[meteorite-list] The Fragmentation of Comet 57P/du Toit-Neujmin-Delporte

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:00:09 2004
Message-ID: <200207241705.KAA24872_at_zagami.jpl.nasa.gov>

http://www.ifa.hawaii.edu/~yan/57p.html


                        The Unstable Lives of Comets:
           The Fragmentation of Comet 57P/du Toit-Neujmin-Delporte

           Yanga R. Fernández, Scott S. Sheppard, David C. Jewitt
                            University of Hawai`i

               (Click here for a more technical description.)
                   http://www.ifa.hawaii.edu/~yan/57ptech.html

Summary: Recent observations from Mauna Kea with the University of Hawai`i's
2.2-meter telescope have revealed a zoo of tiny mini-comets strung out in a
line trailing behind the comet 57P/du Toit-Neujmin-Delporte. At some point
in the recent past this comet has apparently suffered a significant
catastrophe, violent enough to break off many pieces of its nucleus. The
event was probably triggered by thermal stresses within the nucleus due to
it being warmed by sunlight. While it is not uncommon for one or two
companions to be seen near a comet that has fragmented, these recent
observations have revealed at least 19 companions, a rare finding. A similar
situation was found with observations a few years ago of comet LINEAR C/1999
S4, taken by the Hubble Space Telescope. In that case the fragments were
very short-lived, and they could be followed for only a few days. Here we
should be able to observe the fragments for several weeks and watch how the
fragments move and evolve, which should give us insight into the
constitution and fragility of this comet and hopefully comets in general.

Details:

Motivated by the initial report of a single, previously-unknown companion
associated with Comet 57P/du Toit-Neujmin-Delporte, we obtained deep imaging
of the comet's environs to take the census of any population of fragments
that might exist near the comet. We used the Univ. of Hawaii 2.2-m telescope
on Mauna Kea and a charge-coupled device (CCD) to make a digital map of the
sky around the comet. The observations were performed on the nights of July
17/18 and July 18/19, 2002 (Hawaii Standard Time).

We were delighted to find a zoo of fragments strung out in a line extending
almost 30 minutes of arc away from the comet itself. For comparison, the
diameter of the full Moon also covers 30 minutes of arc. So far we have
confirmed the existence of 18 fragments, which brings the total number of
companions to 19. The discovery has been announced by the Central Bureau for
Astronomical Telegrams, the internationally-recognized official
clearinghouse for reporting cometary discoveries. We identified fragments by
taking successive images of a field and detecting their motion against the
background stars. A mosaic of the relevant mapped region is shown in Figure
1, with the location of the fragments circled. The comet's head is at left.
At the distance of the comet, the mosaic spreads over about 1,000,000
kilometers (about 620,000 miles).

                              [mosaic of 57P]

Figure 1. Mosaic of the region near comet 57P, the head of which is at left,
and the discovered fragments (circled and labelled with official
designations). North is up and East is to the left, the standard orientation
in the sky. The comet was observed while it was almost 180 degrees away from
the Sun, so the Sun, Earth, and the comet are almost along a straight line
in space. As projected onto the sky, the comet's motion with respect to the
Sun is almost due East; the fragments all trail along behind the comet and
have virtually the same motion.

What are the brightnesses of these fragments? One way to express this is
relative to the faintest object that most well-sighted people can see with
the naked eye. Component "B", the first fragment found and the brightest, is
about 250,000 fainter than this, and the dimmest components, "K" and "L",
are fully 10,000,000 times fainter than this.

The fragments show a wide variety of activity levels and condensations. We
cannot yet be sure what the sizes of the fragments are but the brightest
ones are probably less than a few hundred meters (few hundred yards) across.
For comparison, Diamond Head crater is several hundred meters across. The
smallest fragments are probably no more than a few tens of meters across,
roughly the size of a house. A gallery of the 18 objects that we found is
shown in Fig. 2.

                           [montage of fragments]

Figure 2. A close-up of each of the 18 new fragments discovered by us. The
labeling system is the same as in Figure 1. There is great diversity among
the fragments' physical characteristics.

What could have caused this fragmentation? The cometary nucleus is basically
a conglomerate of water ice and rocky material. When the comet is within
roughly 400,000,000 kilometers (250,000,000 miles) of the Sun, the sunlight
is strong enough to start evaporating the ice in large quantities. (For
comparison, Earth is 150,000,000 km (93,000,000 miles) from the Sun.) Since
the ice and rock are intimately mixed, the warming and evaporating ice
produces great thermal and physical stresses on the body of the nucleus.
Under normal circumstances, only vapor and tiny dust grains are all that fly
off the surface of the nucleus -- and here on Earth we see a comet with a
long tail, for example as widely seen in the late 1990s with comets
Hyakutake and Hale-Bopp. Occasionally though the stress is great enough that
entire chunks of the nucleus are also forced off. Our observations shown in
the figures show the end state of this event. Now while the basic idea is
thought to be understood, the details are still uncertain, basically because
we do not know many fundamental structural properties of cometary nuclei. In
the case of this comet we cannot yet determine even when the fragmentation
took place; further observations are necessary. With sufficient data
fragmenting comets can provide a laboratory for us to witness major
evolutionary events and can help us understand a comet's basic constitution.

More Trivia/Background: Comet 57P/du Toit-Neujmin-Delporte was discovered in
1941 by the 3 people for whom it is named. The "57P" just means it is the
57th comet in the list of comets that have been seen on two of their
passages around the Sun. (The first comet in this list, "1P", is the famous
Halley's Comet.)

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Received on Wed 24 Jul 2002 01:05:05 PM PDT