Death in sunlight: Halloween comet fell short of our expectations

On October 28, comet C/2024 S1 (ATLAS) passed the perihelion of its orbit. It fell short of expectations and turned out to be much dimmer than astronomers had hoped.

Hubble telescope image of the disintegrated comet C/2019 Y4 (ATLAS). About what C/2024 S1 (ATLAS) looks like right now. Source: NASA, ESA, D. Jewitt (UCLA), Q. Ye (University of Maryland)

We’ve covered C/2024 S1 (ATLAS) several times before. It is a near-solar comet in the Kreutz family discovered on September 27, 2024, a month before passing perihelion, which is quite a lot by the standards of such objects. Not surprisingly, the find has caught the attention of astronomy enthusiasts around the world. Calculations have been made showing that at perihelion, C/2024 S1 (ATLAS) may reach -5 to -7 stellar magnitude. This is greater than the luminosity of Venus.

But it was all only a theory.  Whether a comet could become bright or not depended on the strength of its nucleus and its ability to withstand powerful solar radiation. And the Kreutz family comets have never been particularly strong. In addition, the perihelion orbit of C/2024 S1 (ATLAS) passed at a distance of only half a million kilometers from the solar surface, which initially made its survival rather doubtful.

However, even with all these factors considered, the behavior of C/2024 S1 (ATLAS) surprised many. Its brightness increased less than expected. Then, images were published that showed characteristic signs of the comet’s nucleus collapse. Astronomers then “wrote off” C/2024 S1 (ATLAS), concluding its early demise.

Then C/2024 S1 (ATLAS) surprised everyone. Between October 16 and 19, the comet produced a series of flares. Its brightness has increased almost 40 times to +8.5 magnitude. The comet also got a tail.

But, as is now clear, these flares were the agony of C/2024 S1 (ATLAS). The comet’s nucleus disintegrated, releasing gas that temporarily increased its brightness. When the comet came into the field of view of the SOHO observatory on October 26, it turned out to be much dimmer than astronomers had hoped. Its maximum brilliance was only +4.5 star magnitude.

Comet C/2024 S1 (ATLAS) in the SOHO observatory image. Source: ESA/NASA SOHO C3, Karl Battams

In fact, C/2024 S1 (ATLAS) is no longer incorrectly identified as a single object. At the moment, the comet is a “junk pile” — a cluster of debris that is traveling in a common orbit. Given how close they flew to the sun, the probability that we will see some remains of C/2024 S1 (ATLAS) again is close to zero. Unfortunately, it can now be definitively stated that the Halloween Comet fell short of expectations.