A meteoroid is a bit of debris, usually planetary, hurtling through the solar system. If it's bigger than a boulder, than it's an asteroid. If any part of a meteroid makes it to Earth intact rather than burning up as it barrels through Earth's atmosphere, then it's a meteorite. A meteoroid's visible path through the atmosphere is a meteor. An astronaut struck by a meteoroid is a goner. A meteroid the size of a tomato seed can pierce a space suit.