Hertford Astronomy Group's Alan Willison explains all you need to know about comets.
Comets are like cats: they have tails, and they do precisely what they want.
David H Levy – co-discoverer of comet Shoemaker-Levy in 1993.
Hatfield is famous for the Comet – two of them in fact.
First there was the DH88 Comet, which is the one shown outside the Comet Hotel, and then there was the world’s first jet passenger airliner, the DH106 Comet.
Why De Havilland chose the name Comet is something that I haven’t been able to uncover (answers on a postcard please) but it certainly implies something of a heavenly body which I can’t argue with for either aircraft.
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Celestial comets have been known about for centuries in many cultures. For example, the Greeks and Romans saw them as signs of something good or bad that had happened or was about to happen. That seems to cover all bases.
The Chinese similarly recorded the details of lots of comets and as far back as the Han Dynasty (206BC – 220 CE). They noted the paths, the appearances and disappearances of many of them. They associated the differences to different types of disasters, so comets don’t seem to have a very good record.
Even Hollywood has tuned into the doom associated with comets with movies such as Deep Impact (1998), Lifeforce (1985), Night of the Comet (1984), The Day of the Triffids (1963) and Greenland (2020) to mention but a few. Fortunately, Earth is saved each time.
But what exactly are comets and why do they hold such fascination?
Comets are cosmic objects composed of dust, ice, and various organic compounds. They originate from the outer regions of our solar system, often referred to as the Oort Cloud or the Kuiper Belt.
As these icy bodies journey closer to the Sun, they begin to heat up and release gas and dust particles, creating a spectacular glowing coma surrounding their nucleus.
What makes comets truly mesmerising is their unpredictable nature. Some comets only appear once in a lifetime, while others make regular appearances in our night sky. Their long tails that stretch across vast distances add an ethereal beauty to their presence.
In recent years, space missions like Rosetta and Stardust have brought us closer than ever before to these celestial wanderers.
These missions have provided invaluable data about comet composition, structure, and behaviour.
At the heart of every comet lies its nucleus, a solid and icy core made up of various volatile elements. This nucleus releases gas and dust as it heats up when approaching the Sun.
Surrounding the nucleus is a magnificent coma, a glowing cloud of gas and dust that envelops the comet. This diaphanous halo can span thousands of kilometres in diameter and is responsible for creating one of the most iconic features - the tails!
Comets boast not just one, but two distinct tails. The first is known as the dust tail - composed of tiny particles released from the nucleus. As sunlight hits these particles, they scatter light and create a mesmerising trail that stretches behind the comet's path.
The second tail is called the gas tail - formed by ionized gases escaping from the nucleus due to solar radiation pressure. Unlike its dusty counterpart, this tail appears bluish in colour and often points directly away from the Sun.
So, where do comets come from and how do they get here?
The Oort Cloud, a vast region located on the outskirts of our solar system, serves as the nursery for these enigmatic cosmic wanderers.
Comets are the remnants of the construction of our solar system. They are the bits that didn’t get formed into planets or asteroids.
Think of them as the pieces of rubble and detritus that you find on a new build.
Comets are believed to reside in this icy reservoir, which is home to countless frozen objects.
As they orbit around the Sun, gravitational forces from nearby stars and galactic tides can perturb their paths, propelling them towards the inner regions.
But what happens when a comet ventures closer to our neighbourhood?
Enter the Kuiper Belt - a region beyond Neptune that acts as a cosmic gateway for these celestial nomads.
Some comets get trapped within this gravitational dance between planets and continue their journey on elongated elliptical orbits.
As they approach our Sun, comets begin their spectacular transformation. The intense heat causes volatile substances within their icy cores to vaporize, creating magnificent tails that can stretch across vast distances in space.
In our search to learn about space there have been missions to visit comets and, indeed bring back samples to Earth.
One of the most recent was the Rosetta mission which placed a lander on Comet 67P/Churyumov-Gerasimenko in 2014. The lander discovered organic molecules on the surface of the comet.
