Comment on ``Strangelets as Cosmic Rays beyond the   Greisen-Zatsepin-Cuzmin Cutoff''
  Madsen and Larsen proposed (astro-ph/0211597) that strangelets - stable lumps of strange quark matter (SQM) - are candidates for the highest energy cosmic rays. They point out that the expected properties of strangelets make them easier to accelerate and less prone to energy losses than protons or ions. The purpose of this Comment is to point out that if ultra-high-energy cosmic rays (UHECRs) are indeed strangelets, their flux would guarantee that all neutron stars must be strange quark (SQ) stars, while no ``conventional'' (i.e., composed of nucleons and other baryons) neutron stars can coexist. This seems highly unlikely in view of various observed phenomena in neutron stars. 
Shmuel Balberg (Hebrew University, Jerusalem, Israel)