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"We are just beginning to try and figure out quantitatively how all this might smooth a surface," Dr. Thomas said in the May 17, 2013 New Scientist. "For the smaller craters, it's like if you're filling a bucket, eventually your bucket gets full, but if you keep pouring cups of water into the bucket, you can't tell how many cups of water beyond full you've gone. Looking at the larger craters at the subsurface might give us insight, because that 'bucket' isn't full yet," Dr. Soderblom added. A moon is defined as a natural satellite that orbits a larger body--such as a planet--that, in turn, orbits a star. The moon is kept in its position both by the gravity of the object that it circles, as well as by its own gravity. Some planets are orbited by moons; some are not. Some dwarf planets--such as Pluto--possess moons. In fact, one of Pluto's moons, named Charon, is almost half the size of Pluto itself, and some planetary scientists think that Charon is really a chunk of Pluto that was torn off in a disastrous collision with another object very long ago. In addition, some asteroids are also known to be orbited by very small moons.