On July 2 the Australian amateur Vello Tabur discovered his second comet. Comet C/1997 N1 (Tabur) was then a 10m object with a 1.5', weakly condensed coma near the borders of Lepus and Eridanus. CCD-observations showed a 2.5' tail in PA=210° (IAUC 6692). The first orbital elements (IAUC 6694) indicated a perihelion distance of only 0.38 AU. Assuming a standard formula for the brightness development the comet was expected to reach 5.5m at perihelion, but around this date would not have been visible from earth, because earth and comet were at opposite sides of the sun. European observers were expected to have their first glimpse around the start of September, with the comet estimated to be around 8m. Comet Tabur, however, did not fulfill this expectations but rather followed its successor of 1996. During July the comet did not increase in brightness, even though it approached the sun. And CCD-observations by G.J. Garradd showed it to get more and more diffuse by the end of July. Several searches for this comet in September did not succeed and no photograph showed any trace of it.
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On July 22 Justin Tilbrook discovered its first comet in the constellation Corvus. Comet C/1997 O1 (Tilbrook) then was a 10m object with a 1.5' coma (IAUC 6705/07). It went through perihelion in mid-July but until now could only be detected very low above the European horizon. Not until mid-November will it climb higher, then in the morning sky. However, the observations so far published indicate a low value for n. Thus, the comet seems to fade very slowly giving a chance to European amateurs with greater instruments at the end of this year.
In October Comet C/1997 O1 (Tilbrook) could already be glimpsed from the northern hemisphere at very low altitudes. Starting in mid-November, however, this comet will quickly attain altitude in the morning sky. The estimates published so far hint towards a very small value of n, indicating a slow decline in brightness. The brightness evolution can be described by the formula m = 8.5m + 5×log D + 5×log r, which however is still very uncertain, due to the small number of observations. Comet Tilbrook showed a medium condensed (DC 3) coma of 130.000 km diameter.
Total Brightness and Coma Diameter
Andreas Kammerer
