On May 26 comet C/1998 K5 (LINEAR) was announced. This comet was reported to be of magnitude 18 (later revised to 16m) and star-like in the constellation Ophiuchus. The first hint to a cometary nature came from calculations, showing the object to be moving on a highly eccentric orbit. The first observations thereafter, however, did not show any more. But in the course of very deep exposures a very weak, 0.5' long tail could be discerned (IAUC 6923). But what a surprise it was when additional observations yielded an object not far from the earth (distance about 0.3 AU)!
Until now the observers of the german comet section reported 14 observations. Adding 115 international observations the resulting brightness behaviour is extraordinary. The heliocentric magnitude increased until about 40 days after perihelion, yielding a total brightness of 12.3m. Since then the heliocentric magnitude decreased, but much slower than the increase took place.
t < 40d: m = 15.3m + 5×log D + 0.047×|t-T|
t > 40d: m = 13.0m + 5×log D + 0.010×|t-T|
Total Brightness and coma diameter
Not until the second half of August the DC value fell below DC 9, followed by reports of several observers of a mignon coma (>0.3'). It therefore seems very plausible that the comet did not exhibit a visually visible coma until this time. Perhaps not coincidentally this reports start about 40 days after perihelion, when the evolution of the brightness changed. This strange behaviour could be the result of only one active region, which was quite weak during the earth passage (star-like object with "tail"), but getting more active in the following weeks, producing a mignon, visually visible weak coma. The activity topped about 40 days after the perihelion passage, thereafter decreasing slowly, due to the increasing sun distance. In this scenarion the "coma observations" prior to mid-August actually would have been observations of a tail/jet emerging from the active region, an interpretation confirmed by photographs. Until year's end the coma diameter was 0.3' (10-15.000 km) with the coma highly condensed (DC 7-8).
Andreas Kammerer
