On Oct. 30, 1998 Frank Shelly announced that an object found by LINEAR, actually is a comet. Comet C/1998 U5 (LINEAR) was moving through the constellation Gemini, estimated to be 14m and showing a diffuse coma and a 2' long tail to the west (IAUC 7044/45). Further observations yielded a perihelion passage of 1.2 AU in the second half of December (IAUC 7044/45).
This comet exhibited a very high activity during the two weeks following discovery, as is indicated by the 65 observations by members of the Fachgruppe Kometen and 130 international observations. Reported as being 14m at discovery it was estimated as 8.3m only two and a half weeks later. Although estimates by professionals are notorously too faint by up to 2-3m, a plot of the heliocentric magnitude clearly indicates an outburst at discovery. Thereafter, however, the activity was still high. The evolution after the outburst can be described with the following formulae:
But not only the brightness increased rapidly. During the first two weeks the apparent coma diameter rose rapidly from 2' to 11'. After that it rapidly shrunk down to 6', followed by a slower decrease to 2.5' in January and to 1' in May. The evolution of the absolute coma diameter is an additonal indicator for an outburst. During the first 14 days it rose from 70.000 km to 275.000 km, then dropped to 200.000 km, thereafter decreasing more leisurely to 100.000 km until May. The degree of condensation was estimated as DC 5 at first, decreasing rapidly to DC 2-3 with a current value around DC 2. Visual observations of a tail were made primarily around November 1998 (length: 0.1° = 300.000 km).
Total Brightness and Coma diameter
Andreas Kammerer
