We establish the feasibility of measuring the neutron lifetime via an alternative, space-based class of methods, which use neutrons generated by galactic cosmic ray spallation of planets surfaces and atmospheres. Free neutrons decay via the weak interaction with a mean lifetime of around 880 s. This lifetime constrains the unitarity of the CKM matrix and is a key parameter for studies of Big-Bang nucleosynthesis. However, current laboratory measurements, using two independent approaches, differ by over 4$sigma$. Using data acquired in 2007 and 2008 during flybys of Venus and Mercury by NASAs MESSENGER spacecraft, which was not designed to make this measurement, we estimate the neutron lifetime to be $780pm60_textrm{stat}pm70_textrm{syst}$ s, thereby demonstrating the viability of this new approach.
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