Last updated: 10 Dec. 2013 UT

COMET C/2012 S1 (ISON): when will it fade out? => On the 28th of November!

Nicolas Biver

10.24 Dec.: Comet was not seen this morning (C8x67, 7x50 Binoculars) from Pico-Veleta under good conditions (SQM~20.9):
m1 > 7.6 for Dia=20' (7x50 B) m1 > 10.8 for Dia=5' (C8x67) m1 > 12.5 for Dia=2' (C8x154)
In the celestron, limitimg magnitude was aroud 13.2, a vague cloud with low surface brightness can still not be excluded, though.

6.24 Dec.: Comet was not seen this morning (T407x58, 7x50 Binoculars): m1 > 8 if it had been condensed with a nucleus (i.e. for dia=2') - there may have been something at the predicted position of this brightness but the observation was a bit short due to clouds.

5 Dec.: no new observations, but some other clues on nucleus size and fate of the comet

1st of December: in CBET 3731 explanation of the appearance of the comet over the last 3 days was provided. In agreement with the failure to detect (significant) gaseous emission Nov. 28-30 from IRAM, the comet likely underwent its final outburst early on the 28th when it peaked aroumd mag.-2 in Lasco-C3 images and since then we observed the dispersion of the dusty debris from macroscopic to microscopic size. The integrated amount of material (based on outgassing measurements) released the past two months is on the right order of magnitude to say that the nucleus was sub-km in size and barely made it to perihelion.

Game Over!

But don't worry there is still the nice comet C/2013 R1 (Lovejoy) to observe: m1=5.0 and outgassing around 3 tons of water per second

29.7 Nov.: Various interpretations of the Lasco-C3 (and STEREO-COR2 A and B) images can be done. I think the fact that there is now a stronger condensation with double tail for what is left from the comet does not necessarily mean there is a nucleus outgassing again (No outgassing detected from IRAM today). Geometrical effects must be taken into account:
The comet (or what is left of it) is now slowing down, so dispersion along its orbit is shrinking (orbital velocity divided by 3 in 28h)
The comet is now moving more towards us (with an angle of 45° at Tp+24h, versus 81° at perihelion), decreasing also the apparent dispersion of debris/dust trail and tail
The phase angle has increased (127° at TP+24h versus 42° at perihelion) favouring surge of brightness due to forward scattering (cf CBET 3723, by Joe Marcus)

Meanwhile there is emission of finer dust particles which forms the new tail in PA ~80° (not the trail in PA~160°), likely from the disintegrating fragments.

28.7 Nov.: The comet is at perihelion now (check latest SOHO-Lasco C2 and C3 images, cf links below) but seems to have disintegrated to me - in any case no outgassing was detected earlier today at the predicted nucleus position
27.4 Nov.: The comet is not dead yet and still outgassing a bit of HCN as seen fro IRAM, with large uncertainty on its exact value - maybe half of that of 15 Nov.... but the comet is 5x closer to the Sun
25 Nov.: Big drop in activity reported from IRAM observers (20->25 nov., QHCN divided by ~ 20), and JPL#51,52,53 orbits with large non gravitational accelerations are bad signs for the comet health or nucleus integrity...!
24.3 Nov.: Comet lost in twilight...: m1>3. Although with finally a good clear night/morning, I failed to see ISON this morning, either with 50mm binoculars or 407mm telescope (x58 or x135). I did point where it was supposed to be but the sky was too bright (searched from 6:15 to 6:55 UT). Stars down to m=5.8 where seen 3deg above the comet in the telescope. ISON was very likely fainter than Alpha_1 Librae - Saturn and Mercury were easily seen naked eye.

