"Not all comets are created equal." That's the latest theory from NASA. Scientists believe that one day, they will be able to examine the dust and debris within the tail of a comet. When they do, it will be like going back in time, because this latest theory states that while many comets were formed during the "Big Bang," some may have formed later during the evolution of the solar nebula. It's believed that the comets will show their age by the kind of dust they hang around with.