Pebble, a smooth, rounded stone usually less than 2 inches (64 mm) in diameter. (A larger stone of this type is called a cobble, or cobblestone.) Pebbles are fragments of any kind of rock that have become worn by rubbing together in rivers or on beaches. A loose accumulation of pebbles is called gravel. Conglomerate is rock composed of pebbles cemented together.
Every day, astronomers unravel a little more of the universe's inner workings, but the jury is still out on 95 percent of its contents.
Eros is 21 miles long and 8 miles thick -- a mini planet! Explore the mission that landed on this asteroid after orbiting it for a year!