Asteroids also have received considerable study by the scientific community. Not as much is known about comets. As a rule, comets are considerably larger than asteroids. The composition of a comet is a mixture of nebulous, gasses, ice, dust and space debris. One scientist called the composition of a comet as similar to a dirty snowball because the composition is so diverse and changeable. Scientists have gained significant knowledge about the composition of asteroids and separated them into classes including class S which comes of the part of the belt that is closest to Mars, classes C, D and V which are classified by composition and a class called Centaurs whose flight patterns take them closer to Jupiter and Uranus. Children have enough trouble imagining the size of earth itself, much less something on such a grand scope as outer space. But when it comes to astronomy, we do better when we fall into deeper and deeper levels of awe at what we see up there in the night sky. Some amazing facts about what the children are looking at can add to the goose bumps they are already having as they gaze eyes skyward. In it, he began to suggest that astronomy should be considered a separate science from astrology. It was quite a revolutionary book because it also was the first scientific document to suggest that the earth was not the center of the universe and that astronomy should be focused strictly on the observation and recording of events in the cosmos. Other than the Olympic movement, international cooperation to achieve great strides for human kind in space seems to go forward without interruption even when the nations cooperating in those projects are virtually at war back on the surface of the earth. It is a strange thing to watch as Russian, American and other astronauts work together like brothers on space missions even as their home nations are busily pointing missiles at each other back at home. All of a sudden space is not just three dimensions but the dimension of time becomes exportable and the twisting and maybe even travel through time seems almost possible. The frontier of space is as much a journey of the mind as it is of distance. When Steven Hawking showed us the mysteries of black holes, all of a sudden, time and space could collapse and be twisted and changed in those intergalactic pressure cookers.