  The dwarf galaxies around the Milky Way are distributed in a so-called vast
polar structure (VPOS) that may be in conflict with Lambda CDM simulations.
Here, we seek to determine if the VPOS poses a serious challenge to the Lambda
cold dark matter paradigm on galactic scales. Specifically, we investigate if
the VPOS remains coherent as a function of time. Using the measured Hubble
Space Telescope (HST) proper motions and associated uncertainties, we integrate
the orbits of the classical Milky Way satellites backwards in time and find
that the structure disperses well before a dynamical time. We also examine in
particular Leo I and Leo II using their most recent proper motion data, both of
which have extreme kinematic properties, but these satellites do not appear to
drive the polar fit that is seen at the present day. We have studied the effect
of the uncertainties on the HST proper motions on the coherence of the VPOS as
a function of time. We find that 8 of the 11 classical dwarfs have reliable
proper motions; for these 8, the VPOS also loses significance in less than a
dynamical time, indicating that the VPOS is not a dynamically stable structure.
Obtaining more accurate proper motion measurements of Ursa Minor, Sculptor, and
Carina would bolster these conclusions.
