Comets, Lonely Space Travellers.
About 25 years ago I was on a fishing boat travelling from North East Scotland to Norway. It was April 1997. The mate of the boat, who like myself had an interest in astronomy called me in the middle of the night.
It was calm with no clouds, unusual for the North Sea. At the top of the bridge, we lay down and looked up at the night sky, and there it was, Comet Hale-Bopp.
We had seen the comet on previous nights but this was different. The comet and the stars were so clear, there were more celestial objects visible than we had ever seen before, even the odd shooting star (meteor) streaked across the crystal clear sky.
The reason all the heavenly bodies were so clear and easy to see was the lack of light, therefore, no light pollution, just complete darkness.
Photo by Jeremy Bishop on Unsplash
This must have been similar to the clear skies our ancestors had viewed long before we had towns and cities illuminated with artificial light. Of course, we have come to depend on that artificial light.
Although Hale Bopp and its long flowing tail appeared bright in the night sky, it was actually 197 million kilometres from the Sun at its closest approach.
Comets are cosmic snowballs, leftover from the formation of the solar system roughly 4.6 billion years ago. They are made of ice, rocks and frozen gases, as they approach the Sun they warm up and release water vapour, rocks and gases that stream away from the nucleus, this is what we call a comet’s tail, the tail can stretch away from the Sun for millions of kilometres.
Comets have highly eccentric elliptical orbits, (oval-shaped) paths. Comets close to the Sun can leave a trail of debris behind them, which can lead to meteor showers as they enter the Earth’s atmosphere and burn, leaving streaks of light behind them.
Short-period comets take 200 years or less to make one orbit around the Sun, long-period comets can take hundreds of years, but some can take millions of years to complete one journey around our local star.
It is thought that short-period comets originate from a band of icy objects known as the Kuiper belt which lies farther out than Neptune’s orbit. Among the eight planets in the solar system, Neptune is the farthest from the Sun.
The Oort cloud is a very distant spherical region containing trillions of icy bodies that circle the Sun, long-period comets are thought to originate from that area of space. The start of the Oort cloud could be 100,000 times the distance between the Sun and the Earth. So it is not surprising that some long-period comets can take millions of years to complete one orbit around the Sun.
The distance between the Sun and the Earth is called an astronomical unit.
In 1986 my two young children and I watched the short-period comet Halley as it made its closest approach to the Sun on its long elliptical journey through space.
There is a chance that my children will see it again. In 2023 in an area in space farther away than Neptune it will turn around and start on that long trek back towards the Sun, arriving here in 2061. Depending on when you were born, some people will see Halley’s comet twice in their lifetime.
Photo by Justin W on Unsplash
Comet Halley was named after English astronomer Edmond Halley.
Comets are usually named after their discoverer, comet Shoemaker-Levy 9 got its name from Eugene and Carolyn Shoemaker and David Levy.
No one will ever see that comet again, Shoemaker-Levy 9 collided with Jupiter in July 1994, providing the first direct observation of a comet impacting a solar system object.
Astronomers have found what could be the largest comet ever seen, it is named Bernardinelli-Bernstein. It has an estimated diameter of between 100 to 200 kilometres and will make its closest approach to the Sun in 2031, but there is no need to be alarmed, the closest it will get to the Earth will be roughly 1.6 billion kilometres.
Throughout recorded history, people have looked up at the night sky, and witnessed comets, meteor showers, eclipses and a few supernovae. It could have been fascinating for some and terrifying for others. But now we have a far better understanding of our solar system and the Universe as a whole.
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