When Will We Know?


Comet ISON is now in the LASCO C3 field of view and is already beginning to saturate the detector. This marks a dramatic rise in brightness in the past 24 hours [Image credit: ESA/NASA, annotations by Karl Battams]
My Twitter Feed has been blowing up lately with hundreds of new followers (thank you!!!!) and an insane number of questions. It's really hard for me to keep track of them all as they pour in like a firehose, so I'm going to try and address a few of the questions I've seen. But first, an update about comet ISON!

ISON as of this morning

Most of you reading this must have seen the latest SOHO images which are updating in realtime and, as of a short time, showing us a very healthy sungrazing comet! And indeed that's our bottom line for right now: Comet ISON has started to act like a Sungrazing comet. What does this mean? Well it means that ISON is now in a very near-Sun region of the solar system and is experiencing levels of solar radiation that your average comet is never going to have to deal with. Accordingly, its surface is boiling away furiously, releasing tremendous amounts of ice, dust and gas and brightening up enormously.

In the above LASCO image, you'll see that I have pointed to a saturation spike in the comet's nucleus. This mean that it is becoming too bright for the current exposure time on those images (we'll fix that, don't worry) and the pixels on the camera are "bleeding". We've seen a lot of bright comets, planets and stars in this camera and by experience alone we know that this corresponds to a visual magnitude of around +0.5.

Will ISON Survive? When will we know?

This is the question that I get the most on Twitter, in email, and from reporters. Unfortunately the answer is not particularly satisfying: we will not know if ISON will survive until it actually does so, or gets vaporized before our very eyes! And even if we do see it emerge from the solar atmosphere tomorrow, it will not necessarily mean that the comet's nucleus is in tact. In 2011, we witnessed comet C/2011 W3 (Lovejoy) pass through the million degree solar corona and emerge the other side, seemingly in one piece. But about two weeks later we realized that during that close passage by the Sun, its nucleus suffered a catastrophic breakup and had actually completely dissolved within a day or so of perihelion. Nonetheless, the comet was a spectacular sight for Southern Hemisphere observers, so this does bode well for ISON.

The rapid brightening we are seeing now in LASCO does not offer us any evidence at all as to whether it will survive or not. All it does tell us is that there is still a lot of volatile material centrally located at the comet's head. We don't know if there is a coherent nucleus or not, though I would lean towards thinking there is at least still some solid chunk in there.

If ISON survives, when might we see it?

If ISON does survive, it will probably become visible to experienced amateur astronomers by Monday or Tuesday, but for most observers and those wanting a naked eye object, it will be closer to the end of next week and into the second week of December that you'll want to start checking the skies.

Is ISON a threat to Earth?

I know many of you might roll your eyes here but some people are genuinely concerned. So, for the record, comet ISON will never be any kind of threat to Earth WHATSOEVER. On Dec 26 is will pass about 0.44AU from Earth, which is about the same distance that Venus sits at. There is a possibility in January that Earth might pass through ISON's tail but it is critical to realize that Earth does this all the time. Every meteor shower we see - and there are lots of them - is a result of Earth passing through a debris trail in space. They are completely harmless. I really don't think that we will even notice a meteor shower in our skies but if we do, it will only be a handful of shooting stars every few hours. Barely noticeable and completely inconsequential.

I also see concerns that ISON might change course as it swings by the Sun. Again, this is absolutely not the case: its orbit is basically fixed, except for incredibly minor adjustments that might come from outgassing and jets.

Please do not be concerned about this comet. Be excited! If you're reading this blog post then you are witnessing and experiencing a truly spectacular first-time-in-history astronomical event as we all sit glued to our computer, tablet and cell phone screens and watch the first known Sungrazing Oort Cloud Comet in history plunge in to the Sun's million degree atmosphere. Whether you are a comet expert, a professional or amateur astronomer, or just a curious member of the public, you now know just about as much about the fate of this comet as us "experts" do, and if you watch online with us, you will find out exactly at the same time we do whether the comet gets fried tomorrow!

You may never witness something like this again in your lifetime, so please enjoy this event. I know I will!

Keep up-to-date on the latest ISON and sungrazing comet news via my @SungrazerComets Twitter feed. All opinions stated on there, and in my blog posts, are my own.