20.3 Nov.: The comet is indeed brighter in twilight m1=4.2!

19.3 Nov.: According to Trappist Belgium team, the comet is agin in outburst
16.2 Nov. comet fading a bit from its outburst, and outgassing a few times lower than yesterday

15.2 Nov.: BIG OUTBURST! comet ISON is a again brighter: m1=5.1 !

14.2 Nov.: BIG OUTBURST! comet ISON is (just) visible to naked eye!! Outgassing is 15x in 2 days
13.2 Nov.: OUTBURST! comet ISON is twice brighter and 2-4x times more productive than yesterday!
Is it a step increase in activity or the final burst before disruption as could be suggested if steady activity was due to reducing nucleus size?
11 Nov.: slow updates as I am spending most of the time (9h/day) gathering comet observations...!
Update on light curve and production rate curve: no very good signs of increased activity...
Will it disintegrate within the next 15days, beyond rh=0.7 AU as other comets did?
Think of switching your projects to comet C/2013 R1 (Lovejoy):
its brightness is steeply increasing (now m1=6.2 on 8.2 Nov.) and it may reach m1=4.5-5 in one month!
Where is the comet: map
Latest magnitudes
Drawings, Images
links to other observations images / websites
Its trajectory, Orbit
What can be its brightness evolution
What is its activity, outgassing rate
What is the size of its nucleus
How can it look like in the sky after perihelion

Where is the comet? - Ou trouver la comete?

Map for September-October 2013 ! !

Map for Novemember 2013 - morning horizon

Map for Decemember 2013 - morning horizon

MPC orbital elements and ephemeris
JPL Horizon ephemeris generator
Apparent trajectory in the sky reltive to the Sun:

(with optimistic magnitudes estimates)

(Wider view with optimistic mags.)

How Bright is it? Quelle est sa luminosité actuelle?

Personnal magnitude estimates:
10.87 May 2013 UT: m1=15.9+-0.5 or > 15.0, Dia=0.3', T407x350, Craponne s/Arzon; SQM=21.4; not really seen,
3.15 Sep. 2013 UT: m1=12.7+-0.1:, Dia=1', DC=3, T407x135, Ablis; twighlight, SQM=20.6->19.2, barely seen, uncertain
3.16 Sep. 2013 UT: m1=13.2+-0.2, Dia=0.8', DC=2, T407x350, Ablis; still difficult to see
4.14 Sep. 2013 UT: m1=12.7+-0.2, Dia=1.2', DC=3, T407x135, Ablis; SQM=20.6->19.8, hard to see
4.15 Sep. 2013 UT: m1=12.9+-0.1, Dia=1.0', DC=3, T407x350, Ablis (limiting mag~15.2)
4.15 Sep. 2013 UT: m1=12.6+-0.2, Dia=1.3', DC=3, T407x135, Ablis; twighlight, not easy to see
9.151 Sep. 2013 UT: m1=12.7+-0.2, Dia=1.2', DC=2, T407x135, Ablis; SQM=20.6->20.0, not very easy
9.154 Sep. 2013 UT: m1=13.0+-0.1, Dia=0.9', DC=2, T407x135, Ablis; clearly seen (limiting mag~15.0)
24.19 Sep. 2013 UT: m1=11.3+-0.3, Dia=1.5', DC=2, T407x135, Versailles; SQM=18.1, marginal
24.19 Sep. 2013 UT: m1=11.7+-0.2, Dia=1', DC=2, T407x350, Versailles; bright sky, difficult
01.20 Oct. 2013 UT: m1=11.2+-0.5, Dia=1.5', DC=3, T407x135, Versailles; bright sky, very uncertain: very small holes in the clouds in the twilight - the sky was overcast the rest of the night and morning... Comet in conjunction with Mars and the Moon!
04.193 Oct. 2013 UT: m1=11.5+-0.2, Dia=1.7', DC=3, T256x97, Crespieres (SQM=19.9);
04.185 Oct. 2013 UT: m1=11.9+-0.2, Dia=1.5', DC=3, T256x253, Crespieres; The comet is definitely seen but needs adverted vision. Brighter inner 0.5', core elongated PA=270°, beginning of tail 3' in PA=300°
Note that comets C/2013 R1 (Lovejoy) (m1=10.0) and 2P/Encke (m1=10.6) were quite brighter
13.17 Oct. 2013 UT: m1=10.8+-0.1, Dia=2', DC=4, tail = 0.1° in PA=300, T407x58, Ablis (Good but cold observing conditions, SQM=20.7); At last the comet could be clearly seen in direct vision with its long and narrow tail, and a central nuclear condensation estimated at m2=14.8
22.19 Oct. 2013 UT: m1=10.2+-0.2, Dia=2.5', DC=3, T407x58, still relatively weak
23.19 Oct. 2013 UT: m1=10.2+-0.2, Dia=3', DC=3, T407x58, m2~13.5+-0.2 (T407x350), still weak but with moonlight and city lights (And C/2013 R1, 2P and C/2012 X1 are 2x to 6x brighter...)
29.15 Oct. 2013 UT: m1=9.7+-0.2, Dia=4', DC=3, T407x58, 0.2° tail in PA 300°. The comet is clearly seen under relatively dark skies but it fails to brighten significantly

