16770

We request 4 orbits of HST STIS and COS observations of comet 67P-Churyumov-Gerasimenko (hereafter 67P) during its close approach to Earth to obtain large-scale coma measurements for direct comparison with the localized Rosetta spacecraft results. We will measure the bulk production rates (mol-s) of the main volatiles (H2O comma CO2 comma and CO) comma search for extended sources of volatiles in the coma comma and investigate the sulphur chemistry of 67P. HST.s sensitivity and spectroscopic abilities in the Far Ultraviolet make it uniquely capable to simultaneously measure the Far-Ultraviolet atomic and molecular emission of chemically coupled or otherwise interdependent species in the 100-15 comma 000 km of the coma (i.e. CS comma S comma and S2 CO comma CO2+ and OH). Observations that can connect the results for the inner coma with that of fragment species more typically observed at scales of 1000s of km (A.Hearn 2017) are essential to interpret large surveys of chemical abundances of comets. The details of how comets formed still need to be worked out by the modellers comma but we need to make sure that the inputs for those models have had observational biases removed. The observation should be executed between October 14 and December 20 comma 2021 comma which brackets the close approach of the comet to Earth on November 2 at 0.42 au. We plan to dedicate two consecutive orbits for exposures with STIS G230L exposures comma and two consecutive orbits for exposures with COS G160M. The STIS exposures and COS exposures should be executed within one week. For the STIS observations comma we request that the slit orientation is aligned within 30 deg perpendicular to the sun-comet direction comma which is allowed by the HST roll angle range during our observing window.