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Title - The Lingering Death of Periodic Near-Sun Comet 323P/SOHO
By - Man-To Hui, Quan-Zhi Ye et al, 2022
Summary - The near-sun comet 323P (not a sungrazer) showed no cometary features on its way to perihelion, in late Dec 2020. After perihelion, it developed a long narrow tail mimicking a disintegrated comet. The ejecta, comprised of at least mm-sized dust, during which 0.1-10% of the nucleus mass was shed due to excessive thermal stress and rotational disruption. 2 fragments of ∼20 m in radius were seen in HST observations from early March 2021. The 172 m wide nucleus has a rotation period of 0.522 hr, which is the shortest (& hence, fastest) for known comets in the solar system and implies cohesive strength ≳10-100 Pa in the interior. 3d shape model is given in the paper. Velocity of dust ejection : 20 m/s. Such great speeds have never been previously seen in ejection of dust grains of similar sizes at comets or active asteroids, except in the case of (596) Scheila, whose transient activity was caused by an impact event. While the comet may have been an ordinary Jupiter-family comet over 1 kyr ago, its observed mass loss cannot be explained by sublimation of cometary volatiles. Rather, it is most likely triggered by the rotational instability, plus the enormous thermal stress induced by the huge temperature gradient within its nucleus around perihelion. Comet 323P/SOHO's perihelion is only 0.04 au, orbital period is just over 4 years & was discovered in 1999.
News - Phys.org, Science Times, My Twitter thread on it
Phys.org - "But Jupiter-family comets themselves originated as Kuiper-Belt objects and contain many frozen volatiles. They should form a noticeable tail when heated by the sun. But that's not the nature of 323P's debris tail."
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Article - Comet 323P / SOHO fragmentation
By - Phil Plait, 2022
Extracts - Oddly, it’s not a very active comet, and only seems to show activity when it’s very close to the Sun. That indicates that unlike most comets, it doesn’t have a lot of ice in it. Otherwise it would start sporting a tail by the time it crossed the orbit of Mars on its way in at least. It showed a long, narrow tail, much like what’s seen after a comet disintegrates; comets are very fragile, and when they’ve lost enough ice that holds them together they can fall apart, becoming a cloud of rubble and debris surrounded by an expanding dust cloud. Hubble images showed two pieces had broken off, both roughly 40 meters across judging from how bright they were. Mind you, the solid part of the comet itself, called the nucleus, is somewhat less than 200 meters across, so these were significant chunks.
Given how hot the comet gets as it skims past the Sun, it’s unlikely to have any water ice left even deep in its interior, and hasn’t for a while. So why did it fall apart? Taking a rapid series of observations provided a clue: The comet gets brighter and dimmer on a very short timescale, probably only a half hour. That’s likely due to its rotation. If it’s elongated, then as it spins we see its broad side and then its narrow end, so it gets brighter and dimmer. The amount it changed brightness indicates it’s about 1.4 times longer in one axis than the other, so it’s a bit-potato-shaped, as so many small solar system bodies are.
But that’s weird. A half-hour rotation would make it the fastest spinning comet known, and would have to have an unusually high tensile strength to keep from flying apart due to centrifugal force. It’s possible the forces of sunlight spun it up that rapidly, called the YORP effect. The colors of the comet changed after perihelion, too. The nucleus reflected more red than green light after it passed the Sun — we say it got redder — but when looking at the red versus near-infrared light it didn’t change much at all. The dust ejected, however, got less red, which is also weird; 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 freakish,” which I don’t think I’ve ever seen in a science journal paper) and it’s not clear why it behaved this way.
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