A discovery in a record-breaking distance from the sun was managed by the NEAT team on Aug. 24, 2001. Comet C/2001 Q4 (NEAT) of magnitude 17.5m, situated near the border of Fornax/Eridanus/Cetus, was then at an distance of 10.1 A.U. from the sun, nevertheless showing an 8" (55.000 km) coma! According to the current elements it will pass perihelion not before May 16, 2004 at a distance of 0.96 A.U. from the sun. On its way towards perihelion the comet will pass earth in a distance of only 0.321 A.U. on May 7, 2004. Of course it is still a very long time until then, so only very crude perspectives can be drawn about the performance it will give. According to the last scientific results this comet seems to be not a new one, so n=3 is the most promising scenario. In this case comet C/2001 Q4 will achive a maximum brightness of about 3.5m during the days of earth passage. According to my empiric formulae it should then show a coma of about 30' and a 6° visual tail. Supposing a more optimistic scenario (n=4) maximum values 1.0m/60'/25° result, in the case of a more pessimistic scenario (n=2) the maximum values read 6.0m/15'/1°. In every case, this comet will not become visible for European prior to the date of closest approach to earth, displaying only the declining part of its apparition to these observers.
The comet did not satisfy the very high expectations. It seems that for southern hemisphere observers it was a rather noticeable naked eye object, but not a prominent one. For mid-European observers it was too low at the time of maximum brightness. In the days following the increasing altitudes were compensated by the decreasing brightness; thus the comet was only noticeable when an observer knew its position.
The brightness evolution according to 116 reports by 10 members of the German Comet Section and 1215 international reports shows two main phases, pre- and postperihelion, respectively, centered rather symmetrically around the date of perihelion:
t < -90d: m = 3.7m + 5 × log D + 10.2 × log r
-90d < t < 0d: m = 5.6m + 5 × log D + 2.7 × log r
0d < t < +85d: m = 5.8m + 5 × log D + 4.9 × log r
t > +85d: m = 4.1m + 5 × log D + 12.0 × log r
The resulting maximum brightness was 3.0 mag around May 8. The brightness did not evolve smoothly within the mentioned periods. For instance, the heliocentric brightness showed a local minimum between April 20 and May 5, thereafter increasing rapidly until perihelion.
Evolution of the heliocentric brightness pre-perihelion
Evolution of the heliocentric brightness post-perihelion
The apparent coma diameter measured only about 1' during the first seven months of the apparition. Not until February 2004 a significant increase was noticed. After that the coma increased steadily, reaching its maximum of 25' in mid-May (closest Earth approach at 0.321 AU was on May 7). Thereafter it decreased rapidly, reaching 10' in mid-June. Since then the decrease became more moderate. At the start of September the estimates were around 7', reaching 2.5' at the beginning of November and 1.0' at the end of January 2005. Thereafter the decrease grew again more rapidly, reaching values of 3' at the start of November. The absolute coma diameter measured 150.000 km during the first months. Until the last week of March it increased to 550.000 km, only to decrease to a temporary minimum of 325.000 km until May 7 (probably biased due to perigee and thus not real). Thereafter it increased again, reaching 450.000 km on May 20, 550.000 km at the start of June and finally the absolute maximum of 750.000 km during the last week of August. After that maximum the coma diminshed rapidly to 475.000 km in mid-September. Since then the decrease slowed: in mid-October the coma still measured 400.000 km, reaching 325.000 km at the start of November and 175.000 km at the end of January 2005. The degree of condensation remained constant at DC 5 until the beginning of March, thereafter increasing to DC 6-7 at the end of April. During the following weeks the degree of condensation decreased very continuously, reaching DC 3-4 at the start of August. Since then the decrease slowed, reaching DC 2-3 at the start of November and DC 1 at the end of January 2005.
Total Brightness and Coma diameter
Although a few visual tail observations were reported during fall 2003 (tail length of a few arc minutes), none was reported during the following months. Starting in mid-March a tail was reported again by the visual observers. The visual gas tail reached its greatest length around May 15 with 8° (9 Mio. km). It had diminshed to 2° (5 Mio. km) at the end of May and to 0.5° (2.5 Mio. km) at the end of May. In binoculars it was well recognizable, but not bright. A few observers reported a stubby dust tail in the first half of May. The tail was visible in binoculars throughout the month, but became considerably fainter in June. In March the tail pointed south, turning to a south-eastern direction at the time of perigee and further east during the following weeks.
Andreas Kammerer
