Though we are relatively safe from cosmic rays, they represent a long term threat to astronauts since they can easily pass right through a spacecraft. When the Apollo crews returned from the moon with dents like these in their helmets, laboratory tests were carried out, using artificial cosmic rays, to design adequate shielding for future trips. The photograph is of a silicone rubber replica (magnified 800 times) of the tracks etched in one of the test helmets.