Could the remains of a ‘dead’ comet still be in the solar system?

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The fate of a comet expected to pass near Earth remains a mystery five years after its dramatic breakup in the inner solar system – but some astronomers think part of it may still be there.

At the beginning of 2020, astronomers discovered the icy travelerknown as C/2019 Y4 ATLAS, and predicted that it could provide a night sky spectacle that would excite everyone during the COVID-19 pandemic lockdown: a comet visible to the naked eye as it passed within 23 million miles (37.5 million kilometers) of the sun, about a quarter of the distance that Earth orbits our star. But then the comet broken into dozens of piecesleaving would-be observers hanging – and leaving astronomers wondering if there might still be anything substantial left of our unfortunate icy visitor.

A team of astronomers led by Salvatore A. Cordova Quijano of Boston University hoped to answer that question by scanning the fall 2020 sky. And according to their recent article in The Astronomical Journalthere may be a half-kilometer-wide piece of the comet still in orbit, spinning outward toward the cold darkness of the outer solar system.

Remnants of the lost comet may still be out there

Cordova Quijano and co-authors Quanzhi Ye and Michael SP Kelley scanned the sky in August and October 2020, looking for any sign of the comet’s remains, to no avail. Observations with the Lowell Discovery Telescope (a 4.3 meter telescope in Arizona) and night images of the Zwicky Transitional Facility (which pans the northern sky every other night, looking for changing or short-lived objects like comets and supernovas) revealed nothing. But that doesn’t mean there’s nothing left of C/2019 Y4; it could just mean that what’s left is smaller than the smallest fragment these telescopes could have seen, which is about half a kilometer wide.

In addition to solving an intriguing astronomical mystery, this new study of C/2019 Y4 offers clues about what happens when comets break up in the intense heat near the sun, as well as an opportunity to study the millennial decline of an ancient family of comets (C/2019 Y4 could be a fragment of a larger comet that broke up thousands of years ago, according to a 2021 study).

“The uncertain fate of C/2019 Y4 raises an intriguing question,” the astronomers write in the study. “How many comets thought to be disrupted were actually completely disrupted, and could any of them have survived with a reduced, inactive nucleus?”

In the case of C/2019 Y4, the answer to this second question could be yes: it is possible that a comet fragment, less than half a kilometer wide, could still trace the long path of its larger relative around it. the sun.

A dramatic story

Comet C/2019 Y4 ATLAS was just a faint speck of light in the distance when astronomers with the Asteroid Last Warning System first spotted it in December 2019. started getting brighter very quickly in early 2020 as it flew toward the inner solar system, and astronomers enthusiastically predicted that it might be visible to the naked eye as it passes closest to Earth in late May.

And then, like all of us, the C/2019 Y4 suddenly collapsed at the end of April 2020.

Subsequently, astronomers used the Hubble Space Telescope and other observatories around the world to track a few dozen pieces of the broken cometin what appeared to be grouped into four main piles of icy debris. But one of those clusters later turned out to be a glitch in the data, and another lasted only a few days before dissipating completely. This left two more piles of debris, called Fragment A and Fragment B.

The last glimpse of C/2019 Y4’s icy debris by astronomers was on June 8, 2020, in images from NASA’s STEREO spacecraft, nine days after the comet’s closest approach to the sun. At the time, the comet’s core definitely seemed “completely disrupted,” Cordova Quijano and colleagues wrote. The question that remains is what happened to the nucleus after these observations.

Right now, Fragment A is likely nothing more than a slowly spreading cloud of gas and perhaps a few grains of dust. During the first three days after breakup, the pieces of the old comet core that made up Fragment A appeared to lose about 70% of their mass (because, again, ice sublimates and small pieces tend to sublimate faster than large ones).

Just before perihelion in late May 2020, the largest piece of Fragment B measured about 0.75 miles, or 1.2 kilometers wide. At the time of Cordova Quijano and colleagues’ observations in late August and mid-October 2020, it was clear that “fragment B had undergone further major decay,” but it remains unclear exactly to what extent. Cordova Quijano and her co-authors were unable to spot any trace of Fragment B in their Lowell or Zwicky data, which could mean that nothing remains — or that the remaining fragment is less than half a kilometer wide.

“We cannot conclude, from the available data, whether significant fragments still exist,” they wrote. “The observed decay events produced long-lasting fragments as small as 0.3 kilometers in diameter, which is below our detection limit.”

How to catch the next one

For astronomers, the spectacular breakup of C/2019 Y4 offered a rare opportunity to observe a comet disintegrate. So far, they have only been able to observe this dramatic phenomenon a handful of times: three confirmed and only four suspected. Of these four comets, astronomers have no real idea what happened after the breakup – such as whether any large chunks survived long enough to escape the hot inner solar system – and according to Cordova Quijano and colleagues, this is mainly due to a lack of follow-up observations to confirm the fate of the comets.

The researchers wrote that about two or three months after each comet passed “behind” the sun from our perspective and then reappeared, it should have been easier for telescopes to see. This would have been an ideal time to search for surviving fragments – or lack thereof. Such observations would have confirmed the disappearance of comets and also helped determine whether smaller pieces of their broken cores would continue to orbit the sun as mini-comets.

“For C/2019 Y4, a deep search just after solar conjunction (as immediately after the initial shallower search in early August 2020) could have conclusively determined the state of the remnant,” they wrote in their recent paper. “Similarly, dedicated in-depth research would be helpful in closing cases like those of the other three comets and provide insight into comet disruption dynamics.”

It’s a little too late to do so for C/20129 Y4, but the study gives a warning to astronomers to prepare for these types of observations the next time a comet collapses on its way through the inner solar system.