Comet C/2012 S1 ISON Closest to Earth

Hubble C/2012 S1 ISON
NASA’s Hubble Space Telescope provides a close-up look of Comet ISON (C/2012 S1). Credit:NASA, ESA, J.-Y. Li
Today comet C/2012 S1 ISON will pass closest to Earth. No worries here, close is relative in space and in this case means a comfortable 0.43 AU (64,200,000 km or 39,900,000 miles) from Earth. The comet should be easily visible well up in the dawn sky as an unaided eye object in northern Hercules.

Update: Yes, this post was written and scheduled well before the comet disintegrated at perihelion. If there is anything left it is closest to the Earth today. Many amateurs, large scopes, even Hubble have attempted to recover the remains. No one has reported finding anything.

Comet ISON is Still Dead

The definitive results are in from the Hubble Space Telescope… Comet ISON is still dead.

ISON SOHO LASCO C3
What remains of comet C/2012S1 ISON after perihelion in the SOHO LASCO C3 imagery
On the 18th the Hubble team imaged the expected coordinates of comet C/2012 S1 ISON and found nothing. Given the sensitivities of the instrument and the exposure length, anything brighter than 25th magnitude would have been detected. There was some uncertainty in the position, thus several different locations were imaged with Hubble.

25th magnitude is a lot deeper than amateur attempts at recovery, though the amateur efforts likely covered a great deal more area than Hubble with wider fields of view. Still, there have been no reports of any remains detected by any searcher.

Does this rule out any surviving fragments?

We can’t completely rule out the possibility that something is left of the comet. After all, it was seen after its passage close to the Sun, but disappeared not long after. This material would still exist, but is likely very diffuse gas, dust, and very small pieces spread over an extremely large area. – Zolt Levay, The HubbleSite Blog

I think it is pretty safe to call this comet dead.

ISON Fades

What is left of the comet is rapidly fading.

During the hours after perihelion the comet brightened again, providing hope that some large fragment had survived. Many commentators were speculating that news of the comet’s demise was greatly exaggerated.

A full sequence of comet ISON passing through perihelion on Nov 28, 2013. Movie prepared from the SOHO LASCO C3 imagery.
I will hold to my original opinion, the comet was substantially destroyed. I may be wrong, but that is the fascination of these things, we observe and we learn.

Watching further is providing further evidence that no substantial fragments remain, the flareup was brief, ices originating from the debris cloud providing one last burst of activity.

Over the last twenty-four hours even this has faded, what is left of the comet continuing to disperse and fade. It will be interesting to see what remains when the comet is at last far enough from the Sun for large telescopes to examine the debris field.

Will anything be visible to small telescopes? Possibly, but if there is anything it will be quite faint, something barely seen visually, or photographic only. I am certain that the amateur community will attempt observations. Again, it will be very interesting to see what is found.

Observing Comet ISON after Perihelion

When will we again see comet C/2012 S1 ISON in the morning sky?

Much depends on how bright the comet has become, thus how far it must be from the Sun’s glare before we can see it well. If the comet has become truly spectacular we may see the tail rising before the comet quite early. If it has disintegrated, we may see nearly nothing.

It is probably on the morning of Dec 3rd that we can start looking for the comet to be above the horizon at dawn. On this morning the comet will be 14° away from the Sun, rising at 05:58 HST as seen from the island of Hawai’i. On the 4th this will be 05:51 and 16°, on the 5th the angular separation will be 19° while rising at 05:43HST.

When planning your comet viewing keep in mind that the comet will rise nearly 25° north along the horizon when compared to where it was rising before perihelion. This is closer to due east, at about azimuth 100°.

Unlike some comets, the comet will not emerge into the evening sky after perihelion. It remains in the morning sky for earthbound observers. It will eventually be visible in the evening sky, but not for some time, a few weeks or more depending on the observer’s latitude. The high inclination of the comet’s orbit will take the comet through the northern constellations, into the circumpolar sky at the end of the year.

What morning will you first see the comet?

ISON After Perihelion
Comet C/2012 S1 ISON rising after perihelion at 6am on the morning of December 3rd, 2013

What Remains of ISON

Fragmented? Probably.

There seems to be something left of comet ISON, perhaps some large fragments. Dimmer than it was going in, more spread out. But something is generating a coma that shows up in the SOHO LASCO C3 imagery.

