ISON – More Questions Than Answers

Eagerly awaited, watched almost live from space cameras and dissected endlessly since it appeared to break up, Comet ISON is gem of a comet - even if it is now an ex-comet (and that’s by no means certain either).

We watched with baited breath as the NASA & ESA images were downloaded to let us see in unprecedented real time (or as near to real time as technology and the speed of light permit!), We tweeted our updates and thoughts via @awesomeastropod and the debate continued on our Facebook Group.

Out of the frying pan

The images from 6pm Universal Time/Greenwich Mean Time on the 28th November 2013 onwards didn’t look good for ISON. With the panoply of spacecraft observing the sun, we could see the comet approaching the will it / won’t it time when we could ascertain if it would survive its fiery passage and make its merry way back from the sun to hopefully provide a spectacular show in our night skies. What we were looking for was a clear image of survival, similar to Comet Lovejoy in 2011:

What we actually saw was the comet fade dramatically as it reached the sun’s corona (the outer atmosphere of the sun), as individual filaments of charged plasma battered the celestial wanderer.  

By 8pm UT/GMT it was clear to all observers and experts that no sign of ISON in the Solar Dynamics Observatory spacecraft images and no obvious sign of reemergence on the Solar and Helioshperic Observatory spacecraft (SOHO), meant ISON had not survived perihelion. NASA had called it: the patient was dead.

ISON’s alive?

However, it wasn’t over. The patient wouldn’t give up entirely.

SOHO 8pm

A movie of the widefield SOHO camera was now starting to reveal the presence of the comet - brightening every time it passed though a solar flare:

COR2-A animation of C2012-C1 solar flyby

This gave us some hope that an unexpected phenomenon had happened and we were all too hasty in reporting ISON’s demise.

So where does this leave us now? ISON has survived its close approach to the sun - all the more spectacular as the sun is at its most active point in the 11 year solar cycle. But it carries a lot of scars and battle damage. What survived perihelion is nothing like the 2km comet that made the Promethean attempt on 28th November 2013.

In all likelihood, we have a much smaller nucleus - the solid core of the comet - or even fragments of rock & dust that are continuing along the trajectory of the comet (this trajectory will not change, no remnants will hit Earth). This means we will not get that Comet the Century in early and mid December, but we may be able to see it with amateur-grade telescopes and astrophotography equipment. And if a significant proportion of the nucleus did survive with much of its surface water ice stripped away by the sun’s incessant force, we can still hope for more cracks to appear in the nucleus and begin venting more gases again. So while it’s unlikely that we will get a naked eye - or even binocular - cometary display in December, all hope is not yet lost.

But what we do have is a nice example of that mid point between the cometary conclusion dichotomy - will it survive perihelion or will it burn up. ISON may have been big and tough enough to have made it, but those raging solar flares and Coronal Mass Ejections beat it up too badly, leaving only the battered remains for scientists to analyse.

If it hadn’t been the year of solar maximum, who knows? Perhaps we’d be getting that beautiful naked eye comet, we’d all hoped for.

So, what’s your favourite theory for the bizarre behaviour of this sungrazing comet from the outer-outer solar system?

Comet C/2012 S1 ISON - A Timeline

21st September 2012
An object with an apparently eccentric orbit was discovered beyond the orbit of Jupiter by Vitali Nevski & Artyom Novichonok, using a 16” reflecting telescope in the International Scientific Optical Network (ISON). The data was sent to the Minor Planet Center for verification.

24th September 2012
Following two clear nights for more data on the object to be accrued, Gareth Williams of the Minor Planet Center (MPC) was able to calculate its exact orbit and the MPC declared it was a comet with the name C/2012 S1 ISON.

Comet ISON’s orbit showed it to be a visitor from the outer reaches of the solar system - probably from the Oort Cloud (a theoretical ring of left over debris from the birth of the sun & planets that cocoons the solar system 1,250 times further than the orbit of Pluto).

ISON’s orbit also showed that it would be a sungrazing comet. It would get painfully close to the sun on Thanksgiving 2013, 28th November. Passing just 2 million km from the sun (Scorched Mercury is 46 million km at its closest point to the sun) would, therefore, be a make or break scenario for ISON - quite literally.

1st October 2013
Comet ISON crossed the orbit of Mars on its way through the inner solar system.

1st November 2013
Comet ISON crossed the orbit of the Earth and was visible in amateur telescopes though falling far short of the predicted brightness. Note: predicting new comet brightness is an exercise in futility. The size, composition and rate of gas release as the sun heats it up, all have a bearing on the brightness on any given day and the Minor Planet Center only make brightness predictions with very heavy caveats.

3rd November 2013 (T minus 25 days)
A brightening occurs. Until this point, ISON had remained far dimmer than the predicted light curve.

15th November 2013 (T minus 13 days)
Inside the orbit of Venus, ISON had another sudden surge in brightness over a 48 hour period that made it a binocular object in the pre-dawn sky and brought it closer to the predicted light curve.

27th November 2013 (T minus 1 day)
After a week of steady brightening that saw it become bright enough to see with the naked eye (but too difficult in reality, due to it’s low position in the sky and ever increasing proximity to the sun), a Coronal Mass Ejection appeared to hit Comet ISON. A CME is a massive-scale release of charged particles that even at the distance the Earth is from the sun, still causes bright auroras, have damaged satellites and fried electricity grids.

The images from NASA’s STEREO spacecraft showed the comet passing through a giant loop of solar plasma but, as the comet didn’t appear to brighten and there were no visible changes to its appearance, it’s more likely that the 2D images made it look like the CME hit the comet when, in reality, the CME fired off into space at an angle that avoided the comet. ISON survived to live another day… but would that be it’s last day?