Meteoroids are so small that they are no bigger than a grain of sand. Scientist have decided that meteoroids once were produced by comets that orbited the sun. Some of the satellites we have recovered from space over many years have had little indentions that were caused by little meteoroid collisions. When meteoroids enter Earth's atmosphere they become visible to the naked eye and are classified as meteors. They are visible because they fly so fast and when they hit the surface of the atmosphere, they glow blue or even white. Meteors almost always burn up in the atmosphere, and are only seen for a few seconds. Sometimes a meteor will hit earth and then are termed meteorites.