ISON SOHO LASCO C3
What remains of comet C/2012S1 ISON after perihelion in the SOHO LASCO C3 imagery

The Final Fate of Comet ISON

Like many I was watching as comet ISON passed through perihelion. A better show than many other holiday offerings. It is clear that the comet has failed to survive perihelion passage. Disrupted by tidal forces and the extreme solar heating, the comet appears to have come apart even before closest approach.

I have assembled a better resolution animation of the demise of ISON from SOHO LASCO C2 imagery. Fifty frames at the full 1k x 1k resolution of the camera that shows the event from entry to exit from the field of view. Click on the image below for the full resolution version.

Note the lack of a distinct coma, simply a smeared out debris cloud. A cloud of debris and fragments continues along the original orbit. How large are some of these fragments? This is a question that may have to wait for a few days, allowing observations from a larger telescope further from the Sun.

Best laid plans? Not much hope remains for dawn photos as the comet emerges from the Sun’s glare. There remains comet C/2013 R1 Lovejoy in the morning sky, perhaps another photo session?

ISON Perihelion from SOHO
SOHO C2 camera view of comet C/2012 S1 ISON apparently disintegrating just before perihelion. (click on image for animation)

So Long ISON?

It does appear that comet C/2012 S1 ISON has come apart at perihelion. Imagery shows the comet coma dimming and smearing out as if the nucleus has totally disrupted. Even worse, the SDO imagery programmed to cover perihelion very near the sun show nothing. The SDO cameras are very good at this sort of thing, it should show traces even if the nucleus had been stripped of the tail by the solar wind.

So long ISON?

SOHO C2 ISON Animation
SOHO C2 camera view of comet C/2012 S1 ISON apparently disintegrating just before perihelion.

Comet C/2012 S1 ISON at Perihelion

Today comet C/2012 S1 ISON will pass through perihelion, its closest approach to the Sun.

At a mere 1,800,000km (1,100,000miles) this will be a close pass indeed. As perihelion is measured from center to center, the distance is even closer if you consider the 695,500km (432,200mile) radius of the Sun. Subtracting the solar radius you realize the comet will pass a mere 1,100,000km (680,000miles) above the surface of the Sun. At this distance the intensity of the solar radiation will be nineteen thousand times more intense than a sunny day on Earth. Hot indeed!!

C/2006 P1McNaught
C/2006 P1 McNaught while 5° from the Sun on Jan 14,2007
While the comet is so close to the Sun volatile gasses will be streaming off the comet in huge quantities creating an extremely bright comet. It is fairly likely that the comet will be visible in the daytime. If so, it will appear much as C/2006 P1 McNaught appeared in January 2007 just a few degrees from the Sun, potentially visible to the unaided eye.

At closest approach the comet will be less than a degree from the Sun, difficult to pick out. An observers best bet will be as it approaches and as it moves away from the solar disk. As the comet nears perihelion it will approach the Sun from the west, best seen in the dawn sky. After perihelion it will exit the Sun’s vicinity to the north, favoring northern hemisphere observers.

The comet should be spectacular in the cameras of the dedicated solar observation satellites. Check out the real time views from SOHO or Stereo.

Observing ISON through Perihelion

Plotted below is the path of comet ISON through perihelion. The image is zenith up on the morning of November 28th from the island of Hawai’i. The actual moment of perihelion will be Nov 28.77501UT (18:36UT or 08:36HST).

A few things are notable… The obvious one is how close the comet will get to the Sun. Not just in absolute terms, which is really close. But rather how close it will look to us. The comet will be under 30 arc-minutes from the center of the Sun, recalling that the Sun is about 30 arc-minutes across. The comet will not pass behind the Sun from our point of view. While we may not be able to see it while lost in the solar glare, it will remain in the view of those solar monitoring spacecraft that are near the Earth.

Separation will help in trying to spot the comet during the day. During the 27th, 28th and 29th the comet will be very close to the Sun. On the morning of the 29th the comet will be only 4.5° from the Sun. Best bet to attempt a daytime peek may be on the 30th or later, when the comet will again be more than 7.5° from the Sun. Look for the magnitude estimates and be prepared to give it a look.

ISON at Perihelion
The orbit of comet C/2012 S1 ISON as it passes through perihelion on November 28th, 2013

ISON, Mercury and Saturn in the Dawn

Not quite the dramatic comet in the dawn shot I was hoping for. The comet is just barely able to compete with the dawn glow. Still, a beautiful morning.

Waiting to see what fate holds in store for this dirty snowball as it travels through the hell of the solar corona. I will try another photo session after perihelion.

ISON in the Dawn
Comet C/2012 S1 ISON, Mercury and Saturn in the dawn over Hilo