Yvonne McCallister, age 12, of Omaha, Neb. for her question:
How does a comet start?
A comet becomes news when it shows its golden face in our skies. It may be a small pale comet, trailing just a wisp of golden hair. It may be a huge comet, brilliant enough to be seen at high noon and trailing its golden tresses halfway across the sky. Whether it is large or small, the comet lasts but a few weeks. However, its life does not begin or end with this short period of glory. It comes from a humdrum life and, when the display is over, goes back to a humdrum life.
This humdrum life may last a hundred or two hundred years. During this time it chugs around its orbit, which is a long, narrow oval path around the sun. The orbits of the planets are circles; more or less, with the sun in the center. For example, Earth's orbit is almost a circle so it stays more or less the same distance from the sun. Not so a comet. The sun is at one end of its long, narrow orbit. The rest of its path is far, far out in the cold.
The comet bursts into glory only when it travels the end of its orbit which loops around the sun. Only then does it light up with the sun's radiance. It speeds up and the tail or trailing hair is thought to be gases and fine dust glowing with sunshine.
The comet itself is just a loose bundle of stones, big and small. Large, showy comets visit our skies very rarely. Even so, the orbits of some of them have been charted and we know where they go when their razzle dazzle is over. One of the big ones is Halley's Comet, which loops around the sun end of its orbit once in 75 years. This dazzler appeared in the year 1910 and is expected back in 1985.
Meantime, it has chugged far, far out, almost to the edge of the Solar System. By 1915 it had passed the orbits of Mercury, Venus, Earth, Mars, ‑. Jupiter and Saturn.
Around 1930 it passed the orbit of Neptune, still traveling away from the sun, though much more slowly. In 1950 it was making the U‑turn around the far end of its orbit. It has now started its return journey and, in the next ten years, it will pass the orbit of Neptune on its long journey to pay another visit to the sun.
Right now Halley’s Comet is a cold, dark bundle of stones. So it will remain until it has passed the orbit of Mars. Then it will begin to reflect the glory of the sun. Its head glows with reflected light and maybe the pressure from the sun causes particles to stream out behind it. The tail streams always away from the sun, even when the comet loops around. As it leaves the sun the glowing tresses stream ahead of it.