On Dec. 10, 2010 Leonid Elenin discovered a comet of magnitude 19.0 in Virgo using a 45cm-remote-telescope. Comet C/2010 X1 (Elenin) showed an 8" tear-dropped coma with a central condensation of magnitude 20.5 and a 10-12" tail in p.a. 298°. This comet will reach perihelion on September 2011, expected to reach 6 mag. This brightness should stay constant until Oct. 20, when the comet will pass earth at a distance of 0.22 AU (IAUC 9189 / MPEC 2011-A38). For mid-European observers the comet will become visible above the eastern morning horizon (situated in the constellation Leo) during the first week of October. Until the end of the year it will move through Cancer, Gemini, Auriga, Taurus into Aries, thereby fading to 14 mag.

The expectations aroused by this comet deflagrated at the end of August 2011. There will not be an interesting binoculars object in the mid-European autumn morning sky, since the comet disintegrated. First indications of this process were noticed by Michael Mattiazzo (Australia) while comparing his photos of Aug. 17 and Aug. 22, which showed a significant decrease in brightness. On Aug. 27 the false nucleus appeared elongated and quite diffuse, reminding him to the disintegration of comet C/1999 S4. R.H. McNaught was able to confirm this. Using the 0.5m Uppsala telescope no central condensation was discernable (IAUC 9226). During the following days the comet became more and more diffuse in appearance and fainter, which was impressively documented by Michael Mattiazzo (see his website). On Sep. 14 the comet could only barely be detected. However, in September the comet could only be glimpsed low above the horizon.

The reappearance ot the comet after perihelion was eagerly awaited by all comet enthusiasts. On Oct. 9, Juan Gonzales announced the visual detection of the remnant from his mountain site, estimating the brightness of the extremely diffuse 6' patch of light as 10.7 mag, noticing also a 0.2° tail. However, the skepticism was great, because his observation was supported by only one additional visual sighting, whereas no image, taken with large instruments, showed any sign of it. At this time, however, the comet was still low above the horizon and the Moon brightened the sky considerably. Then, on Oct. 21, Juan Gonzales announced a further sighting. He reported an extremely diffuse, tail-like object of 0.4° length with a total brightness of roughly 10.2 mag, oriented in p.a. 310°. The object showed two nearly identical areas of similar brightness (one of them situated near the expected position of the coma) of 7' diameter, which were marginally more condensed. Finally, on the same day and the following observers also succeeded in capturing the remnant in deep images. On these the remnant is documented as a 90x10' extremely diffuse patch of light, only marginally brighter than the sky background, without any condensation, being better defined on the sunward side.

The apparition of this comet is documented by two observations of two members of the German Comet Section and by 115 international observations. According to these the brightness increased steadily from 15.0 mag to the maximum of 8.2 mag on Aug. 15, describable by the formula

After Aug. 15 the diagram clearly shows the rapid decline of the brightness due to the disintegration. The observations after perihelion, shown in the diagram, belong to the remnant and should better be regarded as an tail than a coma. The steep decline of the coma near perihelion is primarily caused by the deteriorating observing conditions.

Total Brightness and Coma Diameter

During the apparition the apparent coma diameter increased from 0.5' to almost 4'. The absolute coma diameter increased from 40.000 km during the first days of April to 250.000 km on Aug. 1. After that date the shrinkage begun, with the coma diameter measuring not more than 125.000 km on Sep. 2, most probably a consequence of the disintegration but partly also due to the increase of the solar wind. Interestingly the coma become ever more diffuse during the first weeks of the apparition. The DC value decreased from DC 6 around Apr. 20 to only DC 2-3 in mid-July, followed by a brief increase to DC 4-5 until Aug. 15. Thereafter the DC value decreased drastically to DC 1 on Sep. 1 and DC 0 on Sep. 5.

Degree of Condensation

According to Z. Sekanina the disintegration of the comet started on Aug. 16 (+/- 4 days) with a short-lived dust-emission event, which coincided with the brightness maximum. Because the brightest area of the remnant lies on the line-of-variation, it apparently represents the location of the most sizable debris ejected during the mid-August dust-emission event. Since then the fragments, being in the order of several meters at the beginning, soon must have given birth to ever smaller fragments in a cascading fashion. The largest debris surviving at the end of October is perhaps in the centimeter range. The mass of the dust cloud comes out to be in the order of 10⁹ kilograms (CBET 2876).

Andreas Kammerer