  We present the K2 light curves of a large sample of untargeted Main Belt
asteroids (MBAs) detected with the Kepler space telescope. The asteroids were
observed within the Uranus superstamp, a relatively large, continuous field
with low stellar background designed to cover the planet Uranus and its moons
during Campaign 8 of the K2 mission. The superstamp offered the possibility to
obtain precise, uninterrupted light curves of a large number of MBAs and thus
to determine unambiguous rotation rates for them. We obtained photometry for
608 MBAs, and were able to determine or estimate rotation rates for 90 targets,
of which 86 had no known values before. In an additional 16 targets we detected
incomplete cycles and/or eclipse-like events. We found the median rotation rate
to be significantly longer than that of the ground-based observations
indicating that the latter are biased towards shorter rotation rates. Our study
highlights the need and benefits of further continuous photometry of asteroids.
