In March 2024, early risers should be able to catch comet C/2021 S3 (PanSTARRS) through a pair of binoculars as it makes its close approach to Earth.
Comets, often referred to as ‘dirty snowballs’, are icy bodies that travel through space, leaving a trail of dust and gas in their wake when they approach the Sun.
C/2021 S3, discovered by one of the PanSTARRS telescopes located on Mount Haleakala (Hawaii) in September 2021, is predicted to reach between magnitude 7 and 9. (The lower the magnitude, the brighter the comet appears.)
It won’t be visible to the naked eye, but with a decent pair of binoculars, we should be able to spot it. However, the ultimate brightness of a comet is somewhat hard to predict - comets are tricky and like to do their own thing.
But don’t worry, Comet C/2021 S3 poses no risk to the Earth and will pass us safely.
- Related: The Six Brightest Comets of 2024
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When can you see the PanSTARRS comet tonight?
The comet is making its closest approach to the Earth on 14 March 2024. It reached perihelion, the point in its orbit closest to the Sun, on Wednesday 14 February 2024 and has been visible from the US and UK from this date.
Towards the latter part of the month, from 18 March to around 29 March, the Moon will start to interfere as we approach its full state on 25 March. Starting on the other side of the sky, the Moon will gradually move closer to the comet towards the end of the month.
If you’re keen to spot the comet in the later part of March, particularly as it passes across the Coathanger asterism (more on how to find the coathanger below), then you’ll want to look up at around 3am GMT. Essentially, as soon as the comet is above the horizon. As we approach dawn the interfering Moon will scupper proceedings.
How to see the comet
Comet C/2021 S3 is travelling towards the northeast, roughly sweeping between the large constellation Ophiuchus above and the smaller Scutum and Aquila below. Both Ophiuchus and Aquilla straddle the celestial equator, while Scutum is just to the south.
Up until around 18 March, look towards the southeast in the early morning, before the sky starts to lighten. By this time, the comet will be visible above the horizon and the Moon relatively out of the way.
Towards the end of March, around 3am is when you'll see the best views. Also at this time, Comet C/2021 S3 will pass over the small asterism, the Coathanger Cluster. And yes, it looks exactly like the name! Here’s how to spot it:
- Locate the Summer Triangle: Start by identifying the Summer Triangle. This is an easily recognisable asterism and is formed by the bright stars Deneb in Cygnus, Vega in Lyra, and Altair in Aquila.
- Find the constellation Vulpecula: Once you've located the Summer Triangle, look for the constellation Vulpecula. It is located between Cygnus and Sagitta, and resembles a stretched-out-M. You can find it by extending an imaginary line from the star Altair in the Summer Triangle towards the north.
- Navigate to the Coathanger: Once you've found Vulpecula, look for the Coathanger asterism. It's quite distinctive and looks like a coathanger or upside-down question mark. The Coathanger is an asterism within Vulpecula and is easy to spot once you're in the right area.
If you’re struggling to find the coathanger asterism by star hopping, you can download a stargazing app to help. Here are all the best astronomy apps.
Here’s how YOU can help astronomers discover more comets
Astronomers from the University of Reading are asking for photographs of Comet C/2021 S3 as part of a citizen science project studying solar wind.
Photographs from amateur astronomers will help the researchers to improve forecasts of space weather, and the effects of solar wind on technology.
Sometimes referred to as ‘cosmic windsocks’, the tail of a comet can tell us a lot about the strength and direction of solar wind. For instance, if the tail detaches or wobbles, then we can infer an increase in activity.
Send your pictures, along with the date, time, and location of the image, to researcher Sarah Watson: s.r.watson@pgr.reading.ac.uk. The team are particularly interested in looking at observations of the broken tail.
The comet is not expected to brighten to naked-eye visibility, so you’ll need a small telescope or a camera with a big lens if you want to photograph it.
Why do comets brighten as they approach the Sun?
Comets become more active as they approach the Sun. The intense heat and solar radiation from the Sun can cause the icy nucleus to vaporise, releasing dust and gas in a sudden outgassing. This process forms a glowing coma (a cloud of gas and dust) around the nucleus and a bright tail that reflects sunlight.
This tail can stretch for millions of kilometres and is affected by a combination of processes. Solar wind, for example, which is composed of charged particles, can interact with these gases, creating an ion tail that points away from the Sun.
Then you’ve got solar radiation pressure, which pushes the dust particles away. This can form a separate dust tail that often lags behind the ion tail. The combination of sublimation (where solid ice turns directly into gas), ionisation, and radiation pressure all affect how the comet’s tail looks as it orbits the Sun.
And, the closer the comet is to the Sun, the more intense this activity becomes, making the comet appear brighter in the night sky.
There are currently 3,922 known comets (and fragments) in our Solar System.
But they can also dim…
However, this isn’t always the case. If the comet runs out of volatile materials, it may lose the ability to produce a bright coma and tail. Consequently, the comet appears dimmer as it approaches the Sun. Added to that, comets may develop a crust, preventing materials from escaping altogether.
For comet C/2021 S3, scientists have hypothesised that it may have already developed a mineral crust on its surface. If so, then this crust would be protecting the volatile-rich interior from the Sun, and the comet would have a much lower albedo (reflectiveness). In other words, it won’t be as bright as some of the other comets this year. If, however, the underlying substances continue to evaporate and create enough pressure to break through the crust, we may get a flare. Which would be fun.
When is the next comet?
Comet C/2021 S3 is one of a few bright comets that we’re keeping an eye on this year. The next is Comet 12P/Pons–Brooks (Comet Pons-Brooks) aka the ‘Devil Comet’ thanks to the distinctive ‘horns’ that started to flare up towards the end of 2023.
Easily visible with a telescope or binoculars, it may potentially become the first comet of 2024 with naked-eye visibility as it starts to brighten towards the end of March 2024.
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