Comet ISON—or whatever is left of it—is fading fast, and the latest images from the NASA/ESA Solar and Heliospheric Observatory (SOHO) LASCO-C3 camera may be showing us our final images of the “dud of the century.”
The following update is from Karl Battams of the Naval Research Laboratory, who is also the solar spacecraft lead for NASA’s Comet ISON Observing Campaign and the leading authority on all things ISON. You can read the full version of his latest blog post here. The below excerpt touches on many questions we all have about ISON and what may happen in the coming days.
“My @SungrazerComets Twitter feed, and my email accounts, are all blowing up with questions about comet ISON. Many of them have already been answered, and many of them have unsatisfying answers, but I’ll do my best.”
- What happened to comet ISON? Is it still alive?
Great question, and we recommend you find a comet expert to answer that. … [crickets] … Sigh, OK I guess that’s us. Well we don’t have a very clear answer yet but there are a couple of things we can say for sure. First, during its passage through the Sun’s million-degree corona, its dusty/gassy coma got very much burned away, though clearly some fine dust survived (which is the fine cloudy stuff you see being pushed away from the Sun). Second, something did emerge from the corona. It could be a comet, or just the remains of what once was. We can’t tell right now. Certainly there’s lots of dust, and Matthew and I hesitantly lean towards thinking that there’s something there producing dust, but that could be a small nucleus, or it could be a pile of rubble and comet chunks that will dissipate in the coming days. The key thing we need to find out — and as yet we have no data about this as ISON is too faint and close to the Sun — is whether there is any gas being produced. If there’s gas, there’s almost certainly an active nucleus; if there’s almost no gas then probably no nucleus.
- If there is a nucleus, how big is it?
There is no way we can tell this from the spacecraft data we have right now. We will need to wait for Hubble to be able to observe the comet, which will be in mid-to-late December, I believe. What I can tell you is that however big ISON’s nucleus was a few weeks ago, it is much smaller now!
- Will it be naked eye visible? When? How bright?
This is definitely the toughest question but also the most frequent. 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.
- Has ISON changed course? Is it now a danger to Earth?
I’m still getting lots of questions about this, so it’s important to address. It is also by far the most absolute and definite answer I can give. The answer is “NO” — whatever is left of ISON, be it nucleus, rubble, or dust, — the large chunk(s) will continue to follow the same orbit that we always said they would. The passage through the Sun, and any possible fragmentation, will not have made a difference. This means that ISON is absolutely NO THREAT to Earth in any way whatsoever. The comet, or its remains, will now pass harmlessly out into space, never to be seen again. Folks also continue to be worried about a “meteor shower” at Earth so that’s worth mentioning. As I understand it, Earth will probably pass through the remnant trail of comet ISON’s tail some point early next year. The net result of this will be at least one or two, if not a handful, or extra shooting stars in the sky over a couple of night. And that’s it. Just shooting stars, just like we see every single night on Earth. Any dust that Earth encounters will be absolutely tiny and stands zero chance of reaching anywhere near the Earth’s surface. Indeed, Earth passes through numerous comet tails every year — that what the Leonids and Perseids are, for example. So don’t worry about this, and don’t even expect to notice it.
Congratulations: you now know as much as we do! No, seriously. We still have way more questions than answers, and it’s going to take a while longer to get things figured. In terms of a timeline, in a few days time I think we’ll be able to make a good call on the naked eye visibility of ISON (keep your expectations low, please…). In maybe a week or so I’m guessing we’ll fully understand why ISON didn’t put on a show in SDO (they might have already figured this out – I don’t know – but that’s their story to tell, not mine). Give us three or so weeks at least to (hopefully) get Hubble images to say if there’s anything left of ISON, and how much, if so. Then expect results from the Comet ISON Observing Campaign to continue appearing for at least, oh, five years? Maybe ten… “