This article was originally posted on the NASA Comet ISON Observing Campaign website and is reposted here with permission.
On Wednesday, amateur astronomers online were very quick to point out that comet ISON had entered the field of view of the NASA STEREO-A Heliospheric Imager 1 ("HI-1A") camera. It wasn't much to look at, as we only had available our low-resolution "beacon" data, but nonetheless you could clearly see a bright streak on the left-hand side of the HI-1A images. It was nice and reassuring to see the comet arrive in the camera but we had been expecting it for a long time, and indeed had been watching it in the wider field "HI-2" camera on that spacecraft since October 11, 2013.
When our STEREO imagers first caught sight of it, the comet was barely registering magnitude 12 on the brightness scale, and many astronomers were understandably feeling edgy as it labored slowly in its rate of brightening. But just over a week ago, comet ISON suddenly and spectacularly began to live up to our lofty aspirations, experiencing a dramatic outburst that saw it brighten from a 7th magnitude "green fuzzball" into one of the most visually stunning comets we've seen in years, now shining brightly at 3rd or 4th magnitude!
The stage was set, and timing couldn't have been better for ISON to make its appearance into the higher-resolution HI-1 camera on STEREO-A -- and that's what I'm excited to share with you now. But of course, no great scene in space is complete without a cast of supporting characters, and ISON has chosen a great set of companions for this scene.
I hope you appreciate that there's a lot of awesome happening in that short animation! So much so, actually, that I could write pages of comments about it. However, I'll try for brevity here...
First thing you're probably saying: "well that was rather short! We want more!!". Yep, sorry. Come back in a couple of days' time and you'll have a lot more to look at, but for now all we have are those few frames. But just in that brief period I hope you can see what we might have in store for you...
What we're looking at here is a cropped section of a 20-degree field of view, taken from a spacecraft that is far from Earth. We can see not only comet ISON entering the field of view but also Comet 2P/Encke, Mercury and that little place we like to call home!
The dark "clouds" of stuff you see coming from the right are density enhancements in the solar wind, and these are what are causing all the ripples you see in comet Encke's tail. I can pretty much promise you that we're going to see ISON's tail doing that in a couple of day's time, but on a much larger scale! (ISON is closer to STEREO-A than Encke is -- this has little to do with relative sizes of the comets). Indeed, these kinds of solar wind interactions are exactly what I wrote about a couple of weeks ago, and they give us valuable information about solar wind conditions near the Sun.
And this leads me very nicely to the last -- and, to me, most exciting -- thing I'll say for now (brevity, remember...).
Notice the motion of the two comets. Notice the direction Encke is moving in, and then the direction ISON is coming in at. Then note that ISON is moving quite a bit quicker than Encke (both true in reality and because ISON is closer to the camera). See where I'm going? Or where the comets are going??
No they're not going to hit each other -- in reality they are millions of miles apart -- but as seen from the STEREO-A spacecraft, they are going to get very close! How close, I don't know. Tomorrow I hope to give you that answer but it involves math and software, and it has been a long day. But regardless, we are probably a couple of days away from seeing two comets almost side-by-side in that camera, with long tails flowing behind them in the solar wind. To say that such an image will be unprecedented is rather an understatement.
Scientifically it's a huge bonus too! If you read my "comets in the solar wind" blog post, you'll understand why we're interested in watching those tail wiggles, and how they teach us about CME's, solar wind, etc. Basically the comets act like a remote probe or windsock, and we can watch their reaction to the solar outflows and learn what's going on. So now consider this: instead of having one solar probe, we now have two of them in slightly different locations in space. This is a very big deal. I was going to describe more about why, but this post has gone on enough for now.
This is the beginning now, folks. I know we've said it could be the end for ISON, but in terms of excitement and amazing space-based images, this is the beginning. Don't stray too far from this website, particularly later next week. I've lived through one of these events before. It's stressful, chaotic, tense, sleepless, intense and often frustrating. It's also truly thrilling! Matthew and I will be on hand to guide you through it all, so get your comfy chairs ready... it's all about to kick off!