Comet ISON: What exactly happened?

As the media try to make sense of the status of the “Comet of the Century” it’s difficult to filter out exactly what is going on. Has ISON been destroyed in its encounter with the sun, or has some remnant of it survived?

On the 21st of September 2012, a comet was spotted which was quickly predicted to become the “Comet of the century” with initial estimates of its brightness predicting that it would peak brighter than the full moon! Shortly afterwards, these estimates were refined and the peak brought down; eventually it became just visible with binoculars, and was captured by observatories around the world.
I was lucky enough to be part of the team from the University of Birmingham imaging the comet from the 20″ telescope at Wast hills, successfully capturing an image of ISON as it passed us by. (We also managed to image the comet much earlier in the year, whilst it was at magnitude 15- see this article for the details on that)
The image, shows the constituent parts of the comet; it’s solid ice-core, subliming coma and the increasingly spectacular dust tail. Following the progress of the comet so closely, it became difficult not to be a little nervous during it’s encounter with the sun.

Of course, even as the comet disappointed in terms of its peak brightness, it still promised to generate a stir as study of the orbit predicted that it would pass 1,165,000 km over the sun’s surface. This gives ISON the nickname “sun-grazing”, which nicely summarises this behaviour. The reason it caused so much of a stir is that this type of comet does not regularly appear, and it a rare opportunity for scientists to study the constituent parts of the comet as they become partially vaporised during the encounter. To what extent the comet would be vaporised was beyond prediction, as there are many factors to consider in such an encounter- the smallest fracture on a comet’s surface may or may not cause it to break up in the energetic interaction.

So how did we prepare to watch this exciting event? Of course, anything so close to the sun cannot be viewed easily with “normal” telescopes, like the one we used. Instead, the STEREO solar observatories, in orbit around the sun, captured the approach of the comet, the LASCO coronagraphs on the SOHO space observatory imaged the approach (giving the images in most of the press releases) and the Solar Dynamics Observatory was steered to face the critical area around the sun to maximise our chances of capturing the moment.

So what did we see? Nothing. At least, nothing of the interaction itself. LASCO captured the approach of the comet, but the critical moment seems to have occurred while the comet was in our “blind spot” between LASCO and SDO! As a result, there was a huge amount of speculation as to what exactly happened to the comet. After no sign of it was seen for just over an hour, the media was quick to go on suggestions that the comet had been destroyed- it was clear that nothing major had survived the encounter.

Of course, shortly after these suggestions came out, ISON gripped us all again as a small part of it began to emerge on the other side, exactly where it should, but just a little too dim to have been visible on the earlier images. What may be a small fragment of ISON is currently being studied, as observatories attempt to determine the size of this object and how much of the comet is left- it may even be that what we see is just a collection of dust left by the comet, perhaps part of its tail which continued the path around the sun.

As groups around the world try to determine what happened to ISON, it’s interesting to speculate what this means for the scientific community; personally, I hope we will see some exciting analysis conducted to find the composition of the comet- it’s comparable to an exploded view of the object, and gives a great opportunity for us to find out more about the internal composition. Comets are very important in terms of our understanding of the formation of the solar system and other star systems, so it could be a vital clue for our development in this field of research.

Only time (and research) will tell how the comet has reacted to the event, so keep an eye out for updates!

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