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73P/Schwassmann-Wachmann

Originally discovered in 1930 during an approach to within 0.06 AU of Earth, it was lost again until accidentally recovered in 1979 at its most favourable apparition since 1930. It was observed again at its 1990 return and was then widely observed at its return in 1995/96 to have split into a number of separate fragments. At its next return in 2001, three of the fragments were seen again, designated as C, B and E.

(See more details of the previous apparitions on Gary Kronk's Cometography 73/P page, including an image from 2000 Nov 28 by Kenichi Kadota showing the three main components).

The comet was recovered at its 2005/2006 apparition by Carl Hergenrother at Mount Hopkins on 22 Oct 2005 and this was identified as the principle component (C) seen at the previous two returns.

Then on 06 Jan 2006, J. A. Farrell of Jemez Springs, NM (H02) picked up a fragment, identified as likely to be fragment B from 1995/2001 returns, some 26' W of the main comet. Roy Tucker, Tucson, AZ (683) then picked up a third component (designated G) on 20 Feb 2006 further west again.

Four more fragments were found by Richard Kowalski and Rik Hill using the Mt. Lemmon 1.5-m telescope on 04 & 05 March 2006, further west from fragment G, these being designated H, J, K & L.

Following up on fragments H, J, K & L at Great Shefford on the morning of 23 March 2006, a further two fragments were found, now designated as M & N, also observed just a few hours later by Eric Christensen from Mt. Lemmon. A mosaic of the field that morning, covering fragments G, J, M, H, N and L is shown below with close-ups of the individual components.

The image here is of the main component C, taken on the morning of 20 March 2006. The scale is the same as of the other fainter components, twice the scale of the mosaic field.

Here, nearing its closest approach about 7 weeks later, component C is over 6 times closer than in the previous image and is viewed almost side on. A sunward pointing fan of material can be seen, with the main tail stretching from the point-like inner coma in the opposite direction.

The field of view covered on 23 March 2006 from Great Shefford is shown here, with the individual components of 73P/Schwassmann-Wachmann indicated. The main component (C) was 2.3 to the southeast (lower left) of fragment G, or about 4.5 times the 30' width of the mosaic, with the other major fragment (B) 21' to the southeast of G.

Fragment K, discovered on 05 March 2006 should have been about 2/3rds along a line joining J and H, but was not found.

The mosaic was constructed from four separate sets of images, some of which were affected by high cloud, causing uneven background fogging.

Field of fragments G, J, M, H, N, L of 73P/Schwassmann-Wachmann
Close-ups of the individual fragments are shown here:

Fragments G & J of 73P/Schwassmann-Wachmann

Fragments H, M & N of 73P/Schwassmann-Wachmann:

Animation: 23 March 2006:
03:01-03:12 UT: 21 x 30s exposures (total 10m 30s)
03:27-03:44 UT: 33 x 30s exposures (total 16m 30s)
(Note the unequal total exposure makes the objects on the first frame appear fainter than the second frame)

Here an animation of 5 nights of images of fragments h, m & n shows the pieces slowly drifting away from each other as they approach the Earth.

Image details used in animation:

t + days 2006 UT Exposure
0 Mar 23.13 27min
5 Mar 28.14 20min 40s
10 Apr 02.17 7min 20s
13 Apr 05.12 22 mins
17 Apr 09.06 29min 20s
Notes:
h starts with a faint tail, then fades and becomes very diffuse. Centre of light appears to be offset from motion at t + 13d.
m appears to vary in brightness. It is generally diffuse and may have a short tail at t + 17d.
n is almost invisible (at mag +21) on the first two images, although there does appear to be a very faint tail at t + 0d, but then undergoes a dramatic 3 magnitude outburst peaking at t + 13d, fading rapidly by t + 17d, though the tail is still very obvious.
By 18 Apr 2006, about two weeks after its 3 magnitude outburst, fragment N could be seen to have split  in two, with the leading fragment appearing as a headless mini-comet with short 15" long tail, mag +19.6 N and the trailing fragment being 13" diameter, but with no tail, mag +20.6 N.
Fragment M had expanded into a very diffuse and unconcentrated patch of light about 15" in diameter while fragment H had faded almost out of view (dimly seen on the right-hand edge of the animation below):

