Bad astronomy | Comet 323P/SOHO came too close to the Sun and partially broke apart

Comet 323P/SOHO was discovered in 1999 and has an orbit of fire and ice: in its 4-year deeply elliptical orbit around the Sun, it goes out almost to the orbit of Jupiter, but then drops to just a hair more than 5 million kilometers from the scorching surface of the Sun. If you were on the comet at that point, you would have to raise both hands to block the sun, and it would be like looking into a blast furnace.

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

323P/SOHO was discovered in images taken by NASA/ESA’s SOHO solar mission as the comet approached the star. Strangely, it’s not a very active comet and only appears to show activity – getting brighter due to dust being ejected as it heats up – when it’s very close to the Sun. This indicates that, unlike most comets, it does not have much ice. Otherwise, it would start sporting a tail the moment it crossed the orbit of Mars on its way, at least.

Given that 323P comes so close but managed to survive several previous flybys, when it revolved around the Sun again in 2021, astronomers were ready for it. Just a few weeks before perihelion – the closest approach to the Sun – they observed it with the massive Subaru telescope and found nothing really wrong. At that time, it was just a little closer to the Sun than Earth, and it didn’t show signs of activity like before.

Perihelion was on 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 the CFHT, Gemini, Lowell Discovery Telescope, and Hubble, to take a look. [link to paper].

And it looked different.

It showed a long, narrow tail, much like what you see after a comet disintegrates; comets are very fragile, and when they lose enough ice to hold them together, they can collapse, becoming a cloud of rubble and debris surrounded by an expanding cloud of dust.

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

Given how hot the comet gets as it passes the Sun, it’s unlikely it has any water ice left even deep within its interior, and it hasn’t for some time. So why did it fall apart?

Taking a quick series of observations provided a clue: the comet gets brighter and darker over a very short time scale, probably just half an hour. This is probably due to its rotation. If it’s elongated, as it rotates, we see its wide side and then its narrow end, so it gets brighter and darker. The amount of changed brightness indicates that it’s about 1.4 times greater on one axis than the other, so it’s a bit potato-shaped, like so many small bodies in the solar system.

But this is strange. A half-hour spin would make it the fastest spinning comet known, and it would have to have an unusually high tensile strength to keep it from breaking apart due to centrifugal force. It is possible that the forces of sunlight rotate it so rapidly, called the YORP effect.

This spin could have been why it broke apart in 2021. The extreme temperature changes in just a few months as it approaches the Sun and retreats again would likely generate massive cracks in the rocky body due to thermal expansion and contraction. As it approached the Sun, some large cracks must have formed, and the rapid rotation pulled the trigger on a pretty decent cosmic rockslide. They estimate that somewhere between 0.1 and 10% of the comet’s mass was stripped away.

The comet’s colors also changed after perihelion. The core reflected more red light than green after passing the Sun – we say it got redder – but when looking at red light versus near-infrared light, not much has changed. The ejected dust, however, turned less red, which is also strange; the dust scatters the blue light and lets the red through, so you would expect the tail to be red and stay red. Nothing like this has ever been seen before (astronomers actually wrote, “The object’s color was weird”, which I don’t think I’ve ever seen in a scientific journal) and it’s unclear why it behaved this way.

There will be more chances to see him as he’ll be back like this in early 2025 and maybe see if he’s still acting weird. But there won’t be many more chances. Over time, Saturn’s gravity is slowly making the orbit even more elliptical and bringing it closer to the Sun. If it doesn’t disintegrate before then, it will almost certainly collide with the Sun in a few thousand years.

Spectacular comets are also ephemeral in nature. If they are shiny it is because they are active and losing material as they heat up. They can only do this for so long before they either fall apart or just become dead comets, which are more like asteroids. So whenever you can see a comet, take the opportunity. It might not be back for a long time, or it might not be back.

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