We present the most sensitive ultraviolet observations of Supernova 1987A to date. Imaging spectroscopy from the Hubble Space Telescope-Cosmic Origins Spectrograph shows many narrow (dv sim 300 km/s) emission lines from the circumstellar ring, broad
(dv sim 10 -- 20 x 10^3 km/s) emission lines from the reverse shock, and ultraviolet continuum emission. The high signal-to-noise (> 40 per resolution element) broad LyA emission is excited by soft X-ray and EUV heating of mostly neutral gas in the circumstellar ring and outer supernova debris. The ultraviolet continuum at lambda > 1350A can be explained by HI 2-photon emission from the same region. We confirm our earlier, tentative detection of NV lambda 1240 emission from the reverse shock and we present the first detections of broad HeII lambda1640, CIV lambda1550, and NIV] lambda1486 emission lines from the reverse shock. The helium abundance in the high-velocity material is He/H = 0.14 +/- 0.06. The NV/H-alpha line ratio requires partial ion-electron equilibration (T_{e}/T_{p} approx 0.14 - 0.35). We find that the N/C abundance ratio in the gas crossing the reverse shock is significantly higher than that in the circumstellar ring, a result that may be attributed to chemical stratification in the outer envelope of the supernova progenitor. The N/C abundance ratio may have been stratified prior to the ring expulsion, or this result may indicate continued CNO processing in the progenitor subsequent to the expulsion of the circumstellar ring.
The far-ultraviolet (FUV) channel of the Cosmic Origins Spectrograph (COS) is designed to operate between 1130{AA} and 1850{AA}, limited at shorter wavelengths by the reflectivity of the MgF2 protected aluminum reflective surfaces on the Optical Tele
scope Assembly and on the COS FUV diffraction gratings. However, because the detector for the FUV channel is windowless, it was recognized early in the design phase that there was the possibility that COS would retain some sensitivity at shorter wavelengths due to the first surface reflection from the MgF2 coated optics. Preflight testing of the flight spare G140L grating revealed ~5% efficiency at 1066{AA}, and early on-orbit observations verified that the COS G140L/1230 mode was sensitive down to at least the Lyman limit with 10-20 cm^2 effective area between 912{AA} and 1070{AA}, and rising rapidly to over 1000 cm2 beyond 1150{AA}. Following this initial work we explored the possibility of using the G130M grating out of band to provide coverage down to 900{AA}. We present calibration results and ray trace simulations for these observing modes and explore additional configurations that have the potential to increase spectroscopic resolution, signal to noise, and observational efficiency below 1130{AA}.
