Quazen

Halley’s comet requires 75 years to travel its elongated orbit. But for most of the time it is in the outer reaches of the solar system – a cold, dark object invisible to earthbound astronomers. But when Halley approaches the inner planets, the warmth of the sun drives off a cloud of gas and dust thousands of miles wide. The pressure of sunlight and the solar wind – a stream of electrically charged particles constantly flowing out from the sun – sweep the gas and dust out a great tail, millions of miles long.