Artist depiction of the MESSENGER spacecraft in orbit around Mercury.

The MESSENGER spacecraft designed to investigate Mercury (pictured; artist’s impression) has also provided clues about the lifetime of the neutron. Credit: NASA/JHU/APL

Particle physics

The vanishing-neutron mystery might be cracked by a robot in outer space

A mission to explore the planets also furnishes insights into the small stuff.

A spacecraft that whizzed past Venus and Mercury more than a decade ago could help to solve one of physics’ most enduring puzzles: the lifetime of the neutron.

Neutrons are long-lived when combined with other particles in the nuclei of stable atoms, but when they are free-floating they decay within minutes. Laboratory experiments have disagreed on how long it takes free neutrons to die. One leading method yields an estimate of around 880 seconds; the other, around 888 seconds.

Jack Wilson at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, and his colleagues looked off-Earth for an answer. In 2007 and 2008, NASA’s MESSENGER spacecraft measured neutrons in space as it flew past Venus and Mercury. The particles form when cosmic rays hit the planets’ surfaces, knocking neutrons out of atomic nuclei.

The scientists compared the number of neutrons MESSENGER observed with what might be expected to be formed from such cosmic-ray collisions. The observed abundance can be attained if neutrons live around 780 seconds (plus or minus 70 seconds), the researchers say.