How long does a comet last, and why do they disintegrate?

By Will Gater

Published: Friday, 27 January 2023 at 12:00 am

Back in the early months of 2020, a comet – formally catalogued as C/2019 Y4 (ATLAS) – was beginning to brighten as it drew ever closer to the Sun.

The headlines had all the usual excitable language you’d expect, while enthusiastic social media posts spoke of possible naked-eye wonders to come.

Out in the depths of space, though, the icy wanderer had other ideas.

As it raced towards the Sun, astronomers started to notice ominous changes in its appearance; rather than brightening steadily as it neared perihelion in May this year – when it would be at the closest point to the Sun in its orbit – the comet’s glow was faltering.

""
Comet ATLAS photographed by José Chambó, Mayhill, New Mexico, USA, 26 March 2020

Soon, close-up images by astrophotographers showed the head of ATLAS – ordinarily where a solid, icy nucleus would hide among a shining cloud of gas and dust – was starting to elongate.

The comet was falling to pieces. By 20 April, professional astronomers had pointed the orbiting eyes of the Hubble Space Telescope to the ensuing chaos, revealing fragments of the comet drifting through the darkness.

""
Hubble Space Telescope images showing the break up of Comet C/2019 Y4 (ATLAS) in April 2020. Credit: NASA, ESA, D. Jewitt (UCLA), Q. Ye (University of Maryland)

If amateur astronomers in the Northern Hemisphere felt any disappointment at what might have been, that was soon tempered by the naked-eye spectacle of Comet NEOWISE in the following months; while it graced the northern horizon with its magnificent dust tail, the fizzling away of ATLAS seemed quickly forgotten by most.

Even in its death, however, ATLAS had a story to tell scientists – one not just about the fickle nature of comets, but also about the deeper history of our planetary neighbourhood.

""
NEOWISE mosaic, by Petr Horálek and Tomáš Slovinský, Slovakia, 9 July (right) to 3 August (left) 2020.

Why do comets brighten in the first place?

Comets brighten because, as they venture close to the Sun on their long, elliptical orbits, the warmth of our star causes ices to sublimate (change straight from a solid to a gas) from their frosty surfaces.

As this process unfolds, a cloud of material – known as a ‘coma’ – forms around the nucleus.

Some comets also develop tails, giving them the striking appearance these icy objects are so well-known for.

""
José J. Chambó captured this image of Comet C/2022 E3 (ZTF) and its long tail on 24 December 2022 from Mayhill, New Mexico, USA.

How visible and how bright a comet gets in our night sky depends on several things.

Such as how big its nucleus is and how close it gets to Earth and the Sun when it sweeps through the inner Solar System, explains Professor Laura Woodney, a planetary scientist and comet expert based at the California State University, San Bernardino.

The age of the surface of the nucleus also plays a role.

“We think that the more perihelion passages a comet has around the Sun, the more processed the surface becomes due to sublimation of ice, given that the larger rocky particles either fall back to the surface or are never lifted in
the first place, forming an insulating crust over time.

“Thus short-period comets with their frequent solar passages are less spectacular,” she says.

""
The European Rosetta mission revealed plumes of dust and gas erupting from the surface of Comet 67P/Churyumov-Gerasimenko. Credit: ESA

For certain comets making their first journey into the inner Solar System – what astronomers call a ‘dynamically new’ comet – the odds can be somewhat against them getting bright too.

“Statistically speaking, first-time visitors that approach the Sun within 2 AU [Astronomical Units, where 1 AU is the distance between Earth and the Sun] are more likely to fizzle out. But again, outliers are not uncommon. So we don’t really know,” says Dr Ye Quanzhi, an asteroid and comet researcher at the University of Maryland. 

Anyone who remembers the saga of Comet ISON – a sungrazing comet that many hoped would become a spectacular sight in late 2013, but actually flopped – will recall the rollercoaster of emotions that comes with monitoring a dynamically new comet as it approaches perihelion.

""
A Hubble Space Telescope image of Comet ISON (C/2012 S1) captured during 10 April 2013, when the comet was 386 million miles from the Sun. Credit: NASA, ESA, J.-Y. Li (Planetary Science Institute), and the Hubble Comet ISON Imaging Science Team

ATLAS, though, was neither a short-period comet nor dynamically new; its crumbling highlights just how hard it is, still, to anticipate the behaviour of these icy travellers.

Indeed, it’s just one of several comets that astronomers have witnessed breaking up over the years, including Comet 73P/Schwassmann-Wachmann in 2006 and more recently Comet 332P/Ikeya-Murakami in 2015.

""
A Hubble Space Telescope image showing Comet 332P/Ikeya-Murakami disintegrating as it approaches the Sun, late January 2016. Credit: NASA, ESA, and D. Jewitt (UCLA)

“[Disintegration] happens regularly, both to long- and short-period comets and we aren’t good at predicting why yet,” says Woodney.

“That’s one of the interesting questions comet scientists are currently working on understanding right now.”

Might there have been clues from ATLAS’s past, though, that pointed to its fate? 

The comet caught the eye of Ye Quanzhi right from the moment it was found, back in December 2019.

“The preliminary orbit closely resembled another comet that appeared almost 180 years ago, C/1844 Y1,” he says.

“The resemblance is a bit too great to be explained by pure coincidence, and it was pretty clear that the orbital period of ATLAS is much, much longer than 170 years.

“So this raised the interesting possibility that we are seeing a pair of comets that share a common progenitor and are likely to be the end-product of a prehistoric break-up event.”

Analysing Comet ATLAS with Hubble

""
Hubble captures the break-up of comet C/2019 Y4 (ATLAS). Credit NASA, ESA, D. Jewitt (UCLA), Q. Ye (University of Maryland)

To explore ATLAS further, Ye and his colleagues requested time on the Hubble Space Telescope to examine the comet.

“Our motivation back then was primarily to look for fragments produced during ATLAS’s last visit, 6,000 years ago,” he says.

The team’s proposal was sent to Hubble managers in January; but between then and when the orbiting telescope would actually turn its optics toward the comet, in late April, ATLAS underwent its spectacular fragmentation.

“We secretly hoped, but wouldn’t have really guessed that ATLAS would break apart a bit before our scheduled observation – that was a really pleasant surprise,” says Ye.

“Our observation happened overnight and I woke up quite early the next day. It felt like opening a gift box when I was downloading the data.”

Another group of researchers had also been allocated observing time as well, and in total three orbits of the space telescope would eventually be dedicated to scrutinising the spectacle occurring out in space.