  We have coordinated Hubble Space Telescope photometry with ground-based
discovery for three supernovae: two SN Ia near z~0.5 (SN 1997ce, SN 1997cj) and
a third event at z=0.97 (SN 1997ck). The superb spatial resolution of HST
separates each supernova from its host galaxy and leads to good precision in
the light curves. The HST data combined with ground-based photometry provide
good temporal coverage. We use these light curves and relations between
luminosity, light curve shape, and color calibrated from low-z samples to
derive relative luminosity distances which are accurate to 10% at z~0.5 and 20%
at z=1. The redshift-distance relation is used to place constraints on the
global mean matter density, Omega_matter, and the normalized cosmological
constant, Omega_Lambda. When the HST sample is combined with the distance to SN
1995K (z=0.48), analyzed by the same precepts, it suggests that matter alone is
insufficient to produce a flat Universe. Specifically, for
Omega_matter+Omega_Lambda=1, Omega_matter is less than 1 with >95% confidence,
and our best estimate of Omega_matter is -0.1 +/- 0.5 if Omega_Lambda=0.
Although the present result is based on a very small sample whose systematics
remain to be explored, it demonstrates the power of HST measurements for high
redshift supernovae.
