  We present a deep optical survey of Uranus' Hill sphere for small satellites.
The Subaru 8-m telescope was used to survey about 3.5 square degrees with a 50%
detection efficiency at limiting red magnitude m = 26.1 mag. This magnitude
corresponds to objects that are about 7 km in radius (assuming an albedo of
0.04). We detected (without prior knowledge of their positions) all previously
known outer satellites and discovered two new irregular satellites (S/2001 U2
and S/2003 U3). The two inner satellites Titania and Oberon were also detected.
One of the newly discovered satellites (S/2003 U3) is the first known irregular
prograde of the planet. The population, size distribution and orbital
parameters of Uranus' irregular satellites are remarkably similar to the
irregular satellites of gas giant Jupiter. Both have shallow size distributions
(power law indices q~2 for radii > 7 km) with no correlation between the sizes
of the satellites and their orbital parameters. However, unlike those of
Jupiter, Uranus' irregular satellites do not appear to occupy tight distinct
dynamical groups in semi-major axis versus inclination phase space. Two
groupings in semi-major axis versus eccentricity phase space appear to be
statistically significant.
