Science 1. Cosmic Rays

2. Cosmic Rays and

3. CRaTER and

4. More CRaTER


CRaTER detects some of the smallest yet also some of the most energetic particles in the universe. Studying these particles, called cosmic rays, is important for astronauts and scientists. This section explains why, by describing what cosmic rays are, how they affect astronauts, and how they affect the Moon.

Cosmic Rays

  classroom to proton cartoon

This cartoon zooms in on a classroom to a hair, to atoms, and to a proton.  Protons and atoms don’t actually look like spheres, but the cartoon gives you the idea of how small they are.  (NASA/Andrew Jordan)

Cosmic rays are a source of radiation in outer space and are dangerous to astronauts. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) is an instrument on the Lunar Reconnaissance Orbiter (LRO). CRaTER detects cosmic rays.

Cosmic rays are subatomic particles. This means they are even smaller than atoms--too small to see! Most cosmic rays are protons, which are particles usually found in the nucleus of an atom. Some cosmic rays are electrons, and some are combinations of protons and neutrons.

Even though we call them cosmic rays, they are not light rays. Instead, they are particles. Unfortunately, they received their confusing name before scientists knew what they are.

Cosmic rays travel at almost the speed of light. By the time you finish reading this sentence, a cosmic ray could have made two round trips from here to the Moon!

Most cosmic rays come from exploding stars in our galaxy, the Milky Way. These exploding stars are called supernovae. The cosmic rays they create are galactic cosmic rays. Galactic means they are related to stars in the galaxy.

Other cosmic rays come from our Sun. The Sun is a star that sometimes has massive eruptions. These eruptions can create solar energetic particles. When these explosions occur, we can detect many more solar energetic particles than galactic cosmic rays.

Ray, the Cosmic Ray

“We cosmic rays are faster than Superman!  Unfortunately, we’re also a little dangerous. 
Go to 2. Cosmic Rays and Humans to learn why.”