  Since its launch in 1990, the Hubble Space Telescope (HST) has served as a
platform with unique capabilities for remote observations of comets in the
far-ultraviolet region of the spectrum. Successive generations of imagers and
spectrographs have seen large advances in sensitivity and spectral resolution
enabling observations of the diverse properties of a representative number of
comets during the past 25 years. To date, four comets have been observed in the
far-ultraviolet by the Cosmic Origins Spectrograph (COS), the last spectrograph
to 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 limited
spatial information in its 2.5 arcsec diameter circular aperture, and our
objective was to determine the CO production rates from measurements of the CO
Fourth Positive system in the spectral range of 1400 to 1700 A. In the two
brightest comets, nineteen bands of this system were clearly identified. The
water production rates were derived from nearly concurrent observations of the
OH (0,0) band at 3085 A by the Space Telescope Imaging Spectrograph (STIS). The
derived CO/H2O production rate ratio ranged from ~0.3% for Hartley 2 to ~22%
for Garradd. In addition, strong partially resolved emission features due to
multiplets of S I, centered at 1429 A and 1479 A, and of C I at 1561 A and 1657
A, were observed in all four comets. Weak emission from several lines of the H2
Lyman band system, excited by solar Lyman-alpha and Lyman-beta pumped
fluorescence, were detected in comet Lovejoy.