30.17 Oct. 2013 UT: m1=9.7+-0.2, Dia=3', DC=3, T407x58, Versailles
31.17 Oct. 2013 UT: m1=9.3+-0.1, Dia=3.8', DC=4, T407x58, 0.23° tail in PA 300°, Ablis
03.21 Nov. 2013 UT: m1=9.6+-0.2, Dia=3', DC=4, T407x58, 0.23° tail in PA 300°, Versailles
05.15 Nov. 2013 UT: m1=8.9+-0.3, Dia=3', DC=4, T407x58, through cirrus - incoming bad weather (from Ablis)
07.22 Nov. 2013 UT: m1=8.3+-0.1, Dia=7', DC=4, 7x50B: with binoculars (makes it a bit brighter) from Granada, under clear skies but moderately light polluted (SQM=18.9)
08.18 Nov. 2013 UT: m1=8.4+-0.2, Dia=8', DC=5, 7x50B, tail 0.7° in PA=300°, from PicoVelta/IRAM (SQM=21.0)
08.18 Nov. 2013 UT: m1=8.5+-0.2, Dia=4', DC=4, C8x67, tail 0.75° in PA=300°, from PicoVelta/IRAM (Coma barely larger than the tail width)
09.17 Nov. 2013 UT: m1=8.4+-0.2, Dia=7', DC=5, 7x50B, tail 0.7° in PA=300°, from PicoVelta/IRAM (SQM=21.1)
10.18 Nov. 2013 UT: m1=8.0+-0.1, Dia=5', DC=6, 7x50B, tail 1° in PA=300°, from PicoVelta/IRAM (SQM=21.0+Zodiacal light)
10.18 Nov. 2013 UT: m1=8.3+-0.1, Dia=3', DC=7, C8x67, tail 0.8° in PA=300°, from PicoVelta/IRAM
11.19 Nov. 2013 UT: m1=8.1+-0.1, Dia=5', DC=7, 7x50B, tail 1° in PA=300°, from PicoVelta/IRAM (many cirrus)
11.20 Nov. 2013 UT: m1=8.4+-0.1:, Dia=3', DC=7, C8x67, tail>0.6° in PA=300°, from PicoVelta/IRAM (between clouds)
11.24 Nov. 2013 UT: m1=8.2+-0.1, Dia=6', DC=6, 7x50B, tail 0.7° in PA=300°, from PicoVelta/IRAM (twighlight)
12.19 Nov. 2013 UT: m1=8.1+-0.1, Dia=4', DC=7, 7x50B, tail 1° in PA=300°, from PicoVelta/IRAM
12.19 Nov. 2013 UT: m1=8.5+-0.2, Dia=2.5', DC=7, C8x67, tail 0.7° in PA=300-310°, (strong wind)
13.22 Nov. 2013 UT: m1=7.4+-0.2, Dia=3', DC=7+, 7x50B, tail 1.3° in PA=300°, from PicoVelta/IRAM
13.21 Nov. 2013 UT: m1=7.7+-0.2, Dia=2.5', DC=7+, C8x67, tail 0.7° in PA=300-310°, central condensation brighter, too
14.21 Nov. 2013 UT: m1=6.3+-0.2, Dia=3', DC=8, 7x50B, tail 1.3° in PA=300°, from PicoVelta/IRAM (before clouds came back)
14.22 Nov. 2013 UT: m1=6.2+-0.3, Dia=2', DC=8-9, Naked Eye
15.21 Nov. 2013 UT: m1=5.2+-0.2, Dia=5', DC=7, 7x50B, tail 1.5° in PA=300°, from Granada Airport (SQM=19.7)
15.24 Nov. 2013 UT: m1=5.2+-0.2, Dia=3', DC=8, 7x50B, tail 1.3° in PA=295°, from Granada Airport (after checking in my bag!)
16.22 Nov. 2013 UT: m1=5.4+-0.2, Dia=3', DC=7, 7x50B, tail 1.3° in PA=290°, from Ablis: very hazy/fog, SQM~19.4 (at best 20.1 zenith after moonset). Nucleus in T256mm x253 is still starlike, lateral jets visible.
20.24 Nov. 2013 UT: m1=4.2+-0.2, Dia=5', DC=7, 7x50B, tail 1° in PA=290°, from Ablis: twilight, SQM~18.6 (19.2 zenith). In T407mm x58-x135 the 2' coma is intense green with strong lateral jets and more neutral colour tail. The coma is visible but more starlike with a sodium filter.
24.27 Nov. 2013 UT: m1 > 3 (Dia < 2') comet not seen 7x50B, T407x58 or x135 (Ablis, twilight)

