Analysis of past comet apparitions

81P/Wild 2

1997


Until mid-July about 130 observations of 11 section members were received for comet 81P/Wild 2. This surprisingly large number of estimates for a comet with a maximum brightness of 9.1 mag can be traced back to the fact that comet Wild 2 was in a comfortable observing position and was an easy object because of the great surface brightness of its coma. Adding more than 250 international estimates represents a healthy basis for reliable results. The brightness development can be best described by the formula

m = 6.2m + 5×log D + 15.3×log r

However, as the diagram of the heliocentric magnitude demonstrates, comet Wild 2 did not show a constant activity parameter n, instead this parameter displayed a significant decrease as the comet approached the sun (seasonal effect?). This means, that the brightness development of comet Wild 2 can only be roughly described by the standard formula. By the way, an alternative dt-formulae does not work better.

Brightness and coma diameter

Development of the heliocentric magnitude

The diameter of the significantly condensed coma increased from about 1' (70,000 km) to nearly 4' (150,000 km) at the beginning of March. Thereafter the estimates decreased to 2.5', whereas the absolute diameter stayed nearly constant. The degree of condensation quickly increased from DC 3-4 at the beginning of this appearance to DC 5 in February and March. Since then it slowly decreases to DC 4.

Tail observations were rare, placing this feature mostly at 2', with very few observers reporting up to 10' (500,000 km); the tail were oriented to the east. Michael Jäger points to the fact, that the small number of tail observations or photographs is very surprising, regarding the brightness of this comet. Furthermore photographs at the beginning of 1997 displayed a significant tail. This evolution can only partly be explained with perspective effects. It is true, that in January and February we saw any tail head-on, but in April and May the situation had changed significantly and any tail should have shown itself clearly to the east. Only in the case of a dust tail with a great curvature could it be blotted out by the coma.

On April 8 Walter Kutschera reported a significantly elongated coma visible in his 54cm-newton (150x). On May 2 Andreas Kammerer reported a weaker and less condensed coma than four weeks ago; he cannot recognize a false nucleus, but instead the coma displays a dense knot of material of about 30" diameter.

Andreas Kammerer

Observations


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