  We present the discovery of two ultra-luminous supernovae (SNe) at z ~ 0.9
with the Pan-STARRS1 Medium-Deep Survey. These SNe, PS1-10ky and PS1-10awh, are
amongst the most luminous SNe ever discovered, comparable to the unusual
transients SN 2005ap and SCP 06F6. Like SN 2005ap and SCP 06F6, they show
characteristic high luminosities (M_bol ~ -22.5 mag), blue spectra with a few
broad absorption lines, and no evidence for H or He. We have constructed a full
multi-color light curve sensitive to the peak of the spectral energy
distribution in the rest-frame ultraviolet, and we have obtained time-series
spectroscopy for these SNe. Given the similarities between the SNe, we combine
their light curves to estimate a total radiated energy over the course of
explosion of (0.9-1.4) x 10^51 erg. We find photospheric velocities of
12,000-19,000 km/s with no evidence for deceleration measured across ~3
rest-frame weeks around light-curve peak, consistent with the expansion of an
optically-thick massive shell of material. We show that, consistent with
findings for other ultra-luminous SNe in this class, radioactive decay is not
sufficient to power PS1-10ky, and we discuss two plausible origins for these
events: the initial spin-down of a newborn magnetar in a core-collapse SN, or
SN shock breakout from the dense circumstellar wind surrounding a Wolf-Rayet
star.
