Beverly Newman, age 12, of Visalia, California, for her question:
What determines the speed of Halley's comet?
Right now, Halley's comet is dawdling along about halfway between the orbits of Neptune and Uranus. In 1986, it is expected to make a razzle dazzle U turn around the sun at about one million miles an hour. The Earth's orbital speed is very slightly faster in December, when it swings a little closer to the sun. The dramatic changes in the speed of the famous comet also are related to the distance it happens to be from the sun.
When the heavenly bodies were formed, they were set going in certain directions at certain speeds. Then the law of gravity took command to keep them in their places. Its pulling force reaches out from the center of every celestial object, getting weaker as it spans outward across the universe. Its strength depends upon mass, the amount of matter packed into the size or volume of a heavenly body.
More than 99 per cent of the material in our Solar System is packed into the massive sun, and its pulling power is enormous. The less massive planets and comets pull back with their weaker quotas of gravity. This cosmic tug of war helps to keep the Solar System in shape. But is not the whole story. If it were, the sun's mighty gravity would pull all its children down into its fiery furnace. This downward pull is counteracted by other bodies' spinning motions around their orbits. A planet's orbital speed is adjusted to match the sun's pull at a certain distance. Its orbit is almost round, and it speeds up only slightly when it comes slightly closer to the sun.
Halley's comet travels around a long, narrow orbit. Its distance from the sun varies from a few million miles to more than 3,000 million miles. Once every 76 years or so, it makes a close U turn around the sun. Then it departs to make a return loop, far out beyond the orbit of Neptune. The speed of the entire round trip is determined by the comet's distance from the sun.
In 1948, it dawdled at a snail's pace around the outer limit of its orbit. At this farthest distance the sun's gravity was weakest. Now it is well inside the orbit of Neptune, approaching the sun. During the next 14 years its speed will accelerate as the distance grows shorter and the sun's gravity becomes stronger. When it passes the orbit of Mars, it will be traveling very fast indeed. If it performs as it did in the past, the close brush around the sun will be made at about a million miles an hour. As it departs, its speed will decrease as the sun's gravity diminishes with distance.
There is always a chance that the famous comet may meet a mishap on its long orbit. If it chances to come close to Jupiter, the gravity of the big planet could change its speed and perhaps its direction. The fast spin might make it swerve clear out of the Solar System. Or the famous comet might be captured to join Jupiter's large family of moons. There its speed would be adjusted to its distance from the giant planet.