This image of C/2024 S1 (ATLAS) was taken on October 11, 2024, by the 0.43-meter X09 telescope in Chile. Notice how stretched out the core appears. Credit: Exoplanetaryscience/WikiMedia Commons
When comet C/2024 S1 (ATLAS) was first discovered in late September, it was almost immediately identified as a member of the Kreutz family of sungrazing comets. But it was highly unusual for a Kreutz comet: Virtually all of these are discovered only in their final hours or days of existence, as they dive toward the Sun and are spotted by the SOHO solar observation satellite. But this one was found more than a month before perihelion, its closest solar approach, and was spotted using ground-based telescopes.
Such early discoveries of Kreutz comets occur only once every decade or so, and they have often turned out to be extraordinarily bright comets. This includes the brightest comet of all in the last century, Comet Ikeya-Seki (C/1965 S1) in 1965. So, there were high hopes that this could prove to be another dazzling spectacle. Predictions based on its distance and apparent brightness once discovered suggested that near or just after perihelion, it could become brighter than Venus, possibly even observable in daylight. Newspaper headlines spoke of a possible spectacular “Halloween comet” if it were to flare up after its perihelion on October 28.
No luck. It quickly became apparent to experienced observers that this comet was not following the expected course of brightening as it approached the Sun. Some observers noted signs as early as early October of an elongation of the comet’s nucleus, a possible indication that it was already fragmenting. And, as you know from the lack of further titles, ATLAS was not a spectacular sight in the late October sky.
Initial enthusiasm
After it was found by the ATLAS observatory in Hawaii on September 27, “I was the first to track it after the discovery,” says amateur astronomer Sam Deen Astronomy. An active amateur who, along with a couple of friends, operates a remotely operated 17-inch telescope in Chile, Deen is particularly interested in scouring archival datasets for comets, asteroids and other transient phenomena.
“It’s especially challenging,” he says, “when the comet was discovered just a couple of days earlier and you have to predict what it will do in the next month, when it could potentially become brighter than the magnitude observed with the naked eye.” , or simply vanish altogether. To do this, he says, he tries to extract any data that may not be obvious but that could help make those predictions.
When he made his first observations, there was still much uncertainty about the comet’s orbit. “It wasn’t even possible to tell whether it was parabolic or not,” and therefore whether this was his first visit to the inner solar system. But it was already clear that it was on a trajectory to pass by the Sun. So, he tried to insert elements into an orbit search program based on the assumption that it was indeed a Kreutz Sun-grazer. “I set the eccentricity and perihelion to be like a Kreutz Sun-grazer, and the rest of the elements fell perfectly into place,” he says, making the identification likely.
Of the more than 5,000 known comets, more than two-thirds belong to the Kreutz family, all of which are believed to be remnants of a much larger comet that shattered a few thousand years ago. “But most of these we discover only a few hours before they are at perihelion,” Deen says. “So, finding one off the ground is a really impressive feat, so I immediately got excited.”
The last time this happened was with Comet Lovejoy in 2011, which became the fifth brightest comet seen in the last 100 years. So there was indeed reason to be optimistic.
But what went wrong?
From fabulous to sparkling
At the time of discovery, comet ATLAS was farther from the Sun than Lovejoy had been, and even further from Earth, yet it was brighter than Lovejoy. This made it seem likely that it would work similarly. The problem was that everyone assumed that the brightness they saw was typical of the object, but in the end this was not the case. In fact, the comet had apparently just had a major explosion, perhaps triggered by the start of its disintegration. So instead of continuing to brighten, in those first few days it actually faded a little bit.
Karl Battams, a computational scientist at the U.S. Naval Observatory whose work includes tracking all Sun-grazing Kreutz comets that enter SOHO’s field of view, says that typically when these comets are first seen so far from the Sun, by the time they arrive within two or three days of closest approach they “increase the brightness by several orders of magnitude.” This suggested, on paper, that “[ATLAS] it could reach visual magnitudes of –4, –6, even –8. You can then choose your number to fit your narrative. But he was a bit skeptical of these predictions from the start, he says.
Having personally studied thousands of sun-grazing comets over the past 20 years, “I’ve seen some strange behavior in more than a handful of them,” Battams says. And early reports seemed worrisome, he says, because of the wide variations in brightness estimates. He feared that perhaps “the reason it appeared out of nowhere was because it was already undergoing some sort of explosion.” And then, “we all saw what happened in the last few days, when it faded to the point where it normally would have taken off in terms of brightness.”
Ultimately, the comet never got brighter than about magnitude 2, and even then it was too close to the Sun for anything but satellite observations. “At that point, actually, it would be generous to even call it a comet,” Battams says. “Even though the object formerly known as Comet ATLAS was so bright, in reality what we were seeing was the remnants of the comet, the cloud of debris, completely vaporizing. At that point it wasn’t a core, it was literally just a cloud of debris.”
Still valuable
Daniel Green, who heads the Central Bureau for Astronomical Telegrams, the official clearinghouse for all comet discoveries, says that despite its disappointing performance, comet ATLAS “was still unusual and scientifically a very interesting comet for those of us that follow the comets.” Because it was discovered long before its disappearance, a much larger database of observations has been assembled that could help understand the entire class of comets.
And since, on average, one of these exceptional early-detected Kreutz comets appears about every decade or so, and many of them have become among the brightest comets ever seen, Green says, “one of these days, hopefully I’ll have a piece of really big [of the original Kreutz parent body] like in 1965, and put on a truly spectacular show.
