On Apr. 16, 2010 Jan Vales, reported the discovery of an unusually bright (12.5 mag) asteroidal object in northeastern Virgo, only 15° from opposition, observing with the 0.60-m reflector at Crni Vrh, Slovenia. While the discovery images show a distinct stellar profile, follow-up images with the same telescope on Apr. 16.84 UT show that the object had become diffuse with a flatter profile than those of nearby stars of similar brightness. The numerous observations reported after posting on the Minor Planet Center's 'NEOCP' webpage soon made it clear that the object was some 2.0 AU from the earth and therefore of absolute magnitude 8-9. The object was not present (down to V magnitude 20) in frames of the relevant region obtained by the Catalina Sky Survey as recently as Apr. 15.4. Several observers in the day after discovery indicated that the new object appeared slightly but consistently larger than nearby field stars of similar brightness- with a coma becoming noticeable and growing in size over the next several days. Images by P. Birthwhistle on Apr. 17.1 UT show the coma to be of size 26x24", increasing to 38x34" on Apr. 18.1 UT. K. Hornoch measured a strongly condensed coma of size 65x50" some hours later. Visual observers reported a magnitude of 11.4 and a 1.0x0.8' coma on Apr. 19. E. Guido and G. Sostero compared the appearance to that of comet 17P/Holmes in the early phases of its Oct. 2007 outburst (IAUC 9137/39 und CBET 2249/53 sowie MPEC 2010-J87).

This suggestion is supported by 7 observations by 4 members of the German Comet Section and 55 international observations (until the beginning of September 2010). They show a strong similarity to the dust outburst of comet Holmes - although at least one magnitude smaller than that. There is, for once, the brightness, which increased in less than a day from fainter than 20 mag to 11.8 mag, which also indicates the maximum brightness. The extremely fast brightening was followed by a considerably slower fading which can be well described by the formula

The coma diameter showed a linear increase during the first 10 days to 1.5' (140.000 km), followed by a non-linear increase to 3' (300.000 km) until May 20. This comes as no surprise since the outer coma layers will arrive much earlier at sky background levels at magnitude 12 than at magnitude 2 (as in the case of comet Holmes). Thus, during the first days the coma increased at a rate of 0.15' = 14.000 km per day. Also nicely fitting the behaviour of comet Holmes is the evolution of the degree of condensation: linear at first, asymptotic at the end, decreasing from DC 9 to DC 1-2.

Total Brightness and Coma Diameter

Degree of condensation

Andreas Kammerer

FGK observations