  The outburst of V838 Monocerotis in early 2002, and the subsequent appearance
of its light echoes, occurred just before the installation of the Advanced
Camera for Surveys into the Hubble Space Telescope (HST). This fortunate
sequence of events has allowed us to obtain spectacular HST images of the
echoes, yielding not only pictures of extraordinary beauty, but also providing
unique scientific information. Our team has used the HST images to provide a
direct geometrical distance to V838 Mon, based on polarimetric imaging, and
limits on the distance based on the apparent angular expansion rates. Several
morphological features seen in the HST images strongly suggest that the
illuminated dust was ejected from the star in a previous outburst, similar to
the current one. In particular, a "double-helix" feature points exactly back to
the star. Moreover, three-dimensional mapping of the outer edges of the dust
suggests an overall ellipsoidal shape, centered on V838 Mon itself. And the
appearance of the light echo in the most recent Hubble images is remarkably
similar to that of a well-known planetary nebula, M27. Future work on the HST
images will include an analysis of interstellar-dust physics, in a situation
where the scattering angle and illumination are unambiguously known, and
visualization of a fully three-dimensional map of the dust distribution.
