The orbit of C/2005 S2 (Skiff)

October 1^st 2005

There has been a great deal of interest in the orbital solution published on MPEC 2005-S78 that suggests that this is a large comet that is currently at r=12AU and that is still 5 years from perihelion. However, in the absence of pre-discovery observations the orbital arc is extremely short and the MPC advice that the orbit is extremely provisional has not been as widely observed as necessary. It is likely that the true orbit is very different to this initial one and the only thing that is certain is that the comet is distant and beyond Jupiter.

Ramón Naves has calculated various orbital solutions that include his own observations and that extend the arc slightly. These give substantially different solutions, but all have various things in common:

• The resultant orbit is prograde and of low inclination rather that the retrograde solution in the provisional orbit;

• The comet is receding from perihelion and that perihelion probably took place in 2004;

• Three of the four solutions give a Halley-type orbit of period 30-80 years. The shorter the calculated period the earlier the corresponding date of perihelion.

The four solutions (ordered from greatest to shortest period only), which are still all extremely uncertain, are given below. The best residuals are found for a period of approximately 30 years.

Solution 1:

Perihelion 2004 Aug 31.795118 TT

Epoch 2005 Sep 27.0 TT = JDT 2453640.5

q 4.333380 (2000.0) P Q

H 8.9 Peri. 169.33306 0.77110419 0.63638850

Node 151.11304 -0.58720443 0.72305654

e 1.0 Incl. 2.39633 -0.24614886 0.26869854

From 21 observations 2005 Sep. 25-30; RMS error 0.727 arcseconds

Solution 2:

Perihelion 2004 May 11.584848 TT

Epoch 2005 Sep 27.0 TT = JDT 2453640.5

M 6.67024 (2000.0) P Q

n 0.01324999 Peri. 163.44486 0.76194981 0.64735336

a 17.6871926 Node 156.18009 -0.59943516 0.71611753

e 0.7236961 Incl. 2.71551 -0.24517333 0.26097758

P 74.39 H 8.4 q 4.8870394

From 20 observations 2005 Sep. 25-30; RMS error 0.730 arcseconds

Solution 3:

Perihelion 2004 Jan 31.800711 TT

Epoch 2005 Sep 27.0 TT = JDT 2453640.5

M 15.30469 (2000.0) P Q

n 0.02533053 Peri. 158.74467 0.74505565 0.66675376

a 11.4825879 Node 159.40452 -0.61918420 0.70153325

e 0.5409397 Incl. 2.96685 -0.24799796 0.25157607

P 38.91 H 8.0 q 5.2712002

From 21 observations 2005 Sep. 25-30; RMS error 0.720 arcseconds

Solution 4:

Perihelion 2003 Nov 24.961760 TT

Epoch 2005 Sep 27.0 TT = JDT 2453640.5

M 21.32112 (2000.0) P Q

n 0.03172605 Peri. 155.82211 0.72984583 0.68338389

a 9.8823147 Node 161.03490 -0.63571706 0.68797926

e 0.4448637 Incl. 3.11309 -0.25137397 0.24427644

P 31.07 H 7.8 q 5.4860309

From 22 observations 2005 Sep. 25-30; RMS error 0.706 arcseconds

Post data:

When additional data from Juan Lacruz (MPC J87) are also added, an almost circular orbital solution appears that looks strikingly similar to the orbit of 29P/Schwassmann-Wachmann 1:

Perihelion 2001 Jan 8.291438 TT

Epoch 2005 Sep 27.0 TT = JDT 2453640.5

M 93.05407 (2000.0) P Q

n 0.05401614 Peri. 106.87699 0.06869555 0.99752626

a 6.9308579 Node 167.03482 -0.93874730 0.06969160

e 0.0530461 Incl. 3.81010 -0.33768982 0.00918834

P 18.25 H 7.3 q 6.5632024

From 25 observations 2005 Sep. 25-30; RMS error 0.752 arcseconds

The tendency of the data suggests that a relatively low eccentricity and perihelion beyond the orbit of Jupiter are probable.