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Since its launch in 1990, the Hubble Space Telescope (HST) has served as aplatform with unique capabilities for remote observations of comets in thefar-ultraviolet region of the spectrum. Successive generations of imagers andspectrographs have seen large advances in sensitivity and spectral resolutionenabling observations of the diverse properties of a representative number ofcomets during the past 25 years. To date, four comets have been observed in thefar-ultraviolet by the Cosmic Origins Spectrograph (COS), the last spectrographto be installed in HST, in 2009: 103P/Hartley 2, C/2009 P1 (Garradd), C/2012 S1(ISON), and C/2014 Q2 (Lovejoy). COS has unprecedented sensitivity, but limitedspatial information in its 2.5 arcsec diameter circular aperture, and ourobjective was to determine the CO production rates from measurements of the COFourth Positive system in the spectral range of 1400 to 1700 A. In the twobrightest comets, nineteen bands of this system were clearly identified. Thewater production rates were derived from nearly concurrent observations of theOH (0,0) band at 3085 A by the Space Telescope Imaging Spectrograph (STIS). Thederived CO/H2O production rate ratio ranged from ~0.3% for Hartley 2 to ~22%for Garradd. In addition, strong partially resolved emission features due tomultiplets of S I, centered at 1429 A and 1479 A, and of C I at 1561 A and 1657A, were observed in all four comets. Weak emission from several lines of the H2Lyman band system, excited by solar Lyman-alpha and Lyman-beta pumpedfluorescence, were detected in comet Lovejoy.

Far-ultraviolet Spectroscopy of Recent Comets with the Cosmic Origins Spectrograph on the Hubble Space Telescope