Other magnitudes, Images?

Nice image of ISON on its first outburst (15 Nov.) (c) JB Devanssay/ P. Dupouy Animation of the comet motion , from Observatoire de DAX
Snapshot pointing at the position of comet ISON on 20.24-Nov.: Mercury is to the lower left, Spica, upper right, and the comet is the little green dot above the plane trail - 8sec exposure.
Drawing of comet C/2012 S1 (ISON) with the 40.7-cm telescope (x58) on 20 Nov. 2013 Zoom in at x135

Drawing of comet C/2012 S1 (ISON) with the 25.6-cm telescope (x42) on 16 Nov. 2013 Zoom in at x253

Drawing of comet C/2012 S1 (ISON) with the IRAM cellestron 8'' telescope (x67) on 14 Nov. 2013

Drawing of comet C/2012 S1 (ISON) with the IRAM cellestron 8'' telescope (x67) on 13 Nov. 2013 Zoom in at x154 - Zoom in at x400

Drawing of comet C/2012 S1 (ISON) with the IRAM cellestron 8'' telescope (x67) on 12 Nov. 2013 Zoom in at x154

Drawing of comet C/2012 S1 (ISON) with the IRAM cellestron 8'' telescope (x67) on 11 Nov. 2013 Zoom in at x400

Drawing of comet C/2012 S1 (ISON) with the IRAM cellestron 8'' telescope (x67) on 10 Nov. 2013

Drawing of comet C/2012 S1 (ISON) with the IRAM cellestron 8'' telescope (x67) on 9 Nov. 2013 Zoom in at x154

Drawing of comet C/2012 S1 (ISON) with the IRAM cellestron 8'' telescope (x67) on 8 Nov. 2013

Drawing of comet C/2012 S1 (ISON) with the 407mm telescope (x135) on 3 Nov. 2013

Drawing of comet C/2012 S1 (ISON) with the 407mm telescope (x135) on 31 Oct. 2013

Drawing of comet C/2012 S1 (ISON) with the 407mm telescope (x135) on 13 Oct. 2013

Image of comet C/2012 S1 (ISON) obtained with the 1m telescope at Pic-du-Midi on 13 Oct. 2012

Image of comet C/2012 S1 (ISON) on 6 Sept. 2013, (c) J.F. Soulier

More can be found here:

Magnitude estimates, astrometry, phtometry, images can be found on the :
French observers comet website and [Image website in construction]
Comet ISON sub-page
Jean-Francois Soulier comet ISON observations
Other images galleries: gallery
Seiichi Yoshida web page ISON picture gallery

Spacecraft images:

Latest SOHO - lasco-C3 image (for Nov. 27.3 to 30.9)
Latest SOHO - lasco-C2 image (for Nov. 28.5 to 29.1)
Latest low res. STEREO images (many other links exist for higher/older res. images)

What about its orbit? Quelle est sa trajectoire?

