Comet 323P/SOHO was discovered in 1999, and has an orbit of fire and ice: In its deeply elliptical 4-year orbit around the Sun, it exits almost to the orbit of Jupiter, but then falls back just a hair more than 5 million kilometers from the hot surface of the Sun. If you were standing on the comet at that time, you would have to raise both hands to block out the Sun, and it would feel like you were looking at a blast furnace.
Needless to say, this will take something away from you. In the case of the comet, it was a few pieces as big as houses.
323P/SOHO was discovered in images taken by the NASA/ESA SOHO solar mission as the comet approached the star. Curiously, it is not a very active comet and only appears to show activity – becoming brighter due to the ejection of dust as it heats up – when very close to the Sun. This indicates that unlike most comets, it does not contain much ice. Otherwise, it would start sporting a tail at least as it crossed Mars’ orbit on its way.
Given that 323P comes so close but managed to survive several previous passes when it swung around the Sun again in 2021, astronomers were ready for it. Just weeks before perihelion – the closest approach to the Sun – they observed it with the massive Subaru Telescope and found nothing really abnormal. At that time, it was only a little closer to the Sun than Earth and showed no signs of activity, as before.
Perihelion was January 17, 2021, and astronomers had to wait for it to put the Sun behind it before they could point more ‘scopes’ at it. Then, in February and March, they used several large telescopes, including CFHT, Gemini, Lowell Discovery Telescope and Hubble to take a look. [link to paper].
And it looked different.
Photo: Subaru Telescope/CFHT/Man-To Hui/David Tholen
It showed a long, narrow tail, much like what you see after the disintegration of a comet; comets are very fragile and when they have lost enough ice to hold them together they can collapse and become a cloud of rubble and debris surrounded by an expanding cloud of dust.
Hubble images showed two pieces had broken off, both about 40 meters in diameter judging by their brightness. Notice that the solid part of the comet itself, called the nucleus, is just under 200 meters in diameter, so these were substantial pieces.
Given the heat of the comet as it passes in front of the Sun, it’s unlikely there will be any water ice left even deep inside, and it hasn’t been for a while. So why did it collapse?
Taking a quick series of observations provided a clue: the comet is getting brighter and darker on a very short timescale, probably only half an hour. It’s probably due to its rotation. If it’s elongated, as it spins we see its wide side, then its narrow end, so it gets brighter and darker. The amount that it’s changed in brightness indicates that it’s about 1.4 times longer in one axis than the other, so it’s kind of potato-shaped, like so many of the little bodies in the solar system.
But it’s weird. A half-hour spin would make it the fastest-spinning comet known, and would have to have unusually high tensile strength to prevent flying apart due to centrifugal force. It is possible that the forces of sunlight caused it to spin so quickly, which is called the YORP effect.
This rotation may have been the reason it broke in 2021. The extreme temperature changes in just a few months as it approaches the Sun and retreats again would likely lead to huge cracks in the rock body due to thermal expansion and contraction. When it approached the Sun, two large cracks must have formed, and the rapid spin pulled the trigger on a pretty decent cosmic rockslide. They estimate that between 0.1 and 10% of the comet’s mass was stripped away.
Comet C/2019 Y4 (ATLAS) observed by Hubble on April 20, 2020 (top) and three days later (bottom). In the previous image, clusters of fragments A (above) and B (below) are clearly visible, but in the later image cluster A has disintegrated even further. Credit: NASA/ESA/Quanzhi Ye/Alyssa Pagan
The colors of the comet also changed after perihelion. The core reflected more red light than green after passing the Sun – we say it got redder – but looking at red light versus near infrared light, it didn’t change much at all. The ejected dust, however, became less red, which is also strange; dust scatters blue light and lets red through, so you’d expect the tail to be red and stay red. Nothing like this has ever been seen before (the astronomers actually wrote, “The color of the object was weird”, which I don’t think I’ve ever seen in a science journal article) and it’s unclear why he behaved this way.
There’ll be more chances to see him, as he’ll be back in early 2025, and maybe see if he’s still acting weird. But there won’t be many more opportunities. Over time, Saturn’s gravity slowly makes the orbit even more elliptical and brings it closer to the Sun. If it doesn’t decay before then, it will almost certainly collide with the Sun in a few thousand years.
Spectacular comets are also inherently ephemeral. If they are shiny, it is because they are active and lose material as they heat up. They can only do so for so long before they either collapse or become dead comets, which look more like asteroids. So whenever you can see a comet, take advantage of it. He may not be back for a long time, or he may not be back at all.