0.04-0.10, the size of 2016 ND21
should be about 15-23 km. From our multi-color observations, we determined
surface colors V-R = 0.69{\pm}0.04 mag, B-R = 1.79{\pm}0.08 mag, and B-V =
1.10{\pm}0.08 mag. The measured colors indicate an extremely red surface of
this object. A very red surface is unusual for comets, which is in agreement
with the fact, that no cometary activity was detected for 2016 ND21. The B-R
color is higher than the typical B-R colors of the red D-type asteroids, but it
is consistent with colors of the red Centaurs and TNOs classified as RR type in
TNOs classification. This result gives a first evidence of a possible outer
belt origin of this small body.
