When cosmic ray particles interact with atoms in our atmosphere, they create huge air showers. These showers may contain millions or even billions of particles. The original cosmic ray spreads its energy out to all of these particles, thus each particle no longer has ultra-high energy and they are not dangerous or harmful. Thanks to our Earth's atmosphere, cosmic rays pose no threat to us at the Earth's surface.

The story is different, however, in outer space. Because there is no atmosphere in space, the cosmic rays keep their original energy until they collide with something. If this happens to be an astronaut, there can be a problem. With constant bombardment of the astronauts body by cosmic rays, these particles could eventually cause permanent damage. Currently, this is one of the obstacles to long term manned-space flight - how to shield astronauts from cosmic rays. If we ever hope to send people to Mars or live permanently in space or on the moon, we will have to solve this problem.