Comet C/2012 S1 (ISON) is a Sun-grazing comet as its perihelion distance will be 0.0124 AU (1.86 Millions km or 1.17 Mkm above the Sun surface) on 28 november at 18:36 UT. It does not belong to the Kreutz group of comets like C/2011 W3 (Lovejoy) (different orbital plane) but shares similarities with the great comet of 1680 (C/1680 V1) and Seki-Lines (C/1962 C1). At perihelion its orbital velocity will reach 380km/s
As far as orbital elements computed in fall 2013 say, this comet is a Dynamically new Oort cloud comet: it is a newcomer from the Oort cloud as its computed orginal aphelion lies in the Oort cloud:
25 November: Non-gravitationnal accelerations:
JPL#52 (obs. till 22 Nov.): A1 = 9.0, A2 = 0.56 (x10-8 AU/day2) - comparable to C/1999 S4
JPL#51 (obs. till 20 Nov.): A1 = 10.2, A2 = 0.55 (x10-8 AU/day2) - comparable to C/1999 S4

Initial semi-major axis a_orig = 68600+-2000 AU (MPEC 2013-W13, obs. till 20 November 2013)
Initial semi-major axis a_orig = 67100+-2000 AU (MPEC 2013-W07, obs. till 20 November 2013)
Initial semi-major axis a_orig = 61400+-2000 AU (MPEC 2013-W01, 16 November 2013)
Initial semi-major axis a_orig = 109000+-6000 AU (MPEC 2013-V07, 2 November 2013)
Initial semi-major axis a_orig = 143000+-11000 AU (MPEC 2013-U73, 28 October 2013)
Initial semi-major axis a_orig = 217000+-24000 AU (MPEC 2013-U17, 22 October 2013)
Initial semi-major axis a_orig = 184000+-24000 AU (MPEC 2013-S75)

Initial semi-major axis a_orig = (-330000)+-100000 AU (Nakano Note 2567, 15 October 2013)
Initial semi-major axis a_orig = 167000+-44000 AU (Nakano Note 2550, September 2013)
Initial semi-major axis a_orig = (-200000) AU (i.e. infinite, Kinoshita)
The actual computed value is the 1/a_orig, inverse of semi-major axis of the orbit before the comet entered the inner solar system (rh > 50 AU or so), but as we can see from one update to the other or depending on the calculator, its value is 0 plus or minus some uncertainty and we can only tell that the comet comes from as far away as the Oort cloud (a~20000-50000 AU).
So a_orig may be overestimated although on the right order of magnitude: is it significant?. Could it be that a_orig overestimate is due to non-gravitational forces not taken into account yet?
While the great comet of 1680 shows a much shorter orbit (aphelion at 860 AU), although in a similar orbital plane, comet Seki-Lines is more like ISON, beiing also a newcomer, in an orbital plane not very far (5°-20° difference on orbital elements) and with also a close approach to the Sun. Its evolution is interesting to compare, although it was discovered less than two months before perihelion.

