by Justin Bray (University of Manchester, U.K.)

High-energy cosmic rays and neutrinos, when they hit the moon, should produce a pulse of radio waves that can be detected from earth. A series of experiments has searched for these pulses with a range of radio telescopes. None of them have detected a pulse, which excludes some astrophysical models of the high-energy neutrino flux, and the more sensitive experiments are now close to detecting the top end of the cosmic-ray spectrum. The photo on the left was taken during the first such experiment with the Parkes telescope, in 1995. (Image credit: Seth Shostak.) The figure on the right shows the positions of the beams on the moon during the most recent experiment, using the Parkes 21 cm multibeam receiver ( Bray et al. 2015 ).