ASK A RESEARCHER: About every 76 years, the famous Halley’s Comet lights up the sky. With it, we found out how comets move and what they consist of.
Currently, you can spot Comet C/2022 E3 ZTF.
Another and more famous comet is Halley’s comet. Some of us may see it twice in our lifetime.
But what do we really know about the iconic comet? How far away is it? And how old is it? That’s what Jon, one of videnskab.dk’s readers, has asked. He is a prison teacher and his students have many questions about comets.
We asked astrophysicist Michael Linden-Vørnle from DTU Space.
Revealing Comet’s Frozen Core
The last time Halley’s Comet flew between the Sun and the Earth and could be seen in the sky was in 1986. Then Michael Linden-Vørnle caught the sight.
– When Halley’s Comet approached in 1986, several space probes were sent out towards it. They were supposed to show us for the very first time what a comet nucleus looks like, says Linden-Vørnle.
– It was ground-breaking and confirmed the idea you had about comets. Namely that they consist of ice, rock and dust, with streams of gas and dust from the core, he explains.
Halley’s Comet will reach its farthest point this year
Comets are said to be deep-frozen remnants from the time the solar system formed, about 4.6 billion years ago. And that makes Halley’s Comet ancient.
It takes about 76 years to travel all the way around the Sun and is likely to reach its furthest point from the Sun by the end of this year. Which is a distance of around 5.3 billion kilometres.
– Comets are active objects, and their orbits are not completely stable. This also applies to Halley’s comet, says the astrophysicist.
– When comets come close to the sun, they are heated, and gas and dust seep out of them. Because it flows out like jets, it can act as an engine that can affect the comet’s orbit. But we know when we will see Halley’s comet in the sky again, he points out.
Appears in prehistory
Halley’s Comet was the first comet found to be periodic. The English astronomer Edmond Halley discovered it in 1705, when he calculated with the help of Newton’s laws that the comet would appear every 76 years.
If we look even further back in history, Halley’s Comet appears several times.
According to historical records from China, Halley’s Comet was observed around 240 BC. And again in the year 1066.
– You can see Halley’s Comet woven into the famous Bayeux Tapestry from William the Conqueror. He wanted to conquer England, and when Halley’s Comet appeared in the sky, he saw it as a sign that the invasion would succeed. Comets have usually been interpreted as something ominous, says Linden-Vørnle.
Countdown
It will be a long time until we see Halley’s return. That won’t happen for another 38 years. In 2061 it will pass between the Sun and the Earth.
Meanwhile, other comets will whiz across the sky. Both some reminiscent of Halley’s Comet and some that have never appeared in human history before.
– If you use binoculars, you can regularly see comets here from the northern hemisphere. But ten years can pass between the arrival of large, clear comets that we can see with the naked eye, explains Linden-Vørnle.
What we see clearly are usually comets that come every few thousand years. They are the so-called long-period comets, such as the famous Hale-Bopp from 1997 and C/2022 E3 ZTF, which appeared at the turn of January/February this year. C/2022 E3 ZTF has not visited since Neanderthals existed around 50,000 years ago.
– The long-period comets come straight out of the Oort cloud, which are repositories of ice and rock cores that lie around the solar system. When comets come from there, they contain a lot of ice that can evaporate and create a cloud around the comet nucleus itself, says the astrophysicist.
The Sun eats Halley’s Comet
Actually, comets are pitch black and only appear as brilliant objects when they approach the Sun and are heated.
– The sun’s influence pushes away the material of gas and dust that seeps out of the comet, and that is what we see as the comet’s tail. So that every time the comet passes through the sun in the inner solar system, it loses part of its mass, explains Linden-Vørnle.
– Halley’s comet loses a few meters each time. But it will probably take a very long time before it ends up as either a dead comet or disintegrates, he states.
Comets can kill us
When comets die, either by being dissolved by the sun or floating around with no more material to leave the nucleus, it has no consequence for us on Earth or the rest of the solar system, says Linden-Vørnle.
But the living can, in the worst case, become the downfall of humanity.
– We are a little worried about comets because there is a possibility that they could hit the earth, just like asteroids. But comets come without warning and can be hard to spot because they are so black. Therefore, we have to deal with them, concludes Linden-Vørnle.
© Videnskab.dk. Translated by Lars Nygaard for forskning.no. Read the original story on videnskab.dk here.
