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. ^ { \prime \prime } 5 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 Ã .
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 Ã Â by the Space Telescope Imaging Spectrograph ( STIS ) .
The derived CO/H _ { 2 } OÂ production rate ratio ranged from \sim 0.3 % for Hartley 2 to \sim 22 % for Garradd .
In addition , strong partially resolved emission features due to multiplets of \ion S1 , centered at 1429 Ã Â and 1479 Ã , and of \ion C1 at 1561 Ã Â and 1657 Ã , were observed in all four comets .
Weak emission from several lines of the H _ { 2 } Â Lyman band system , excited by solar Ly \alpha and Ly \beta pumped fluorescence , were detected in comet Lovejoy .