The Return of C/1999 J6 = C/2004 V9

Many attempts have been made to predict the returns of Kracht or Marsden group comets. All have failed with the single possible exception of the Kracht group comets C/1996 X3 = 2002 S7 = 2008 N4 (MPEC 2009-J14).
The two brightest Marsden group comets C/1999 J6 and C/2004 V9 were linked by Brian Marsden (MPEC 2004-X73) and Z. Sekanina and P. W. Chodas, an orbit for the 2010 return by S. Nakano is here.

The Nakano orbit has T = 2010 May 01.46288 (11:07) TT. The comet will enter the SOHO LASCO C2 field about 24 hours before perihelion.

How bright will it be? I have generated this plot with "Comets for Windows" from Matthew M. Knight's measurements:

C/1999 J6 (black dots) and C/2004 V9 (red dots)

24 hours before perihelion the comet seems to be mag 9 or fainter. Then it will be probably difficult to detect in SOHO LASCO C2. During the similar 1999 apparition the first detection in Knight's data was 1999 May 10 17:06 UT, 21 hours before perihelion. This corresponds to 2010 April 30 14:07 UTC:

If it's found again in the SOHO LASCO C2 images of 2010 April 30 then it will be perhaps possible to image the comet with earthbased telescopes in late May. With Nakano's orbit the comet will make a close approach to the earth about June 05.3 at a distance of 0.14 AU moving with 22.8"/minute. For May 24 the distance from earth is 0.30 AU and the motion is 5"/minute. It could be captured with series of 30 seconds exposures.

Uncertainties: While Nakano's orbit looks good with many observations and small residuals, a close approach to Jupiter on 2008 June 25 has enhanced the errors of any prediction for 2010. Additionally, rather small nongravitational forces can alter the perihelion time by several hours.

Most uncertain is the brightness at the earth approach after perihelion. In his dissertation "Studies of SOHO comets" Matthew M. Knight estimateted (at the low end) "If instead it is inactive and behaves like an asteroid of radius 30 meters it would be about magnitude 19 at closest Earth approach (ignoring phase effects)." The phase angle at closest approach is about 50 degrees, so one can expect a magnitude of about 20 for this case.
Z. Sekanina and P. W. Chodas have a similar estimate for C/1999 J6: "Regarding the comet’s brightness at the time of closest approach to Earth in 1999 June, we obtain an estimated apparent visual magnitude 16, corresponding to a diameter of 35.4 m (Table 20) at an assumed geometric albedo of 0.04 and zero phase; at the actual phase angle of nearly 90°, the magnitude should have been between 19 and 21, which for all practical purposes rules out its accidental detection at the time.".

An apparent magnitude of 20 poses no problem, if one knows where to look. Fortunately the comet will be at a solar elongation of about 90 degrees on 2010 June 01 and that's where WISE observes the sky. Observations after the 2010 perihelion earthbased or by WISE seem to be feasible.

C/2010 H3

Jiangao Ruan found a Marsden group comet in LASCO C2 images of 2010-04-19. Matthew M. Knight measured the brightness in C2 images of April 18 22:30 - April 19 13:54 UT and until April 20 16:18 UT in C3. The lightcurve is similar to those of C/1999 J6 and 2004 V9 "making their identities as the same comet very likely". Brian Marsden computed a common orbit for C/2004 V9 and the new comet designated C/2010 H3 (MPEC 2010-H37). The time of perihelion is 2010 April 19 21:28 TT about 11.6 days earlier than expected. Alan Watson located the comet in STEREO HI1A images beginning at April 20 15:29 - 20:09 UT. Wentao Xu measured the positions in HI1A until April 21 05:39 UT and made an animation showing the comet in HI1A.

I have found a nongravitational solution for C/1999 J6 = 2004 V9 = 2010 H3 with A2 = -0.1898 using only SOHO data.

Karl Battams has extended the observed arc to April 22 10:24 in HI1A with his measurements, that is until 61 hours after perihelion. He remarked that "it drifted in and out of the background noise", possibly a sign of an active region rotating in and out of view.

Closest approach to the earth will be on May 29.45 with a distance of 0.317 AU (moving with 9.5"/minute). Earthbound observations will be difficult due to the greater than expected distance and the nearly full moon. The closest angular approaches to the moon after perihelion are on May 10.24 (3.5 degree, moon mag -9) and June 01.44 (9.1 degree, moon mag -12).

After perihelion the comet will appear in the morning sky. A solar elongation of 30 degree is reached on May 04 (40 deg May 08, 50 deg May 12, 90 deg May 24). After the May 10 conjunction with the moon, the comet will be favourably placed for observers on the southern hemisphere at the beginning of astronomical twilight.

The time for observations from WISE is now around May 24. Perhaps it can be found with groundbased telescopes before that date.

Brian Marsden found an excellent solution (including the nongravitational parameter A1) for P/1999 J6 = 2004 V9 = 2010 H3. He estimates the uncertainty of the resulting ephemeris as only about one arcminute. The magnitude to expect is still very uncertain ("astrometry and photometry would be very desirable").