Most meteorioids begin as small fragments of asteroids that have either shed material or lost mass due to impacts. Other meteoroids can be shed by comets or ejected from planets and moons that have been hit hard enough to accelerate debris into space.
Meteoroids are generally smaller than asteroids, and they can be difficult to detect. They are also given to eccentric orbits at a variety of velocities. For these reasons, meteoroids pose a serious hazard to space travel, as impacts can be sudden, unpredictable and destructive. Near-Earth space is littered with many naturally occurring meteoroids as well as some artificial objects known as "space junk."