Comet trails

The object of the IRAS moving object project was to detect comets and asteroids but one day in August 1983 the fast moving object detection software seemed to find a number of "asteroids" all in the same patch of sky. None of these detections looked right and they could not have been a single object being detected several times as the motion would have been too erratic to be real, so I did not worry too much about them. However, to my surprise the next day several more appeared in a very similar region. This went on for several more days and eventually I tried plotting all the positions onto a map of the sky. The result was amazing, all the objects seemed to lie on a straight line! I went to the data reduction system and looked more closely at the bit of each scan during which these "objects" were detected and soon found that in fact they were not single detections but a wide and very narrow band of infrared emission across the sky. As IRAS scanned across the structure all of the 25 micron detectors registered it but, because we had various filters on the processing to minimise false detections, in only a few cases did the signal get large enough to register as a possible point source and get flagged by the moving object software. A closer look at the positions revealed that the structure pointed straight at the position of comet Tempel-2. It seemed I had discovered a huge "infrared tail" on the comet invisible from the ground.

At first some of the other astronomers were dubious but, in the best traditions of science, I was able to predict that when IRAS returned to the area in a few days the trail would have moved along with the comet and if it did then it must be real. A few days later, to my great relief, this prediction was confirmed and the reality of the trail was proved beyond doubt. A couple of years later Mark Sykes independently discovered a number of similar trails using more refined IRAS data products.

We now know that the trail is caused by largish particles (about a millimetre in size) which fall off the comet but continue around the Sun in almost the same orbit, spreading out slowly ahead and (mostly) behind the nucleus. If the Earth flies though such a trail during its orbit around the Sun the result is a meteor shower.

The trails are very hard to see from the ground but Mark Sykes, Bill Reach and I used the Infrared ISO satellite to make a number of scans of comet trails in 1996. Here is the result of two cuts trhugh the comet Kopff trail. Click here for a copy of the paper describing these results.

An ISO image of two two slices through the Comet Kopff dust trail. The trail is in yellow in this colour image of the infrared (12 micron) data.

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John Keith Davies
Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ
tel: (44) 0131 668 8348 / fax: (44) 0131 662 1668/