HALLEY’S Comet of 1985-6 witnessed by billions of people right around the world, including Pakistan, for several weeks continuously, until it at last vanished into the frigid space, only promising to reappear some 76 years later in the year 2061-62

Comets are occasional visitors towards the centre of the Solar System. After whizzing around the Sun, the comets (one and all), fast at first, gradually lose speed as they move away from it until they almost come to a dead stop and resume their journey once again into the frigid space of the outer space, usually in the vicinity of planet Jupiter.

But Jupiter is not some sort of a limit for them as a few of them go well past Jupiter, and even Saturn. Some even go beyond Uranus. Comet Halley goes just past the orbit of planet Neptune before swinging back to make a laborious journey back toward the mighty Sun, often passing within half a million miles or so away from the earth.

A comet resembles a huge cylinder suspended in the sky. In fact pulled by the gravity of the sun and often that of various planets that it encounters along its path (including the earth), it keeps moving as it approaches Sun. The surface of a comet is never very smooth but pockmarked by craters and other rough features heavily filled with various gases and chemicals, which become bloated and puffed up as these are gradually released by the gradually increasing heat of the ever-approaching Sun. Hence it develops a tail which begins to shine because of the sun’s light.

It often presents a beautiful spectacle in the skies, as was Halley’s Comet of 1985-6 witnessed by billions of people right around the world, including Pakistan. I, together with my companions, will never forget the hundreds upon hundreds of people took to various lakes around Karachi for both the naked-eye and telescopic viewing of a once-in-a-lifetime opportunity that presented itself for several weeks continuously, until it at last vanished into the frigid space, only promising to reappear some 76 years later in the year 2061-62.

In the concluding part on comets in the next issue, we shall dilate upon the history, the technical aspects and where they come from. Later on, a highly absorbing aspect of astronomy pertaining to our part of Solar System, namely, the eclipses, will grab our attention. So, see you next time!

Inside comets

Comets come from two places: The Kuiper Belt and the Oort Cloud

Imagine a place far, far away at the very edge of the Solar System. A place where millions of comets can be seen swishing around in every direction. These icy comets are orbiting the Sun in two different places, both of which are very distant. One place is called the Oort cloud, and the other is called the Kuiper Belt.

Structure

A comet has a nucleus (made of solid ice, gas and dust), a coma (water vapour, CO2, and other gases) and a tail (dust and ionised gases). The coma and tail form only when a comet is near the sun, whose heat activates the volatile materials of the nucleus.

A comet’s core is like a dirty snowball. The nucleus of a comet is composed of ice, dust and rock — debris left over from when the Solar System formed 4.6 billion years ago. Comet nuclei are among the darkest objects in the solar system, reflecting only four percent of light that hits them.

Tail

Many people think that a comet’s tail is always following behind it, but actually the coma, or tail, can either be behind the comet or in front of it. Which way the tail is pointing depends on where the Sun is. That’s right, the Sun’s heat and radiation produce a wind called the Solar Wind, as a comet gets close to the Sun, it begins to melt. The gas and dust that melt off are blown away from the Sun by the solar winds. So if a comet is travelling towards the Sun, then the tail will follow behind, but if the comet is travelling away from the Sun, the tail will be in front of the comet.

Halley’s Comet

Halley’s Comet streaks by Earth every 76 years, and astronomers studied it up close when it last appeared in 1996. The comet had a potato-shaped nucleus, some nine miles long, that contained roughly equal parts ice and dust. Halley’s comet is classified as short-period, meaning it completes its orbit in less than 200 years.