Analysis of past comet apparitions

C/1998 J1 (SOHO)


Comet C/1998 J1 (SOHO) proved to be one of the rare examples of a relatively small object which survived a close encounter with the sun. When it left the field of view of SOHO the attention rose whether it could be found from the ground. Finally, on May 11 N. Biver and H. Dahle were the first who could spot the comet in the bright twilight from Hawaii. They reported a 0.5m, extremely condensed object. On May 14 P. Nation succeeded in Southern Australia, reporting a 2.5m object with a 0.25° tail. On May 16 two additional observers estimated 3m and saw a 0.5° tail. During the short period of theoretical visibility in the bright twilight from Europe only O. Farago succeeded by taking a CCD-exposure on May 10, showing the comet with a 15' long faint tail and four stars of the Pleiades, allowing Jost Jahn to perform astrometry, which yielded a significant improvement of the orbit (IAUC 6906, AFZ 578)! During the following days comet SOHO could be seen under a darker and darker sky from the Southern hemisphere. Until May 22 the reported tail length reached 1° with the unaided eye and 7° with binoculars, whereas the brightness went down to 4m. On a photograph taken on May 19 the gas tail could be traced out to 10° and a 1° long dust tail was visible. On May 23 the comet passed the nebulae around zeta Ori.

Until May 30 the brightness decreased to 5.0m. But then, on June 1 several observers reported the comet to be 1.5m brighter (IAUC 6926). This short-term outburst (lasting 3-4 days) is confirmed by additional ICQ-observations, not yet shown in the diagram. Eventually, radio observations of the OH production rate (× 10^29 molecules/s) support it too: June 1.6 UT: 3, 2.6: 5, 3.6: 4, 4.6: 3 (IAUC 6934).

The brightness behaviour can be best described - excluding the possible outburst - with the formula

m = 6.2m + 5×log D + 6.0×log r,

yielding a maximum brightness of 1.5m (3.5m at times when the comet could be seen under a moderately dark sky), which means, that comet SOHO was third place (together with IRAS-Araki-Alcock) of all comets visible from the ground for the last 15 years! And it was the third bright comet within the last three years!

The coma diameter increased to a maximum of 6' (250.000 km), with the coma itself extremely condensed (DC 9) during the first days, but getting more diffuse until mid-July (DC 2). The absolute tail length reached 15 Mill. km.

Total Brightness and Coma Diameter

Length of ion tail

Andreas Kammerer


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