Comets are unpredictable celestial visitors and there's a lot of uncertainty surrounding forecasted magnitude. However, there are six comets in 2024 that we're keeping a close eye on - here's what we know so far.
This year is shaping up to be an excellent year for bright comets, and there are a few noteworthy candidates that astronomers are keeping their eyes on. Beyond being pretty exciting to spot for the backyard stargazer, comets offer valuable insights into the mysteries of our Solar System's formation and evolution. They also make excellent subjects for astrophotography.
And there’s a very exciting prospect later on in the Autumn; a comet that will be potentially brighter than Neowise, a celestial object which dazzled skywatchers in 2020. Some estimates forecast that Comet C/2023 A3 could even become one of the brightest objects in the night sky!
However, comets are finickity visitors. They’re shrouded in uncertainty, and a lot can happen between now and then that might affect a comet’s brightness. A comet's brightness is measured in magnitude – the lower the magnitude, the brighter the object. A full Moon has a magnitude of -12.6, while the Sun has a blistering magnitude of -26.74.
Here are the comets to look out for in 2024:
Perihelion: 25 December 2023
Closest approach to Earth: 29 January 2024
Magnitude at close approach: 9.5
Visibility: Potentially through medium-size binoculars
Orbital period: 6.4 years
Although perihelion has been and gone, towards the end of January Tsuchinshan 1 might still be visible through medium binoculars as it makes its closest approach to Earth.
Perihelion: 25 January 2024
Best seen: February
Magnitude at perihelion: 8.5
Visibility: Contested, but potentially through medium-size binoculars.
Orbital period: 7.6 years
Comet 144P/Kushida made its closest approach to Earth back on 12 December 2023 when it reached magnitude 15. As the comet approaches perihelion, it will brighten to magnitude 8.5, and although visibility projections vary, it may be visible through medium-sized binoculars.
Perihelion: 14 February 2024
Magnitude at perihelion: 7.1
Closest approach to Earth: 14 March
Magnitude at closest approach: 7.1
Visibility: Likely visible after perihelion to northern hemisphere viewers with a telescope. Potentially visible with medium-size binoculars.
When C/2021 S3 (PANSTARRS) reaches perihelion on 14 February, it may be visible through a telescope in the dawn sky for a short while, but it will be at a relatively low altitude of 23 degrees above the southeastern horizon. As the month progresses, it will start to become visible for longer and will climb in altitude, reaching 36 degrees by 4 March.
Perihelion: 21 April 2024
Magnitude at perihelion: 4.2
Best seen: Late March, after sunset
Closest approach to Earth: 2 June 2024
Magnitude at closest approach to Earth: 6.3
Orbital period: 71 years
Visibility: Potentially naked-eye visibility before perihelion for northern hemisphere viewers, and after perihelion for southern hemisphere viewers.
Comet Pons-Brooks has the potential to become one of the brightest comets of the year. At magnitude 4.2, it’s just on the threshold for naked-eye visibility, but you should be able to see it with a pair of small binoculars in June, when it’s closest to the Earth.
Perihelion: 30 June 2024
Magnitude at perihelion: 7.5
Closest approach to Earth: 20 July 2024
Magnitude at closest approach: 7.6
Orbital period: 69 years
Visibility: Potentially visible with small binoculars near perihelion in June and July.
13P/Olbers is a Halley-type comet that was last visible in 1956, when it reached an apparent magnitude of 6.5, with a tail reaching one degree in length.
Perihelion: 27 September
Magnitude at perihelion: 0.2
Closest approach to Earth: 12 October 2024
Magnitude at closest approach: -0.9
Visibility: Weather permitting, Comet C/2023 A3 will have naked-eye visibility with the potential to become very bright. It will be best seen in the southern hemisphere before perihelion, and the northern hemisphere after perihelion.
Hold on to your hats, because Comet C/2023 A3 in the autumn has the potential to outshine the much-photographed Comet Neowise from 2020. While comet Neowise reached 0.5 in magnitude, Comet C/2023 A3 has the potential to reach -0.9. If so, in terms of brightness that would bring it close to Sirius (magnitude -1.44), the brightest star in the night sky.
Discovered in 2023, Comet C/2023 A3 hails from the Oort cloud, believed to be a vast and distant ‘spherical shell’ full of icy planetesimals and comet-like objects that surround the Sun, planets, and Kuiper Belt Objects. However, the Oort cloud is so far out, that Voyager 1 – travelling around one million miles per day – won’t even reach it for another 300 years. And even then, it will take a cool 30,000 years to get to the other side.
We’ll learn more about Comet C/2023 A3 as it comes closer, so for now – watch this space. No doubt a suitably catchy name will be coined nearer the time.
Understanding magnitude is useful for comets as it quantifies their predicted brightness. It’s a measure of brightness that astronomers use to describe objects in the sky, such as stars, planets and comets, where smaller numbers denote brighter objects.
Apparent magnitude is a logarithmic scale. This means that each whole number increase represents a decrease in brightness by a factor of approximately 2.5. That means something with a magnitude of 1 is approximately 2.5 times brighter than an object with a magnitude of 2, and so on.
To put it into context, Venus, which is the brightest planet we can see, is around -4 magnitude. Back in 1997, Comet Hale-Bopp reached a rather tasty magnitude 0, thanks to its exceptionally large size, while southern sky observers were treated to a dazzling -5.5 magnitude from comet McNaught in 2007.
Generally, comets with an apparent magnitude of +4 or lower are visible to the naked eye without the aid of telescopes or binoculars. As comets are more diffuse than stars, which have a visibility threshold of around +6 in dark skies, they are more difficult to see.
The lower the magnitude, the brighter the comet appears. Some comets can even reach negative magnitudes, making them exceptionally bright and easily visible (spoiler: there's a potential candidate later this year!). However, factors like light pollution, atmospheric conditions, and the comet's proximity to the Sun can also influence its visibility.
Side note: When you see the term ‘bright comet’ being mentioned, this generally means a comet that can be seen through binoculars or a telescope.
It can be difficult to forecast the brightness of comets as these icy visitors are unpredictable, and their visibility depends on multiple different factors. For example, the comet's composition, degree of activity (outgassing and dust production), and the nature of its interaction with the Sun, can all affect how bright a comet will appear to us.
Comets undergo changes in brightness as they approach (or move away from) the Sun, and the extent of these variations depends on the unique characteristics of each comet. These complexities make it tricky to precisely predict how bright a comet will appear at any given time.
In addition, their behaviour can be unpredictable; they can flare in a sudden outburst that releases dust and gas, shed material, break apart, or even disintegrate entirely.
New non-periodic comets, often outshine their periodic cousins, but these are even more unpredictable and can arrive from deep space at any time. Just look at Comet Hale-Bopp in 1997 or The Great Comet of 1811 (C/1811 F1). The latter was visible with the naked eye for a whopping 260 days and famously made it into Tolstoy’s War and Peace, or most recently the stage musical Natasha, Pierre, & the Great Comet of 1812.