Comet ISON: Recovered!

[Please note: this article was submitted AUGUST 12, 2013... not December 8, 2013. I realize the above mm/dd/yy date format is confusing to those that use the dd/mm/yy format. Sorry!]

Well this is rather exciting news: Comet ISON lives on! (we think...)

For several weeks now, ground-based observers have been blind to Comet ISON as our local star was sitting directly between us and the comet. We knew this was a temporary problem, and expected that by the end of August, ground-based observers would begin to detect Comet ISON, so long as it hadn't fizzled out during that time. So now I am delighted to share two pieces of good news: first, that ISON is still alive and well, and secondly that it has been recovered a couple of weeks earlier than I would have expected!


Is this what we think it is?! In the center white square, that fuzzy shape could well be Comet ISON! Image Credit: Bruce Gary.

The above image was recorded by amateur astronomer Bruce Gary using his 11-inch telescope at Hereford Arizona Observatory, and show Comet ISON planted firmly between two very slightly different predictions for it's current location in space exactly where predicted! Yes, the comet looks faint in this image, but it was only a few degrees above the horizon when this image was recorded, and the images are being somewhat washed out by twilight. It's actually a remarkable feat to have imaged ISON this close to the Sun, and Bruce is to be applauded for that! It should be noted that these images are awaiting confirmation, and indeed Bruce himself states very clearly that "[a] final claim that this is Comet ISON should be based on another clear morning's observations showing the expected motion". This is a critical point: while this certainly looks like a comet, does it move the way we would expect it to be moving? If it does, then that's another valuable piece of evidence that we are indeed looking at what we hope it is.

If it looks like a duck, and quacks like a duck...


Motion analysis seems to hint that this object moves very much the way we expect ISON to be moving... [image credit: Bruce Gary]
Thankfully, Bruce is clearly an experienced and diligent observer, and has already taken it upon himself to begin a motion analysis to answer this very question! In the plot opposite (which is only a quickly-made preliminary analysis, hence the question-marks on the axes), Bruce has plotted the right ascension [blue] and declination [red] of his observations versus time. When he draws a line through those red and blue data points, he then calculates the slope of that line to tell him the rate of motion (i.e. distance divided by time). His results are remarkably close to the predictions for Comet ISON, lending significant weight to the likelihood of his observations indeed being that of Comet ISON.

Going back quickly to an earlier remark, I suspect I will get questions about why there are two predictions for the current position of Comet ISON, and why neither seems to be correct. Let me address that briefly. Bruce has subsequently noticed that he had an error with his interpretation of the MPC and JPL observations, and they do actually agree exactly with each other and his observation. However, the following does hold true in that observations can often produce ambiguous orbits.

The orbits of comets are calculated based on observations reported primarily from ground observers who, based on where the comet is relative to nearby stars, report the observed right ascension and declination of the comet. These reports are then sent to the Minor Planet Center (MPC) who then make the observations publicly available. Then, both the MPC and JPL take these observations and use them to calculate an orbit. So why do they get two different answers? Well they don't, or not as such. Both the MPC and JPL agree on the orbital parameters of Comet ISON to with minimal uncertainty, and it's this tiny uncertainty that's highlighted by Bruce's plot.

But you may wonder why is there any uncertainty at all, and that's a valid question. There are a couple of reasons. The first is simply that not all submitted observations of a comet's location are 100% perfect. There can be any number of factors that mean it's hard to say precisely where the center of the nucleus of a comet is on someone's image. Perhaps it's very diffuse, perhaps there's a small calibration error; there are a few minor factors that can lead to small error bars. So when scientists try and calculate the orbit of a comet, they have to find a set of orbital parameters that best fit the submitted observations, given that a perfect fit is going to be impossible. It is during this "best fit" procedure that additional small uncertainties can appear, and two equally convincing "fits" can be obtained with very marginally different results.

Thus when observers like Bruce make observations such as this, particularly after a long drought of data, the MPC and JPL folks find the data enormously useful as it allows them to tweak their result to include the new data. Over time, as more observations are made, the errors in the orbit parameters will become so small as to be irrelevant.

But I digress. I didn't want to get into a debate about orbit determinations, as right now we are celebrating that ISON does appears to still continue to shine, and it looks like it has brightened up somewhat as we would have hoped and predicted. Bruce is going to make follow-up observations at the next opportunity, and at that point we should get absolute confirmation of this detection. (His final words to me in earlier correspondence were "I need sleep!". I think after his great efforts, we can allow him that luxury!)

We are going to need several more observations like this from different observers around the world before we are going to be willing to make an updated prediction on ISON's future behavior, so please don't leap on us for a week or two about that! Just stay tuned to this site and you'll be among the first to know!

UPDATE: Bruce noticed that he'd made a (completely understandable!) sleep-deprivation-enduced error in his interpretation of the JPL and MPC orbit predictions, and that in fact they both agree almost exactly with his observation. So that's even better news! (But my point about uncertainties in orbits is still 100% valid and true.)

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.