Dwight Jauch, age 11, of San Francisco, California, for his question:
What is meant by lone and short period comets?
The study of comets includes a patient tracking job carried on continuously by observers around the world. The paths of hundreds of them have been charted in detail. But astronomers estimate that more than 100 billion other comets belong to the Solar System and we can only guess at the periods of time it takes them to orbit the sun.
A planet is a massive body that travels in a strict orbital path around the sun. The time it takes to complete each orbit may be called its year or period. A comet may be considered a sort of junior size planet because it too travels an orbit around the sun, usually in a fixed period of time. Our notions of the comets have made great strides in recent years but we still tend to think of them only as the long tailed spectacles that appear once in a while in our skies. We now know that these showy visitors are far outnumbered by some 100 billion comets that never come within our range. Most of them pursue their solar orbits in vast crowds near the orbit of Pluto and beyond.
Naturally, investigations are underway to clock the exact speeds of these outer comets and estimate their orbital periods. We know that they travel slowly around im¬mense orbits. They are the truly long period comets. Swamis of them almost certainly have yearly periods longer than Pluto, the outer planet that takes 248 earth years to complete each orbit around the sun.
We tend to be more interested in the comets that travel in long, thin orbits. At one point of its journey, such a comet loops close around the sun. During this arrival and departure it grows a long, brilliant tail and comes within our viewing range. The rest of its orbit is a dreary chug out into interplanetary space to some point where it turns around for its return trip to visit the sun. We clock its period from one appearance in our skies to the next.
More than two dozen well charted. comets chug out across the orbit of Jupiter, turn around and gradually gather speed and glory to make their swoops around the sun. Most of these fellows make their reappearances between five and nine years and we rate them as short period, or short term comets. One of them, discovered in 1819, does not go as far as Jupiter's orbit. It is known as Encke's comet and its short period is 3.3 years. Halley's famous comet travels beyond the orbit of Neptune and returns to pay its respects to the sun every 76 years or so. It is rated as a short period comet.
The paths of more than 500 visiting comets have been charted in detail. Their close U turns around the sun are very risky and a number of them have been observed to disintegrate or break apart. In 1882, a dazzling comet swerved at more than a million miles per hour within 300,040 miles of the sun's surface. It shattered into four pieces and these are expected to return at some period between the 25th and 28th centuries. Meantime the majority of comets travel less risky, long period orbits around the outer edges of the Solar System.