This article presents a measurement of the proper motion of the Ursa Minor dwarf spheroidal galaxy determined from images taken with the Hubble Space Telescope in two distinct fields .
Each field contains a quasi-stellar object which serves as the “ reference point . ”

The measured proper motion for Ursa Minor , expressed in the equatorial coordinate system , is ( \mu _ { \alpha } , \mu _ { \delta } ) = ( -50 \pm 17 , 22 \pm 16 ) mas century ^ { -1 } .
Removing the contributions of the solar motion and the motion of the Local Standard of Rest yields the proper motion in the Galactic rest frame : ( \mu _ { \alpha } ^ { \mbox { \tiny { Grf } } } , \mu _ { \delta } ^ { \mbox { \tiny { Grf } } } ) = ( -8 \pm 17 ,% 38 \pm 16 ) mas century ^ { -1 } .
The implied space velocity with respect to the Galactic center has a radial component of V _ { r } = -75 \pm 44 km s ^ { -1 } and a tangential component of V _ { t } = 144 \pm 50 km s ^ { -1 } .

Integrating the motion of Ursa Minor in a realistic potential for the Milky Way produces orbital elements .
The perigalacticon and apogalacticon are 40 ( 10 , 76 ) kpc and 89 ( 78 , 160 ) kpc , respectively , where the values in the parentheses represent the 95 \% confidence interval derived from Monte Carlo experiments .
The eccentricity of the orbit is 0.39 ~ { } ( 0.09 , 0.79 ) and the orbital period is 1.5 ~ { } ( 1.1 , 2.7 ) Gyr .
The orbit is retrograde and inclined by 124 ~ { } ( 94 , 136 ) degrees to the Galactic plane .

Ursa Minor is not a likely member of a proposed stream of galaxies on similar orbits around the Milky Way .
Nor is the plane of its orbit coincident with a recently-proposed planar alignment of galaxies around the Milky Way .
Comparing the orbits of Ursa Minor and Carina shows no reason for the different star formation histories of these two galaxies .
Ursa Minor must contain dark matter to have a high probability of surviving disruption by the Galactic tidal force until the present .