Non-gravitational accelerations and nucleus size:

It is also possible that non gravitational accelerations due to the outgassing of the nucleus are perturbing the comet: this will be effective if the comet nucleus is small, and was indeed observed for comets C/1999 S4 (LINEAR) and C/2010 X1 (Elenin), also newcomer but which disintegrated:
C/1999 S4 (LINEAR): 1/a_orig = 0.000002 AU-1, A1= +9.11, A2=-1.58 (Kinoshita)
C/1999 S4 (LINEAR): 1/a_orig =-0.000055 AU-1, A1= +9.26, A2=-1.71 (MPC)

C/2010 X1 (Elenin): 1/a_orig = 0.000009 AU-1, A1= -3.19, A2= +5.33 (Kinoshita)
C/2010 X1 (Elenin): 1/a_orig = 0.000013 AU-1, A1= -3.97, A2= +5.90 (Nakano note 2121)

A1 and A2 are in units of 10-8 AU/day2 and are usually of the order of less than unity (e.g. +0.20 and +0.03 for comet 103P/Hartley 2 which is 2.0x0.4 km in size) and need generally to be measured on a long period (several orbits for periodic comets). This suggests that large values mean that the ratio of outgassing rate to nucleus size is very large. As long as non-gravitational parameters are not necessary to fit a correct orbit to the astrometric measuremnts of comet ISON, this is a positive sign of a reasonnably not too small nucleus!
But non-gravitationnal accelerations were introduced very late in fitting those two orbits:
Until rh=1.05 AU, C/1999 S4 (LINEAR) (MPC 40822) and C/2010 X1 (Elenin) (Nakano note NK2114) astrometric measurements did not use any non-gravitational acceleration to fit an orbit: they were introduced in the next orbits (MPEC 2000-O02 and NK 2121, respectively) for observations at heliocentric distances less than 0.83 AU.
MPC orbits:
Great comet 1680 V1
Comet C/1962 C1 (Seki-Lines)
Comet C/2012 S1 (ISON)
S. Nakano Notes:
Oct. 2013 orbit of C/2012 S1 (ISON)
K. Kinoshita Orbits:
May 2013 orbit of C/2012 S1 (ISON) (needs update?)
P.Rocher Notes cométaires (IMCCE):
Oct. 2013 orbit of C/2012 S1 (ISON)

How Bright will it be? Prévisions de luminosité?

Comparison to other dynamically new comets: (English version + Bortle limit) October french version
This illustrates the problem of predicting the brightness of comet. Beyond rh=1.5 AU from the Sun (log(rh)=0.2), comets can have a very different behaviour. Old comets (long period ones, and even more short period comets) can brighten fairly quickly as they approach the Sun beyond 1.5-2.5 AU. Especially for old comets depleted in CO, they "switch on" very late and their magnitude can decrease faster than as 10*log(rh) (brightness increase as 1/rh^4). New Oort cloud comet (C/2006 P1 seems not to have acted as this) switch on farther away - ISON may have done it between 10 and 5 AU - and then step down until water can sufficiently sublime around 1.5 AU. But as we can see, there is no clear trend.
Below rh=1.5 AU, all comets seem to increase brightness in a more similar way: m1 proportionnal to 8*log(rh) or so (brightness increase as rh^-3.2) and "absolute" magnitude at Rh=Delta=1 AU may reflect the true size of the nucleus: km size around mh=6, hectometer size around mh=9 and those later ones are too small to survive approaches significantly less than 1 AU from the Sun and exhaust all their volatile between 0.8 and 0.5 AU from the Sun. If ISON is that small it may fully disintegrate by mid-november.

How active is it? Quelle est son activite?

Measuring its outgassing rate can tell us a bit on its size. Several measurements have been done in March-June 2013 but with only marginal or upper limits on the outgassing measured:
QCN = 2.2E24 molec./s (4 May, rh=3.86) by D.G. Schleicher IAUC 9257
QCN = 1.3E24 molec./s (5 Mar., rh=4.57) by D.G. Schleicher IAUC 9254
QH2O < or = 2E27 molec./s (30 jan. rh=4.94 AU) (SWIFT)
Other measurements: QH2O < 4E26 molec./s (8 Mar. rh=4.54 AU)
Other measurements QCO = or < 2-4E27 (HST, Spitzer, IRAM, March-June at rh~4 AU) CBET 3598 )
QCO2 < or = 2E26 molec./s (13 June, rh=3.35 AU) (Spitzer)
For comparison, we can take comet Hale-Bopp with a nucleus of about 70km diameter and over 20% active area (i.e. ~3000km2) with its measured outgassing rate at similar heliocentric distances (CO2 ~1/3 CO) (Hale-Bopp production rates, Biver et al. 2002, EMP 90, 5), HCN(CN), CO2, CO, H2O were respectively about 350x, >250x, >50x, >100x higher, so we can estimate that the active surface of ISON was probably no more than ~10 km2. It can be the surface of a ~2 km diameter 100% active nucleus, but if most of the outgassing comes from icy grains lifted off the surface (like for 17P/Holmes during its outburst) the nucleus can be smaller.

