As with the Hubble constant, we're still bickering over the lifetime of a neutron. A free or weakly-bound neutron will decay into a proton, in the "beta" process, thus losing mass. Neutrons, as neutral particles, get measured when bound in a nucleus - where they (usually) don't decay. Not so easy to weigh them, or even detect them, when they are flying around. So how long does it take them to beta-decay into protons, i.e. ionic hydrogen?
Durham University may have a solution: measure the outflow from Mercury and Venus. There's a solar wind, from them. The best Venus and Mercury mission so far was MESSENGER 2007-8. MESSENGER wasn't equipped for this particular experiment, but its old data do prove the viability of such an experiment.
They recommend another inner-planet expedition to sort that out. Although - I do wonder, why not Earth. Magnetic field interference?
WONDER NO MORE 10/17/21: Measuring from the Moon.
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