  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 NASA's
MESSENGER spacecraft, which was not designed to make this measurement, we
estimate the neutron lifetime to be $780\pm60_\textrm{stat}\pm70_\textrm{syst}$
s, thereby demonstrating the viability of this new approach.
