Comets have fascinated and terrified humans since history began. Some people have thought they are portents of doom – fiery messengers from the gods. Others thought that comets could signal the winning or losing of wars, the births and deaths of kings, and even the “end of the world”.
With modern science we know comets do not do any of those things. They are not “messengers”. They are wonderfully memorable astronomical objects that on rare occasions become bright enough that we can see their graceful, long tails arched across the night sky.
There are two comets around this month: the romantically named C/2023 A3 (Tsuchinshan–ATLAS), which did not quite get bright enough to see by eye, and the upcoming C/2024 S1 (ATLAS). This one might even be brighter than the planet Venus that lights up our early evening western sky this month in Port Alfred. Then again, there might be literally nothing to see.
Let me explain.
Comets are sometimes called “dirty snowballs”. They are typically a few km in size and are made of rock and ice. The famous Halley’s Comet is oblong and 8km long, so roughly the size of Mt Everest. And there are billions of potential comets. Billions.
They are leftovers from the formation of the solar system 4.6 billion years ago having condensed in the outer reaches where it is cold and where the giant planets, Jupiter, Saturn, Uranus and Neptune orbit the Sun. The gravity of those giant planets flung some of the comets into the Sun, and others far out from the Sun where they now orbit in what is known as the Oort Cloud, named after the famous Dutch astronomer, Jan Oort, who figured out that is where they hide.
The Oort cloud stretches out more than a light year, where the gravity of the Sun is very weak. Over millions of years, occasionally another star passes close enough to disturb the orbits of the far comets. Some of them are pulled off into the emptiness of the galaxy, no longer orbiting the Sun. But some are slowed down, and they fall into the inner solar system.
This typically takes 50,000 years. But time passes, and when the comet – the dirty snowball – comes close to the Sun, the heat boils the snow into vapour that streams off carrying dust and rocks with it. Then, the constant wind from Sun and the pressure from its light blow the dust and water vapour out into long tails that shine brightly by reflected sunlight.
The tail of the comet is not “fiery”; it is cold. But it reflects sunlight just as clouds beautifully do in a lovely orange sunset. The Sun blows the tails away from it, so a comet’s tail does not stream “behind” it as it orbits the Sun. The tail points away from the Sun, so that when the comet is leaving the Sun, it goes tail first!
Then for 50,000 more years the comet usually goes back out to the Oort cloud where it came from. But not always. If the comet passes close enough to one of the giant planets, such as Jupiter, it can be captured in a small orbit about the Sun. That happened to Halley’s Comet maybe 16,000 years ago. Halley now orbits the Sun in 76 years, and each time is passes close, more comet is boiled away. Eventually, there will be no more Halley’s Comet.
The space mission SOHO that has been observing the Sun for decades now has discovered over 5000 new comets. Many of them have dived right into the Sun, or passed so close they were totally vapourised.
That may happen to C/2024 S1, which will pass closest to the Sun on 28 October when it may even outshine Venus. Late October may be the best time to see it – if the Sun doesn’t totally boil it away. We cannot predict this.
Also, the Sun can let off a Coronal Mass Ejection – a big blast – anytime. When those hit the Earth, we get good aurorae. If one hits a comet, it can brighten spectacularly. This is unpredictable.
So stay tuned. If C/2024 S1 gets bright enough to see with your eye, it will be in the news.
- This article was first published in Talk of the Town, October 17, 2024. The newspaper serving the communities of Ndlambe and the Sunshine Coast, with a weekly wrap of Makhanda news, is available at stores from early on Thursdays.
