Ancient humans were afraid of the sight of comets . Many cultures believed that comets were messages sent by gods, and people thought these messages were warning signs for bad behavior. But today science and technology give us a better understanding of comets. Last week, comet Ison ended its million-year-long journey as it flew straight toward the sun, melting most of the comet. It was hoped that comet Ison would survive this trip around the sun. Because once it got near Earth, it would have produced a great show of lights for us. In ancient times, the coming of comet Ison might have scared people. However, now we can track a comet's journey in our solar system. We can also study what they're made of. There is even a project to launch a spacecraft onto a comet to study them further. Although from the sky they look like great giant fireballs, comets are a collection of gas, ice, dust and small rocks. As a comet goes into our solar system, the sun's solar winds blow some of the dust and gas away, giving the comet a tail. The average size of comets is about 10 kilometers in diameter . But amazingly, their tails can be 150 kilometers long! Comet Ison's diameter was about 5 kilometers. Today, people don't fear comets as much as they did before. Western countries have even developed and old tradition. When people see a comet or meteoroid  shooting across the sky, they call it a shooting star, People who get the chance to see one get to make a wish. It is a tradition that lives on today. In ancient times, people thought comets were   _  .
warning signs sent by gods lucky signs for good behavior great shows of lights flying suns