QH2O ~ 1.1E28 molec./s (14 Sep., rh=1.95 AU) by D.G. Schleicher IAUC 9260
This is about 1/100th of Hale-Bopp at the same distance and around 1/6th of that of Hyakutake, so probably compatible with a 1-2km diameter nucleus.
QH2O ~ 1.5E28 molec./s (4 Oct., rh=1.58 AU) by D.G. Schleicher
Again this is on the order of 1/6th or a little less than Hyakutake at the same rh
QOH ~ 2-5E28 molec./s (5-8 Oct. rh=1.5 AU) according to H. Weaver with HST and A. Lovell with Arecibo (preliminary information circulated at DPS)
=> On a better slope this time probably!
QOH=3+-1E28 molec./s average 4-28 oct. (rh=1.3 AU) from Nançay... but much less from Trappist (La Silla, Université de Liège)
QH2O = 2E28 molec./s (8.9 and 21.9 Oct., rh=1.50--1.23 AU) by H. Weaver et al. (CBET 3680)

QH2O = 1.6--1.1E28 molec./s (22--29 Oct., rh=1-23--1.08 AU) by M.Mumma et al. and N. Dello-Russo et al. (IAUC 9261, CBET 3686,...)

These are not very good signs at all: the activity of the comet is not increasing while it is getting closer to the sun: Surface area decreasing? Nucleus shrinking quickly?
8.3 Nov.: QHCN~2E25 with IRAM-30m, still weak (much weaker than Lovejoy)
9-11 Nov: QHCN=1.5 to 2.2E25 with IRAM-30m: fluctuations but no global change from day to day
12.5 Nov.: QHCN~2E25 with IRAM-30m, still weak (while Lovejoy keeps on increasing)
13.3 Nov.: QHCN~5E25 (marginal detection but strong increase)
13.4 Nov.: QOH=4E28 (Nancay) (4x increase - CBET)
14.3 Nov.: QHCN~40E25 (huge increase - CBET), but signal peaks 7-10'' tailward: is it a sign of disintegration? (particles or nucleus affected by outgassing reaction (non-gravitational accelerations) are accelerated tailwards)

Plot of the evolution of production rates (mostly from SWAN, Combi et al. 2009, Makinen et al. 2001, for older comets; assuming QH2O=1000xQCN or 1000xQHCN,... if not OH or H2O direct measurement) of comet C/2012 S1 (ISON) and two other Dynamically New comets:

I have added a simulation of a 250m radius body with low density and high surface production rate to +-fit the QH2O(ISON)... this makes it disappear on 14 Nov.... French version without the model

What is the size of its nucleus? Quelle est la taille du Noyau

Direct measurements with Hubble space telescope (e.g. Li et al. DPS 2013, CBET 3496) or Spitzer could not detect the nucleus but put an upper limit of about 4km diameter.
Outgassing rates depends on its active surface area, but if it is close to 100% as for young comets, then it may be around 2km diameter, and even less if a large fraction of the outgassing observed comes from a cloud of icy grains (e.g. 103P/Hartley 2 displays effective surface area for water sublimation of 200% to 600% in 2010-1997). So it can even be as small as a few hectometers in diameter.

