Pamela Youngwith, age 10, of Milwaukee, Wisconsin, for her question:
What are comets made of?
A first class comet is a real razzle‑dazzle in the heavens. Its golden head outshines the brightest star. Its glowing tail streaks millions of miles over the sky. It challenges the sun for a place in the daylight and may be seen at high noon. Yet this showy performer is much more than a loose bundle of debris.
The only solid part of a comet is its head, or nucleus. This is believed to be composed of pebbles, gravel and dust. The bundle of debris is held loosely together by gravity. We are pretty sure there is plenty of space and maybe more gases between the fragments. For, in telescopic pictures, distant stars may be seen clear through the golden head of the comet.
A comet travels a long, narrow orbit. One end of the long, thin oval loops close around the sun. The other end reaches out maybe beyond a distant planet. We see it only as it comes to loop around the sunny end of its orbit. For only then does it burst into glory. Then the stony head glows bright with borrowed radiance from the sun. The head crowns itself with a huge, filmy halo and a long tail beams out like a searchlight.
The workaday comet, a mere bundle of debris, is inside all this temporary glory. A large, grandpa comet may be 10,000 miles across. On the scales, all 600 odd known comets could not equal one solid planet. It would take 100,000 grandpa comets to equal the weight of the earth.
For their size, comets are the lightest members of the Solar System. Their over‑all density, including the debris and the spaces between, is lighter than air at sea level. This means that the comet is mostly space populated with debris and rare gases. It has been compared to a flying aloud of gravel.
When it comes near the sun, the comet feels heat, radiation and pressure. What's more, it gathers speed. This agitates the flying debris. Fine particles are left behind as the nucleus speeds on Sunbeams exert pressure and push more fragments from the comet. The tail is built from minute fragments and gas particles.
The tail of one great comet is said to have been over 500 million miles long. It reached out to the orbit of Jupiter. In 1910, the tail of Halley's comet was said to be some 15 million miles long. It crossed the orbit of the earth and the world passed through part of the comet's tail. Was everyone clouded in dazzling star dust? Not at all. A few astronomers noted a very faint glow in the sky. Most people noticed nothing at all. This shows the fine, filmy nature of the comet's tail. We see it in the sky, only because it is washed with the splendor of the sun.
As it loops around the sun, the comet behaves like a duchess at court. She may bow low and turn around, but her train must be kept behind and her face towards the king. The comet faces the sun as it loops around. Its tail always points away from the sun.
When the comet leaves the sun, its tail is pointing ahead of it. But its glory is spent before it reaches the orbit of Mars. It slows down and continues its orbit as a dark, cold cloud of flying gravel.