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Despite using Type Ia supernovae (SN Ia) to precisely measure cosmological parameters comma we still do not know basic facts about the progenitor systems and explosions. Theory suggests that SN Ia progenitor metallicity is correlated with peak luminosity comma but not how quickly it fades comma which we use to calibrate the luminosity and measure distances. This effect should lead to an increased Hubble scatter comma reducing the precision with which we measure distances. If the mean progenitor metallicity changes with redshift or population comma cosmological measurements such as the dark energy equation-of-state parameter and the Hubble constant could be biased. Models also indicate that changing progenitor metallicity will have little effect on the appearance of optical-NIR SN data comma but significantly alter UV spectra. These data can only be obtained with HST. Previous HST observations of 2 d_commatwind_comma SN (having nearly identical optical spectra and light-curve shapes) with different UV spectra and peak optical luminosities indicate progenitor metallicity differences consistent with models. To determine the overall impact of progenitor metallicity on cosmological measurements comma we must increase the sample. The community now discovers virgul20 SN Ia each year >2 weeks before peak. With the increased discovery rate of young SN comma we can increase the sample of SN Ia with UV spectra near peak by 50% in a single Cycle. UV observations are critical to the understanding of SN Ia explosions and progenitors and constraining fundamental parameters such as the Hubble constant. This is our best opportunity to further our understanding of SN Ia while directly improving the utility of SN Ia for cosmology.