Astronomers left puzzled over fate of Comet ISON

Astronomers are all scratching their heads today and asking what has happened to Comet ISON after the celestial visitor sprung a string of surprises as it rounded the Sun. We are still waiting to see what, if anything, will emerge to be seen in the pre-dawn sky.

Comet ISON
Comet ISON appears as a peculiar bright smudge with two tails, above the Sun in this image from SOHO’s LASCO C3 camera at 09.22 UT on 29 November. Credit: NASA/ESA

Yesterday, in a day of high drama, the previously rapidly brightening comet suddenly faded as it approached its closest point to the Sun, a position in its orbit called perihelion.

Then, as it raced at more than 300 times the speed of a bullet, less than a solar diameter from the Sun’s furnace, the ice-and-rock head of the comet, called the nucleus, suddenly vanished.

High-resolution images appeared to show just a trail of dust with no sign of the starlike nucleus at all. And nothing was visible in images of the Sun’s vicinity taken by NASA’s Solar Dynamics Observatory, which had been aimed just away from the Sun specially to show ISON.

But then, just as experts were declaring the comet lost and reading the last rites, some manifestation of it came into view again on the other side of the Sun, brightening as it did so and showing two tails streaking away from it.

The dramatic events kept a worldwide audience on tenterhooks as they watched images returned by solar observatories in space, notably the NASA/ESA SOHO satellite which had the comet in view from two cameras, the wide-angle LASCO C3 and narrower-field LASCO C2.

NASA’s and ESA’s SOHO websites were clearly showing the strain of the huge demand to keep up with cosmic events.

All seemed to be well with ISON when it first appeared in the field of view of SOHO’s LASCO C3 on Wednesday, having brightened to around magnitude -2. But as it approached the Sun, the first alarm bells sounded because the comet began fading rather than brightening further. It seemed that astronomers’ worst fears were being realised and that ISON would not survive its daredevil brush with our home star after its journey lasting more than a million years from the Oort Cloud to get here.

Images from SOHO show the remains of Comet ISON moving away from the Sun. Credit: NASA/ESA

In a first reaction, comet authority Professor Alan Fitzsimmons, of Queen’s University Belfast, told Skymania News: “This remnant may be simply large dust grains that did not fully vaporise. By eye it looks like the remnant is further out from the Sun than would be expected from the pre-perihelion orbit. This would imply a significant effect from radiation pressure, and hence dust grains rather than substantial macroscopic bodies. However this is eyeballing only, and what would be required would be a direct comparison with the predicted orbital path.”

Another leading comet expert Karl Battams, of NASA’s Comet ISON Observing Campaign, blogged: “After impressing us yesterday, comet ISON faded dramatically overnight, and left us with a comet with no apparent nucleus in the SOHO/LASCO C2 images. As the comet plunged through the solar atmosphere, and failed to put on a show in the SDO images, we understandably concluded that ISON had succumbed to its passage and died a fiery death. Except it didn’t. Well, maybe . . .

“After perihelion, a very faint smudge of dust appeared in the the LASCO C2 images along ISON’s orbit. This surprised us a little, but we have seen puffs of dust from Sungrazer tails, so it didn’t surprise us enormously and didn’t change our diagnosis. We watched and waited for that dust trail to fade away. Except it didn’t.

“Now, in the latest LASCO C3 images, we are seeing something beginning to gradually brighten up again. One could almost be forgiven for thinking that there’s a comet in the images!”

Battams, who was working with fellow expert Matthew Knight, of Lowell Observatory, added: Matthew and I are ripping our hair out right now as we know that so many people in the public, the media and in science teams want to know what’s happened. We’d love to know that too! Right now, here’s our working hypothesis:

“As comet ISON plunged towards to the Sun, it began to fall apart, losing not giant fragments but at least a lot of reasonably sized chunks. There’s evidence of very large dust in the form of that long thin tail we saw in the LASCO C2 images. Then, as ISON plunged through the corona, it continued to fall apart and vaporize, and lost its coma and tail completely just like Lovejoy did in 2011.

“Then, what emerged from the Sun was a small but perhaps somewhat coherent nucleus, that has resumed emitting dust and gas for at least the time being. In essence, the tail is growing back, as Lovejoy’s did.

“So while our theory certainly has holes, right now it does appear that a least some small fraction of ISON has remained in one piece and is actively releasing material. We have no idea how big this nucleus is, if there is indeed one. If there is a nucleus, it is still too soon to tell how long it will survive. If it does survive for more than a few days, it is too soon to tell if the comet will be visible in the night sky. If it is visible in the night sky, it is too soon to say how bright it will be . . . ”

Jonathan Shanklin, Director of the British Astronomical Association’s and Society for Popular Astronomy’s Comet Sections, commented: “It seems that the comet has survived its close encounter with the Sun, thus increasing the chances of there being something to see in the coming weeks.” Our guide shows the track of the comet – or the comet’s remains – in December.

Update, 30 November: The latest SOHO images show that the comet has dimmed considerably in just a few hours and lost the starlike head it had. This apparent fresh disintegration of whatever remained makes a bright, naked-eye comet unlikely over the coming week.

Battams comments in a new blog post: “We still don’t know if it will be naked eye but based on its current brightness in the LASCO images – which is around magnitude +5 and fading – it does seem unlikely that there will be much to see in the night sky. I suspect that some of the outstanding astrophotographers around the world will be able to get something, but I doubt it will be as spectacular as before perihelion. I hope I’m wrong though.

“I’d guess that a few observers will begin picking up ISON in a couple of days but if – and I do mean IF – comet ISON becomes naked eye visible, it won’t be until near the end of next week (say, Dec 6 or 7). Please don’t get your hopes up, but we all need to keep in mind how ISON keeps surprising us.”

That blog post is well worth visiting for two great movies showing ISON’s solar encounter, viewed from other sides of the Sun by NASA’s twin STEREO satellites.

A fanlike Comet ISON appears much fainter in this view from SOHO’s LASCO C3 camera at 0654 UT on 30 November. Credit: NASA/ESA

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