Comets have been observed and recorded by many cultures since ancient times. They have often been interpreted as harbingers of doom. In the past they were thought to foretell of events as diverse as earthquakes, floods, hail storms, heat waves, poor harvests, epidemics, war and high prices!
But by 1700 most scholars had concluded that these events happened whether there was a comet or not.
Visitors from deep space
Comets are cosmic snowballs of frozen gas, rock and dust that orbit the Sun.
They usually have highly eccentric elliptical orbits. The time they take to orbit the sun is very variable. Some take only a few years, whereas others may have orbits lasting as long as several million years. Short period comets originate in the Kuiper belt which lies beyond the orbit of Neptune. Long period comets are thought to originate in the Oort cloud which extends from outside the Kuiper belt to halfway to our nearest star!
Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. When comets get closer to the Sun they warm up and begin to release gas and dust. This produces a visible atmosphere or coma. A comet’s coma may be up to fifteen times the Earth’s diameter. Some comets can get bright enough to be seen from Earth without the aid of a telescope.
Comet Tails
As a comet approaches the sun, some of the gas and dust in the coma is blown away by solar radiation. This force exerted by the Sun is large enough to create an enormous “tail” from the dust and gas.
The streams of dust and gas form distinct tails, each pointing in slightly different directions. The dust is pushed out of the coma by radiation pressure. It then continues to follow the comet’s orbit around the sun often forming a curved tail. On the other hand the gas in the coma is ionised by the solar radiation. Because these charged particles are more strongly affected by the solar wind they form a separate tail that always points directly away from the Sun. Comet tails can stretch as far as the distance from the Sun to the Earth.
Comet 46P / Wirtanen
Comet 46P/Wirtanen is a short-period comet with an orbital period of 5.4 years. It is relatively small in size, with an estimated diameter of just 1.2 kilometers.
However this one came really close.
On December 16th it passed by at a distance of only 7 million miles from Earth. By then it was neatly positioned between the Pleiades and Hyades star clusters – a really favourable place for us to see it high in the sky after sunset.
This icy space rock was luminous enough to be just visible with the naked eye. It was the brightest comet that we are likely to encounter for at least another five years.
We hope you didn’t miss it!
Meteor Showers
Meteor showers are caused when the Earth passes through the trail of debris left by a passing comet. This debris is moving at extremely high speeds and usually burns up on entering the atmosphere to create shooting stars. Most meteors are smaller than a grain of sand, so almost all of them disintegrate and never hit the Earth’s surface.
Because meteor shower particles are all travelling on parallel paths, and at the same speed, they appear to come from a single point in the sky. That point is called the radiant. Meteor showers are almost always named after the constellation in which the radiant is situated.
