The first comet discovery of 1999 announced the Australian amateur Justin Tilbrook on Jan. 12. His second comet moved rather fast, hinting on a small distance to earth (0.6 AU). Comet C/1999 A1 (Tilbrook) of magnitude 10.5 was positioned in the southeastern part of Aquarius, displaying a coma diameter of only 1' with a hint of a false nucleus. CCD observations on the following day confirmed the small coma and showed a faint broad tail of 3' length and a false nucleus of magnitude 12.5 (IAUC 7084/85). The comet reached a minimum distance of 0.30 AU to earth during the Christmas days, when it should have peaked at magnitude 9.5. Thus, the very late discovery is astonishing. Either it is a rather unusual object (such as C/1998 K5) or it has experienced an outburst shortly before discovery. Based on 40 published observations the brightness evolution is not well documented, but can be roughly simulated with the parameters m0=12.0 mag / n=4. The diameter of the coma was in the order of 2.5'.
Total Brightness and Coma diameter
--------------------On May 26, 1999 the LINEAR-team discovered a comet near the border of the constellations Cygnus/Pegasus. The 17 mag comet C/1999 K8 (LINEAR) showed a 30" coma (IAUC 7182). It will pass perihelion at the start of 2000, expected to reach 14-15 mag. The comet showed a great absolute magnitude and a rather high degree of activity (considering the great perihlion distance): m = 0.5m + 5×log D + 15.5×log r, peaking at magnitude 13.2. The coma diameter increased from 0.5' (120.000 km) at the start of the apparition to a maximum of 1.4' (250.000 km) in November 1999. Thereafter it decreased to 1.2' (220.000 km). The degree of condensation was very constant at DC 3.
Total Brightness and Coma diameter
--------------------On frames of Oct. 14, 1999, an object was found by the LINEAR-team, which revealed itself as a 16.5m faint comet in the constellation Pegasus. Comet C/1999 T2 (LINEAR) showed a short 20" tail at PA=100°. (IAUC 7280/81). Until the end of August 2001 24 observations by members of the German comet section were received which, together with 195 international observations, present the following picture: The brightness evolution can be described rather well by the following formula, indicating a surprisingly low activity factor n: m = 9.2m + 5×log D + 2.9×log r. Thus this comet reached a first maximum of 13.0m in August 2000 and a second one at 12.8m in mid-April. Assuming that the activity factor remains low, this comet may be observable with amateur instruments even through 2002! The coma diameter increased from 0.7' (135.000 km) at the start to 1.3' (200.000 km) in 2000 - with a slight decrease to 1.1' (175.000 km) during summer. During spring 2001 the coma seems to have increased temporarily from 1.3' (175.000 km) to 1.7' (210.000 km). The coma was moderately condensed, the DC-value decreased slightly from DC 4 to currently DC 3. In April/May 2001 K. Hornoch succeeded in detecting an anti-tail of maximum length 7' in his CCD-exposures.
Total Brightness and Coma diameter
Andreas Kammerer
