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 \pm 60 _ { \textrm { stat } } \pm 70 _ { \textrm { syst } } s , thereby demonstrating the viability of this new approach .