[meteorite-list] Comet ISON Recovered

[Ron Baalke]

http://www.isoncampaign.org/karl/ison-recovery

Comet ISON: Recovered!
NASA Comet ISON Observing Campaign
Submitted by Karl on Mon, 08/12/2013 - 14:52

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! 
 
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. 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...

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. 

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!