Comet 73P/Schwassmann-Wachmann - update 20 April

Fig. 4 Comet 73P/Schwassmann-Wachmann fragment A (dia 1) or C (see text) imaged April 19 2006 at 23h 32m UT. A 120 sec. exposure 270mm f/2.8 ISO 400. The bright star to the left is alpha Cor. Bor. mag. 2.2. The star close to the comet is GSC-2029-1065, mag. 7.4. Stars to mag. 15 are visible. Field: 3 arc degrees approx.

Fig. 5 Comet 73P/Schwassmann-Wachmann fragment A (dia 1) or C (see text) imaged April 20 2006 at 00h 38m UT. A 110 sec. exposure 270mm f/2.8 ISO 400. Compare with fig. 4. Field: 3 arc degrees approx

Both fragments were visible last night in 8x 30 binoculars. Fragment A's proximity to alpha Cor. Bor. is making observation difficult for the next 72 hours or so. It is the brighter and more interesting of the two and in 20x80 binoculars shows a tail about 15 arc minutes in length.

The two images taken 66 minutes apart (figs. 4 & 5) clearly demonstrate the rapid motion of the comet in relation to the 7.4 magnitude star GSC-2029-1065. In 30x80 binoculars the star gave the illusion of greater brightness for the comet since for a short time it appeared almost like a nucleus to the coma.

Fragment B is more diffuse and shows a shorter tail.

Some background - the naming of parts.

Comet Schwassmann-Wachmann was the third comet to be discovered in 1930 by Schwassmann and Wachmann then working at the Hamburg Observatory, principally on minor planets. (A feature on minor planets is to follow later.) The comet demonstrated typical characteristics of short period periodic comets, mostly unspectacular in their physical appearance.

Such comets, inhabiting for much of the time the region of space dominated by the giant planets - Jupiter in particular, are subject to considerable pertubatory forces. This results not only in alteration to the fundamentals of their orbits (the elements) but can have major physical consequences. In the case of  Comet 73P/Schwassmann-Wachmann this resulted in 1995 in the comet breaking up into at least five parts.

The subsequent behaviour of the parts is generally one of decline, visually at least, as might be expected. The present case is interesting in that two of the fragments B & C (which I have denoted A in the present article just to add to the confusion!) at this apparition show considerable brightness. In addition, both show the "shape" often associated with comets in most people's minds. It is not to be expected that this will necessarily be the case at the next apparition in about five years time. The circumstances at each apparition may differ significantly for one thing.

At a personal level this comet is of particular significance. Shortly after I had joined the BAA's computing section in 1951 (I was still at school, incidentally) I was tutored in the application of perturbation methods for the elucidation of the elements of periodic comets. I was given Comet Schwassmann-Wachmann to work on but never actually produced an ephemeris for publication. This I did for a number of other comets, however.

In those days our computations had to be done the hard way using large tomes containing mathematical functions to 6-places, sometimes eight. (I have some historically notable books tabulating functions to 61-places!) Calculations that can now be done in a matter of minutes or seconds using modern computers then took weeks if not months.

Incidentally, the naming of parts for Comet 73P/Schwassmann-Wachmann was originally done without regard or knowledge to any future behaviour in the fragments. I have called fragment C, A, in this instance simply because it happens to be the brighter of the two, B and C. Where is the original A, you may ask? In all honesty, at this moment, I do not know.

JV 20/04/06

Comet 73P/Schwassmann-Wachmann

Dia. 1 The sky looking east (Orkney) at 23h 00m UT (GMT) and showing the course of the two fragments A & B of Comet 73P/Schwassmann-Wachmann
Fig. 1 Comet 73P/Schwassmann-Wachmann location images for fragments A (left) & B (right) from a photograph taken 17 April 2006 at 23h 53m UT. A 20 sec. exposure 75mm f/1.4 ISO 350.
Fig. 2 Comet 73P/Schwassmann-Wachmann, fragment A (centre), imaged 17 April 2006 at 23h 42m UT. A 30 sec. exposure 160mm f/1.8 ISO 350. The brightest star in the field is TYC 2025-1339-1 mag 6.37. Field 2.2 degrees approx.
Fig. 3 Comet 73P/Schwassmann-Wachmann, fragment B (centre), imaged 17 April 2006 at 23h 49m UT. A 30 sec. exposure 160mm f/1.8 ISO 350. The brightest star in the field is psi Boo. mag. 4.52 Field 2.8 degrees approx.

This periodic comet (period 5.36 years) has separated into at least three portions two of which are presently observable with 20x80 binoculars.

Opinions differ as to the magnitude these two fragments will reach at maximum around May 15 2006. (Some were predicting naked eye visibility for both. In my opinion this is unlikely.)

From the latitude of Orkney the two objects will be circumpolar between April 28 and May 11. Dia. 1 shows the course of fragments A & B (my nomenclature) from April 19 to May 13.

Observations in poor weather made from Rousay on the 16 indicated for 'B' a magnitude of 9.4. The comet appeared as a faint "cigar" shape about 5 arc minutes in length fading towards the horizon

Images taken the following night indicated for 'A' a magnitude of 9.1 and for 'B' 9.3. The two bodies were clearly captured in the same field using a 75mm f/1.4 with 20 second exposures and 160mm f/1.8 with 30 second exposures. See figs. 1- 3.

Note: For the latitude of Orkney astronomical twilight (see archives) lasts all night between April 25 and August 19. Nautical twilight lasts between 16 May and 28 July.

JV 17/04/06