LRO-LAMP Observations of the Preperihelion Coma of Comet C/2013 A1 (Siding Spring)

Abstract The Lyman-Alpha Mapping Project (LAMP) UV spectrograph on board the Lunar Reconnaissance Orbiter observed comet C/2013 A1 (Siding Spring) from lunar orbit at closest approach. On 2014 September 5, Siding Spring came within ∼0.89 au of the Earth–Moon system and provided an opportunity for LAMP to contribute to the broader campaign of observations. The comet coma was imaged during two consecutive orbits, approximately 2 hr apart. Coma emissions of atomic oxygen were resolved within LAMP observations at distances up to ∼1.4 × 106 km from the comet nucleus. We report an atomic production rate for the observed oxygen of 9.42 ± 0.22 × 1027 s−1. We additionally place upper limits on the column densities of candidate species including noble gases and primary transitions. Leveraging this, we place upper limits on the production of hydrogen and carbon of <3.59 × 1026 s−1 and <8.41 × 1025 s−1, respectively. Additionally, we derive OH and H2O production rates and estimate an upper limit on the production of CO. The H2O (8.17 ± 2.64 × 1027 s−1) and OH (7.53 ± 2.244 × 1027 s−1) production rates are found to be in general agreement with previous studies when production rates are derived utilizing oxygen observations, branching ratios, and empirical formulations. Similarly, the upper limit on the production of CO (<1.33 × 1028 s−1) is found to be in good agreement with previous studies (within ∼10%) when we utilize the upper limit on CO Fourth Positive group emissions.