Animation details: 2006 April 18 00:50 - 01:13 UT  Binned 2x2 and enlarged x2. Field of view 6'x6'
Two frames of 10m 40s and 10min total exposure, 0.40-m Schmidt-Cassegrain + CCD
Fragment L of 73P/Schwassmann-Wachmann:
Component B was observed to be in outburst by John Bortle on 03 Apr 2006 (see Comets-ml message 9485) and images below show the change in appearance during the 48 hours between 02 Apr and 04 Apr 2006. They are to the same scale as the rest of the images on this page (apart from the mosaic): 

Components B (& P) 24 hours before Bortle's observation:
 
Component B in outburst, showing straight thin tail. (Note this image has nearly 7x less exposure than the previous image):
 

Within a few days various observers were reporting that the false nucleus (the normally almost star-like condensation at the heart of the comet, surrounding the invisible real nucleus) was more like a cigar shape, without the usual concentrated centre. Evidently some disruption of the nucleus that caused the brightness surge in early April was becoming apparent.

Here, on 2006 April 12.0 UT the cigar shape is evident, without any obvious brightening at the expected position of the nucleus, the brightest point being just offset to the bottom right from the leading edge, rather than being at the leading edge:

This next image five days later on 2006 April 17.1 UT shows the nuclear condensation is once more visible, but there is also a distinct brightening in the centre of the 'cigar':

A line profile of the intensity of the coma from the above image is shown here, revealing the nuclear condensation (labelled A) and the brightening in the centre of the 'cigar' (labelled B).

Here, an image of component B from 06 May 2006 has been processed to bring out faint detail close to the centre of the coma and a string of small fragments from disruptions to the nucleus in previous weeks can be seen extending to the southwest (see enlargement below). An even more recent disruption to the nucleus has led to another bright patch immediately to the southwest of the nucleus which can be expected to develop into yet more streams of fragments trailing the comet in the following days and weeks. The Digital Development Processing (DDP) algorithm used retains a remarkable amount of detail from the brightest part of the nuclear condensation, the thin straight tail, to the outer reaches of the main coma.

The same image is processed here to bring out the trail of debris more clearly using Unsharp Mask processing. It has been enlarged to twice the scale of the image above and shows a number of small fragments extending to a distance of 130" from the main nuclear area:

Five days later on 11 May 2006, with component 73P-B just three days away from its closest approach to Earth, further exposures were taken, this time at higher resolution (with the CCD unbinned, at an image scale of 1.1"/pixel) and with shorter exposures to stop the comet from trailing. As with the 06 May 2006 image, the individual exposures were stacked together by averaging (to stop the brightest parts of the image from 'burning' out), log stretched to allow a greater dynamic range of brightness to be displayed and then an unsharp mask software filter was applied to enhance the faint fragments drifting away from the main nuclear area. As can be seen below there are a few fragments visible close in, extending up to 40" from the central condensation, as well as four or more very faint fragments about 140" away, about two thirds the way towards the right hand (western) edge of the frame. 



The main component of comet 73P/Schwassmann-Wachmann (73P-C) approached the Earth to within 0.0786 AU on 12 May 2006, but the small, trailing fragments passed somewhat closer during the following 5 days, with the closest approach by a named fragment being 73P-AX to within 0.051 AU (20 times further away than the Moon, or 7.6 million Km = 4.7 million miles) on 17 May 2006.

Further images of 73P/Schwassmann-Wachmann continue on page 2 here
73P/Schwassmann-Wachmann Spitzer main page
73P/Schwassmann-Wachmann Summary of fragments page


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