One way to have a very rough idea of the size of a comet nucleus, is to estimate it from its outgassing rate at 1 AU from the Sun:
about QH2O = 10^28 molec./s of water can be sublimed from the heat received from the Sun by 1km2. And the correlation between visual magnitude and water production rates (Jorda et al. ACM 2008) gives:
log(QH2O) = 30.68-0.245mH
=> a 100% active nucleus of 2.5km radius will produce 2E29 molec./s have a magnitude (at 1AU from Earth) mH=5.6 (Like Kohoutek, C/2002 T7, Hyakutake, C/2006P1 (McNaught)?)
=> a 100% active nucleus of 1km radius will produce 3E28 molec./s have a magnitude (at 1AU from Earth) mH=9 (Like comets C/1999 S4, Elenin, Hoenig, which disintegrated but may have been over 500% "active"):
=> a "500% active" nucleus of 0.4km radius will produce 2.5E28 molec./s have a magnitude (at 1AU from Earth) mH=9.3

Nucleus lifetime:
Assuming a density of 0.5g/cm3, 50% ice, "500% active" (i.e. large fraction of outgassing from icy grains lifted off the surface) nucleus of 800m diameter will exhaust all of its material in 1000 days at 1AU, 260 days at 0.5 AU, 10 days at 0.1 AU. In such a case a 1km nucleus cannot survive to a perihelion distance much less than 0.1 AU.
M. Knight and K. Walsh ( have estimated that if the nucleus is larger than 1km, dense enough and does not rotate too fast, it may survive perihelion (not being destructed by tidal forces) - we may be on the good side, now.

Another upper limit on the nucleus size: diameter < 1 km

in CBET 3720 W. Delamere et al. report on the upper limit on the light reflected by the nucleus as observed by the HiRISE camera on MRO in Mars orbit around 1st of october when the comet was close to Mars, and find mV < 16.5 in 0.2'' aperture and deduce a ~1km diameter upper limit.

Integrating roughly the mass lost (based on the production curve through perihelion) during the mid-september to 29-november period, assuming M_ice/M_dust = 1 and density of 0.35 g/cm3, I find a diameter of around 900m.
so we can conclude that the nucleus of comet ISON was very likely in the 0.5-1km diametre range and completely lost its volatiles as it reached perihelion.

How the comet will look like if it survives until perihelion

Rough simulation of tails appearance:

Here are rough estimates of the possible appearance of the dust tail of ISON:

! ! ! ! ! ! ! ! ! !
Date Tail length Apparent length PA tail (particles age)
01-Oct. 0.017 AU 0.20° 296° 20 days
06-Oct. 0.018 AU 0.26° 296° 20 days
11-Oct. 0.020 AU 0.34° 296° 20 days
16-Oct. 0.023 AU 0.45° 296° 20 days
21-Oct. 0.026 AU 0.61° 297° 20 days
26-Oct. 0.030 AU 0.84° 297° 20 days
31-Oct. 0.035 AU 1.2° 297° 20 days
5-Nov. 0.042 AU 1.8° 297° 20 days
10-Nov. 0.051 AU 2.6° 296° 20 days
15-Nov. 0.065 AU 4.0° 295° 20 days
20-Nov. 0.09 AU 5.9° 290° 20 days
25-Nov. 0.14 AU 8.9° 280° 20 days
01-Dec. 0.03 AU 1.7° 355° 1.5 days
03-Dec. 0.10 AU 6.5° 350° 3.5 days
05-Dec. 0.20 AU 15° 350° 5.5 days
07-Dec. 0.30 AU 24° 355° 7.5 days
09-Dec. 0.30 AU 27° 360° 9.3 days
10-Dec. 0.29 AU 28° 10 days
15-Dec. 0.30 AU 32° 14 days
20-Dec. 0.31 AU 35° 18 days
25-Dec. 0.33 AU 34° 22 days
30-Dec. 0.29 AU 25° 26 days

Links to other ISON dedicated web pages

Comet ISON "NASA" observing campaign
Comet ISON webpage by G. Cannat (in french) - lettre spéciale "Guide du Ciel" sur la comète ISON
Comet ISON webpage by Gilbert Javaux and Cedric Bemer (in french)

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