An apparently asteroidal object of magnitude 18.7m on a cometlike orbit (P=48.5 years), discovered by LONEOS on Aug. 13, 2001 near the border Pisces/Aries showed no sign of cometary activity despite numerous observations. Now approaching perihelion, the object has finally shown cometary activity during January 2002, displaying a 0.8' coma and a broad 0.9' long tail. Comet C/2001 OG108 (LONEOS) will pass perihelion (situated near the earth orbit) in mid-March 2002, approaching earth in mid-April (minimum distance: 0.56 A.U.) (IAUC 7814). According to the very few observations in January a maximum brightness of about 9m can be predicted during those days. Earth crossed the orbital plane on March 31. On Apr. 9 it passed the North Pole at a distance of only 2.5°.
This comet was visually observed until mid-May. A total of 17 reports by 6 group members were received. For the analysis 140 international observations could be included. The brightness evolution was different pre- and post-perihelion, with no sudden change but a smooth alteration from the first to the second one. The heliocentric magnitude decreased more rapidly after perihelion than it increased before. This could be a hint that this object possesses only one active region, which either possesses only a small quantity of gas/dust or approached the unlit area of the nucleus after perihelion. Maximum brightness of 9.8m occured on Apr. 10. The evolution can be best simulated with the following formulae:
Total Brightness and Coma diameter
The coma diameter was less than 1' (60.000 km) at the beginning of this apparition, increasing to 3' (110.000 km) until the first week of April. Thereafter it decreased nearly as quickly and was about 1.3' (60.000 km) in mid-May. During the apparition the coma showed a continuing trend to get more diffuse, with the degree of condensation near 5 at first, decreasing to 2 in mid-May. Tail reportings (up to 3') were extremely rare.
Andreas Kammerer
