Identifying fragments in the Spitzer Space Telescope image
||Ground based positional
measurements of 65 of the fragments from disintegrating comet 73P/Schwassmann-Wachmann
have been analysed to help identify some of the many small pieces
of the comet revealed by a wide angle infra-red image taken by the
Spitzer Space Telescope in May 2006.
On 10 May 2006 a stunning infra-red image of many of the fragments of
Comet 73P/Schwassmann-Wachmann was released by NASA (see original
press release 'Spitzer
Telescope Sees Trail of Comet Crumbs') represented above in false colour
but also available in high resolution
black & white format. The image shows the two brightest
components B & C together with many of the individual fragments of the comet
detected by ground based instruments as just part of an almost
continuous line of warm dust left behind by the disintegrating comet.
The image was a mosaic of three fields, by the Spitzer MIPS
instrument operating at a wavelength of 24 microns, taken over a period
of three days from 4th to 6th May 2006, with a total exposure of 21
During the 2006 apparition of the comet a total of 65 fragments (Table
observed from the ground for two or more nights and received official
designations. The two main fragments designated B & C had been
observed in previous perihelion passages in 1995 and 2000 and were the
two brightest components in 2006. Fragments observed for the first time
in 2006 were designated G, H, J - N, P - Z, AA - AZ and BA - BS. Many
more fragments were reported during 2006 but did not receive
designations because of lack of confirming observations. As has been
shown here and elsewhere, flaring of individual small
fragments by several magnitudes in one or two days or less appears to have
been common, so a fragment observed one night was often completely
invisible when follow-up observations were attempted on subsequent
Figure 1 shows the original high-resolution black & white Spitzer image
at reduced scale, with the main component C identified in the top
of the three fields, with component B being the brightest of the many
fragments visible in the lower half of the image.
To identify the fragments in the Spitzer image with the officially
designated fragments observed
by ground based instruments during the first
half of 2006, the original image above was rotated to align the trail
of fragments horizontally (Figure 2). The fragments coming to
perihelion first are to the left of the image with those to the right
coming to perihelion at correspondingly later dates. The 2006 perihelion
times (UT) of fragments C and B are marked on the rotated image
along with a scale indicating the approximate perihelion times along the
line of variation:
All the published astrometry for the components in 2005 and 2006 was analysed
to determine the perihelion time of each one, so that the smaller components
could be identified on the Spitzer image by interpolation between the
larger, more easily identified fragments. 8674 astrometric positions
were used, 73.5% of those being of the five brightest components C, B
(including AQ which split from B during April 2006), G & R. Of the remaining
60 fainter fragments, 40% were observed for less than 10 days.
Because some of the minor fragments were observed for just a few days,
values of the perihelion time (T) could not be determined
accurately via direct orbit determination. Making the assumption that the time
of perihelion for the trail of fragments was proportional to their
spacing in the sky compared to other nearby fragments, T was determined
for all of the poorly observed fragments by interpolation in Right
Ascension between well observed components with accurately known values
of T. In each
case this was done within the observed arc of the minor fragments,
assuring relative positional accuracy to a few seconds of arc. Table
2 lists all the designated fragments in order of perihelion time
However, there are various factors that make the identification of fragments
in the Space Telescope image problematic, including:
- The extreme variability in brightness of the smaller fragments in
short timescales means that those observed in March and April 2006
cannot be assumed to be identical to fragments identified in
approximately the same location in the Spitzer image.
- The image is a mosaic collated from exposures
made over a period of three days so the separation of the fragments
across the entire image will not always be proportional to the
perihelion time T.
- The Spitzer Space Telescope is positioned in an orbit around the Sun
very similar to the Earths orbit, but trailing the Earth by 0.44 AU.
When the images were taken on 2006 May 4 - 6, the comet fragments were
only about 0.1 AU from Earth, so the perspective from the Spitzer Space
Telescope was appreciably different to that as seen
from Earth (Figure 3).
Figure 3. The positions of the Earth, Sun,
the Spitzer Space Telescope and trail of fragments of Comet 73P/
Schwassmann-Wachmann viewed from north of the Ecliptic on 5 May 2006.
The length of the trail of 73P indicates the extent seen in the Spitzer
image. The extent recorded by ground based telescopes was about 60%
longer. The orbits of Venus and Mercury are also shown.
(Graphic adapted from an original orbit diagram courtesy of the JPL Orbit
To minimise some of the distortions introduced by those factors,
interpolation has been done separately within each of the three Spitzer
frames. Even so, the unambiguous identification of specific fragments,
especially in the western half (right hand side) of Figure
6 is not always possible.
Below are the three sections of the original Spitzer image, with
marks indicating where 53 of the 65 named fragments have been determined
to be. Four other named fragments precede the easternmost image and a
further nine trail the westernmost image and are therefore not included
in the images below.
East (left) side of Spitzer image:
Figure 4. Fragments with perihelion times
from June 6.4 - 7.1 UT 2006.
Image of 73P-BC and 73P-AP
(06 May 2006)
Image of 73P-C (20 Mar 2006.)
Image of 73P-C (11 May 2006.)
Central area of Spitzer image:
Figure 5. Fragments with perihelion times
from June 7.2 - 8.0 UT 2006.
Note the several obvious fragments that were not designated from ground
Image of 73P-B and 73P-P (02
Image of 73P-B and 73P-AQ (17
Image of 73P-B and 73P-AQ (06
Image of 73P-B and 73P-AQ (11
West (right) side of Spitzer image:
Figure 6. Fragments with perihelion times
from June 7.9 - 8.8 UT 2006.
Image of 73P-G, J, M, H, N, L
(23 Mar 2006)
Image of 73P-G and J (23 Mar 2006)
Image of 73P-H, M and N
(23 Mar 2006)
Image of 73P-H, M and N
(23 Mar 2006 - 09 Apr 2006)
Image of 73P-L
(23 Mar 2006)
Image of 73P-H, M, N, BO and
BQ (28 Apr 2006)
Image of 73P-H, N, BO, BP,
BQ (30 Apr 2006)
See the full size image (3621 x
1579 pixels, 621 KB) of all three sections combined.
Summary tables of fragments of 73P can be
All images on this page are Courtesy NASA/JPL-Caltech/W. Reach (SSC/Caltech)
unless otherwise noted. Identification of individual fragments on the
images is by Peter Birtwhistle.
page & Page 2
73P/Schwassmann-Wachmann Summary of