Could comets be responsible for bringing the building blocks of life to our planet Earth?
That is a possibility as it is believed that the Earth would have been bombarded with millions of comets in its formative years. Also, as comets comprise a large amount of ice, they would have contributed much to our oceans.
Spotting and observing comets can be a truly awe-inspiring experience, and with the right techniques and resources, you can enhance your chances of success.
Comets can be naked eye objects but, like many astronomical phenomena they can be underwhelming.
We get so used to seeing fantastic pictures and can be disappointed to see a comet as a small fuzzy blob – if we can see it at all.
However, with a camera and a lot of know how, you can transform that blob into something quite remarkable.
Have a look at the two photos that have been taken by Steve Heliczer and Kevan Noble from their back gardens in Hertfordshire.
Kevan tells us about his picture: “Comet C/2022 E3 ZTF was discovered March 2, 2022. The comet name is derived from the comet naming system approved by the International Astronomical Union. C = comet that is not periodic and may take more than 200 years to pass again, 2022 E3 = Year, Month and 3rd object discovered in the same period, ZTF is the Zwicky Transient Facility that made the discovery.
"A comet is a small body made of ice and dust left over from the formation of the solar system. Comets have a frozen core (nucleus) surrounded by a huge, fuzzy cloud of gas and dust (coma), which appears as the comet approaches the Sun and heats up. A tail is formed when the solar wind pushes gas away from the coma.
"By the way, a comet usually has two tails. The dust tail is broad, white, and made of dust, as you might guess. The ion tail is bluish/green and always points directly away from the Sun.
"This image was taken 30 January 2023, two days before the comet's closest distance of 26.4 million miles from Earth. The comet last passed by Earth 50,000 years ago, according to NASA. So capturing this image is truly a once in a lifetime event."
One essential tool for comet hunting is a telescope. Understanding the proper telescope viewing techniques can make all the difference in your ability to detect these celestial visitors.
From adjusting your focus to finding the right magnification, mastering these skills will greatly enhance your ability to spot comets in the vastness of space.
However, what comets do leave behind is a lot of the debris that is discarded as they get closer to the Sun.
They shed lots of the dust and ice as they warm up and much of that dust remains in the path that the comet took. Sometimes the Earth passes through that debris field and the result is a meteor shower – shooting stars!
In addition to technical know-how, having access to observing schedules and charts is crucial when it comes to tracking comets.
These valuable resources provide information on upcoming comet appearances, their expected brightness, and their location in the night sky at specific times.
By staying informed about upcoming celestial events, you'll be able to plan your observations effectively and maximize your chances of witnessing these captivating phenomena.
Whether you're a seasoned stargazer or just starting out on your astronomical journey, following these comet hunting tips will undoubtedly elevate your experience as an amateur astronomer.
So grab your telescope, consult those observing schedules and charts, and get ready for an unforgettable adventure under the starry night sky.
- If you want to learn more about astronomy, have a look at the Hertford Astronomy Group website www.hertsastro.org.uk where you can see our programme for the forthcoming year.
- You may consider coming to one of our meetings at the University of Hertfordshire site in Hatfield (also held simultaneously on Zoom) and you will be made very welcome. The next meeting is on Wednesday, September 6 about SpaceX Starship.
Photo of the month
The Eagle Nebula (M16) – Kevan Noble
This stunning image is of the famous Eagle Nebula, a star-forming nebula with a young open star cluster located in Serpens. M16 lies near the borders with the constellations Sagittarius and Scutum.
The nebula is best known for the Pillars of Creation region, three large pillars of gas famously photographed by Hubble in 1995.
Also known as the Star Queen Nebula, M16 lies at a distance of 7,000 light years from Earth and has an apparent magnitude of 6.0, which would be just at the limit of naked eye observation under perfect seeing conditions.
The Eagle Nebula occupies an area 70 by 55 light years in size. The name Eagle comes from the nebula’s shape, which is said to resemble an eagle with outstretched wings.
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