Bad Astronomy | Comet 323P/SOHO came too close to the sun and partially disintegrated

Discovered in 1999, comet 323P/SOHO has an orbit of fire and ice: In its deeply elliptical 4-year orbit around the Sun, it nearly goes out to Jupiter’s orbit, then falls back to just a hair over 5 million miles from the sun’s erratic surface. If you were standing on the comet at that moment, you’d have to hold up both hands to block out the sun, and it would feel like you’re staring into a blast furnace.

Needless to say, that will take something away from you. In the case of the comet, it was a few chunks the size of houses

323P/SOHO was discovered in photos taken by the NASA/ESA SOHO . solar emission as the comet approached our star. Curiously, it’s not a very active comet and only appears to show activity — brighter by dust ejected as it warms — when it’s very close to the sun. That indicates that unlike most comets, there is not much ice in it. Otherwise, it would begin to tail by the time it at least crosses Mars orbit.

Given that 323P comes so close but has managed to survive several previous passes, when it orbited the sun again in 2021, astronomers were ready. Just weeks before perihelion – the closest approach to the sun – they observed it with the huge Subaru telescope, and found nothing really wrong. At that time, it was only slightly closer to the sun than Earth, showing no signs of activity, as before.

Its perihelion was on January 17, 2021, and astronomers had to wait for it to put the sun behind it before aiming more “scopes” at it. Then, in February and March, they used several large telescopes, including: CFHTTwinLowell Discovery Telescopeand Hubble take a look [link to paper]†

And it looked different.

It showed a long, narrow tail, just like what you see after a comet disintegrates; comets are very fragile, and if they lose enough ice to hold them together, they can fall apart, become a cloud of rubble and debris, surrounded by an expanding cloud of dust

Hubble images showed that two pieces had broken off, both about 40 meters in diameter, judging by how bright they were. Mind you, the solid part of the comet itself, called the nucleus, is just under 200 meters across, so these were significant chunks.

Given how hot the comet gets as it skims past the sun, it’s unlikely that even deep within the innermost water ice will remain, which hasn’t been the case for a while. So why did it fall apart?

Taking a quick series of observations yielded a clue: The comet is getting brighter and dimmer on a very short time scale, probably just half an hour. This is probably due to the rotation. If it is elongated, when we rotate we see the wide side and then the narrow end, so that it becomes brighter and weaker. The amount the brightness has changed indicates that one axis is about 1.4 times longer than the other, so it’s a bit potato-shaped, like 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 the centrifugal force. It’s possible that the forces of sunlight made it spin so fast, the YORP effect

That twist may have been why it fell apart in 2021. The extreme temperature changes in just a few months as it approaches the sun and recedes again would likely cause massive cracks in the rocky body due to thermal expansion and contraction. As it got close to the sun, some major fissures must have formed, and the fast spin pulled the trigger on a pretty decent cosmic rock slide. They estimate that somewhere between 0.1 and 10% of the comet’s mass was snatched away.

The comet’s colors also changed after perihelion. The core reflected more red than green light after it passed the sun — we say it got redder — but looking at the red versus near-infrared light, it didn’t change much at all. However, the ejected dust turned less red, which is also weird; dust scatters blue light and lets red through, so you would expect the tail to be red and stay red. Such a thing has never been seen before (the astronomers actually wrote, “The color of the object was erratic,” which I don’t think has ever been seen in a scientific journal article) and it’s not clear why it behaved like that.

There will be more chances to see it as it will be back this way in early 2025, and maybe see if it’s still acting strange. But there won’t be many opportunities left. Over time, Saturn’s gravity slowly makes its orbit even more elliptical and closer to the sun. If it doesn’t disintegrate sooner, it will almost certainly collide with the sun in a few thousand years.

Comets that are spectacular are also ephemeral by nature. If they are clear, it is because they are active and lose material as they heat up. They can only do that for so long before they fall apart or just become dead comets, more like asteroids† So any time you can see a comet, take the opportunity. It may not be back for a long time, or it may not be back at all.

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