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Figure 5. ISOPHOT observations of the dust trail of comet P/Kopff in October 1996. The 1-dimensional multi-filter (12, 25, and 60/jm) scans were performed perpendicular to the orbit at separations of —2°, +0.25°, +2°, and +5° ahead and behind of the nucleus.

et al. [27] observed the dust trail of comet P/KopfF at mean anomalies of 0.5° and 1.0° behind the nucleus, and compared the results with measurements from 13 years earlier obtained with IRAS [27]. The new observations were performed at 12 /jm with ISOCAM. The trail was clearly detected towards both positions with a surface brightness of ~ 0.33MJysr_1. This value is about a factor of 2 lower than that predicted from the IRAS data, indicating different levels of activity between perihelion passage before and after the IRAS observations. Comparing the sizes of the P/Kopff trail observed by the two satellites, a significant broadening was observed during this 13 yr period.

With ISOPHOT we also observed the trail of comet P/Kopff at 12, 25, and 60 /jm in October 1996. Figure 5 shows both the observed positions and the derived brightness profiles taken perpendicular to the orbit at different distances behind and ahead of the comet. The figure shows that the trail was clearly detected at 0.25° behind the comet at 12 and 25/jm, and it was clearly resolved by the 3' aperture exhibiting an asymmetric profile. The trail, however, was only marginally detected at larger distances behind the nucleus, and no anti-tail was seen, showing that the intensity of the trail drops relatively quickly behind the comet. The colour temperature determined from the 12 and 25/Jm

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