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The CO 2  production rate has been derived in comets using Cameron‐band (  a  3 Π →  X  1 Σ ) emission of CO molecules, assuming that photodissociative excitation of CO 2  is the main production mechanism of CO in the   a  3 Π  metastable state. We have developed a model for the production and loss of CO(  a  3 Π ), which has been applied to comet 103P/Hartley 2: the target of the  EPOXI  mission. Our model calculations show that photoelectron impact excitation of CO and dissociative excitation of CO 2  can together contribute about 60–90 per cent to Cameron‐band emission. The modelled brightness of (0–0) Cameron‐band emission on comet Hartley 2 is consistent with  Hubble Space Telescope  observations for 3–5 per cent CO 2  (depending on the model input solar flux) and 0.5 per cent CO relative to water, where the photoelectron impact contribution is about 50–75 per cent. We suggest that estimation of CO 2  abundances on comets using Cameron‐band emission may be reconsidered. We predict a height‐integrated column brightness of the Cameron band of ∼1300 Rayleigh during the  EPOXI  mission encounter period.

Model for Cameron‐band emission in comets: a case for the EPOXI mission target comet 103P/Hartley 2