Comet Tsuchinshan–ATLAS comes to Durham?

By Isabella Castel-Branco

‘Comet of the century’, ‘Last seen by the Neanderthals’, … many such claims concerning comet Tsuchinshan–ATLAS are currently flooding news platforms as it has reappeared, visible in the twilight sky. Though it is not the only one in recent times, it has grasped widespread attention due to its promising characteristics. 

In everyday language, terms like comets, asteroids, and meteors are often used interchangeably, as if they were all simply space rocks. However, each category is clearly defined, and understanding these distinctions helps explain why comets are particularly sought after. Asteroids are space rocks ranging from a dozen meters to hundreds of kilometres in diameter. These irregularly shaped objects orbit the Sun, primarily in the asteroid belts between Mars and Jupiter and extending beyond Neptune. In contrast, meteoroids are smaller, pebble-sized space rocks—often fragments of asteroids. When these meteoroids burn up in the Earth’s atmosphere, they become meteors, and any surviving fragments that reach the Earth’s surface are called meteorites. These objects are categorized based on size and position. Comets differ in composition; their nuclei are typically made of ice and dust, giving them a distinctive elongated, blurry appearance.

Comets usually have two different tails: the longer, infamous tail, which gave it its name from the Greek komētēs (long-haired star), consists of evaporating ice and dust. The second, ion tail, is caused by powerful solar winds and points directly away from the Sun as electrically charged particles are ripped off the surface and ejected into space. The ion tail is bright and colourful, usually blue. Both tails span out over millions of kilometres. As comets journey through the Solar System, they create spectacular shows on Earth; the debris they leave behind is encountered by Earth during its orbit, resulting in annual meteor showers when we pass through the comet’s remnants. Comets are particularly interesting objects to study as they are remnants of the formation of the Solar System and hence could give us further clues on its past evolution.

As first suggested by Edmond Halley, comets can be periodic: that is, they can return every set number of years after completing a long orbit around the Sun. It is theorized that the Solar System is surrounded by the Oort cloud, a vast reservoir of small rocky objects, potentially housing billions of comets. This would explain how we regularly have such visitors — the most recent and memorable one for our generation was Neowise in 2020, but we have subsequently had a few, less noticeable ones. If Tsuchinshan–ATLAS is effectively a periodic comet, it has been estimated to have last passed by Earth 80,000 years ago when Neanderthals were still alive.

It has been estimated to have last passed by Earth 80,000 years ago when Neanderthals were still alive

As it approaches the Sun, Tsuchinshan–ATLAS is visible to the naked eye. Monitored instrumentally since its discovery in January 2023, it has only two short observation windows: just before and after its closest approach to the Sun (the perihelion), when it shines brightest, illuminated by sunlight reflecting off its icy surface. The first, from 27 September to 2 October, is visible east just before sunrise, and currently, from 12 to 30 October, visible west just after sunset, according to the Royal Astronomy Society. So far, it has been dubbed ‘comet of the year’ due to its peak apparent magnitude of -4. The more negative a magnitude is, the brighter the object is: the least dim objects we can see are of apparent magnitude +6, while the Sun is approximately of apparent magnitude -26. Nonetheless, being so close to the Sun sets observation limits. While it is brighter when closer, it rises and sets with the day, making it difficult to spot before it vanishes below the horizon.

The comet is not yet easily visible in Durham, as it is too close to the horizon when the Sun sets and, therefore, hidden by hills and trees. However, each day, it will reach a higher altitude (but also consequently grow dimmer). With an updated sky map, a clear view of the western horizon, and a pair of binoculars, amateur astronomers should find it a rewarding object to hunt down. Wishing clear skies to all those who try!

Image by James Peirce Via Flickr