C/1999 X3 = 2004 E2 = 2008 K10

C/2004 E2 was found by me on 2004 March 12 in realtime C2 images 08:06 - 10:30 UTC. It was confirmed by Karl Battams on March 15 as SOHO-746. MPEC 2004-M42 was issued 2004 June 23 with the official designation C/2004 E2 and these orbital elements:

C/1999 X3 was found by Hua Su on 2006 May 26 in archival C2 images of 1999 December 12 13:31 - 16:06 UTC. Karl Battams noted: "Hua Su's non-group comet looks good to me -- well done!! It is fairly small and condensed, and looks to be brightening as it reaches the edge of the image. I would guess at maybe mag 7.5 - 8.0. I think it is too small for C3, but I will check next week.". Later (IAUC 8982, 2008 September 26) the comet was described as "being near the limit of visibility in C2 images". The comet received SOHO number 1157 on May 31. MPEC 2006-L20 was issued 2006 June 02 with the official designation C/1999 X3 and these orbital elements:

Nothing indicated that both comets could be identical.

C/2008 K10 was found by me on 2008 May 31 in C2 images of May 30/31 23:06 - 00:54 UTC. It was confirmed by Karl Battams on June 13 as SOHO-1483 and described as "small and stellar in appearance (mag 7-7.5, brightening as it left the C2 field-of-view but never appeared in C3)" (IAUC 8982, 2008 September 26). MPEC 2008-O16 was issued 2008 July 22 with the official designation C/2008 K10 and these orbital elements:

After I had found C/2001 D1 = 2004 X7 = 2008 S2, I searched for other unrecognized identities among the non-group comets. I looked for comets with low inclinations, short observational arcs and similar time intervalls between their perihelion passages. I found C/199X3, 2004 E2 and 2008 K10 on September 24. The orbital solution converged rapidly with small residuals (RMS = 9.6") and I sent my solution to Brian Marsden at 21:11 UTC. Less than two hours later MPEC 2008-S49 appeared with the identification and these orbital elements:

Brian Marsden noted that "The object passed 0.058 AU from the earth on 2000 Jan. 13, 0.032 AU from Mars on 2004 May 19 and 1.17 AU from Jupiter on 2003 Feb. 1.".

At the apparitions in 1999 and 2004 the object passed behind the sun with phase angles of 21 - 31 degree (1999) and 35 - 20 degree (2004), while in 2008 it passed in front of the sun with phase angles of 162 - 147 degree. With the model of the phase dependence of coma brightness due to dust from Marcus as used by Matthew Knight in his thesis with a dust-to-gas ratio of 0.16 for the C2 orange filter the correction to a normalized phase angle of 90 degree is 2.7 - 1.2 magnitudes for C/2008 K10 and amounts to less than mag 0.05 for C/1999 X3 and C/2004 E2.
The descriptions of brightness for C/1999 X3 are a bit discordant and there is none for C/2004 E2, so I have measured all images of the three apparitions:

My measurements for C/1999 X3 (black dots), 2004 E2 (blue), 2008 K10 (red) without the correction for phase.
C/2008 K10 appears to be a small amount brighter than C/1999 X3. This agrees well with Karl Battam's first statement about the brightnes of C/1999 X3 (maybe mag 7.5 - 8.0) compared with that for C/2008 K10 (mag 7-7.5), which makes C/2008 K10 about mag 0.5 brighter than C/1999 X3.

Same as before, but C/2008 K10 corrected for phase with a dust-to-gas ratio of 0.16

Matthew Knight has noted in his thesis the possibility that the phase correction is too severe for the Marsden (and most likely Kracht) comets and that a larger fraction of the observed light for these comets may come from the gas in the coma, likely sodium. This could apply to other non-Kreutz comets. With a dust-to-gas ratio of 0.05 the light curve of C/2008 K10 looks like this:

Same as before, but C/2008 K10 corrected for phase with a dust-to-gas ratio of 0.05

Same as before, but C/2008 K10 corrected for phase with a dust-to-gas ratio of 0.02

From the limited data used and my brightness measurements it seems that this comet has indeed a very low dust-to-gas ratio. But it is possible that the Marcus model is overcorrecting at large (>150 deg) phase angles (footnote to C/2000 C3, C4, C7 in appendix C of Knight's thesis). As C/2008 K10 was observed at phase angles of 162.4 to 147.4 degree, the true dust-to-gas ratio could be greater than indicated by the light curves above.

Rainer Kracht, 2008 February 20

The Return of 2012

The comet was found again by Peiyuan Sun on 2012 August 20 in LASCO C2 images. As an already known comet it did not receive a SOHO number. 32 positions were measured by Karl Battams in 2015 May and they were published with a pure gravitational solution on MPEC 2015-K01. My best fit for the observations 1999 - 2012 uses a smal nongravitational parameter: A2 = -0.0003. Orbital elements from this solution:

There was a close approach to Jupiter with 0.272 au on 2015 February 12.

Rainer Kracht, 2015 May 21