During the night of Oct. 23/24, 1998 our Austrian member Michael Jäger discovered a suspicious object on photographs taken for comet 52P/Harrington-Abell with his 10"-Schmidt camera. Comet P/1998 U3 (Jäger) was 12.5m bright, strongly condensed, with a 1' coma and a 10' long, fanned tail in P.A.=280°. Confirmation came the following night, by international observers and Michael Jäger himself. Comet Jäger was announced in IAU Circular 7038 on Oct. 25, which already mentioned the possibility that it is of short period. IAUC 7040 on Oct. 27 could already announce a first orbit on the basis of 91(!) precise position measurements.
Improved orbital elements proved that comet Jäger would pass perihelion on Mar. 10, 1999 at a distance of 2.13 AU. They also confirmed the periodic nature. Comet Jäger takes 14.95 years to round the sun, i.e. Michael Jäger has a good chance for observing "his" comet on the next apparition (under slightly better geometric circumstances). It is now clear that this comet experienced a very close encounter with Saturn in July 1991.
Jost Jahn calculated the secular changes of the orbit of comet Jäger: this comet did round the sun on an orbit very similar to Saturn since at least the 15th century, but without close approaches to the ring planet. Then in May 1962 it approached Saturn to within 0.6 AU and on July 18, 1991 it came as close as 0.018 AU to the ring planet. The next close encounters will take place on Feb. 24, 2020 (0.31 AU), Aug. 12, 2198 (0.30 AU) and July 4, 2226 with 0.26 AU (AFZ 657/663).
It is thus reasonable to conclude that comet Jäger comes this close to the sun for the first time, showing a (temporary) increase in its activity. This could explain the rather high brightness parameters:
The analysis, which used 93 estimates of members of the Fachgruppe Kometen and 150 international estimates thus indicates a maximum brightness of 10.5m.
The coma diameter increased - in an accelerating manner - from 1.7' around discovery to a maximum of 3' in January. Thereafter it decreased, first rapidly, then more slowly, to 2'. Regarding the absolute coma diameter this increased from 140.00 km to 190.000 km (ever faster toward the end). The degree of condensation changed from DC 3-4 at discovery to DC 4-5 in January, decreasing to DC 3 thereafter. Visual observations of the mostly west-pointing tail did not show it to be longer than 0.1° (about 2 million km).
Andreas Kammerer
