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We present a measurement of the lifetime of ground state atomic carbon,C(^3P), against ionization processes in interplanetary space and compare it tothe lifetime expected from the dominant physical processes likely to occur inthis medium. Our measurement is based on analysis of a far ultraviolet (FUV)image of comet C/2004 Q2 (Machholz) recorded by the Galaxy Evolution Explorer(GALEX) on 2005 March 1. The bright CI 1561 A and 1657 A multiplets dominatethe GALEX FUV band. We used the image to create high S/N radial profiles thatextended beyond one million km from the comet nucleus. Our measurements yieldeda total carbon lifetime of 7.1 -- 9.6 x 10^5 s (scaled to 1 AU). Which comparesfavorably to calculations assuming solar photoionization, solar wind protonchange exchange and solar wind electron impact ionization are the dominantprocesses occurring in this medium and that comet Machholz was embedded in theslow solar wind. The shape of the CI profiles inside 3x10^5 km suggests thateither the CO lifetime is shorter than previously thought and/or ashorter-lived carbon-bearing parent molecule, such as CH_4 is providing themajority of the carbon in this region of the coma of comet Machholz.

GALEX FUV Observations of Comet C/2004 Q2 (Machholz): The Ionization Lifetime of Carbon