Spectacular Comet Breakup
Mosaic of the region near comet 57P, the head of which is at left, and the
discovered fragments (circled and labeled 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. Image by Yan Fernández, University of Hawaii.
Three Institute for Astronomy scientists have discovered that Comet 57P/du
Toit-Neujmin-Delporte has broken up into at least twenty pieces. They observed
the fragmentation during observations at the University of Hawaii 2.2-meter
(88-inch) telescope on Mauna Kea on the nights of July 17 and 18 after learning
of a recent report that the comet had a previously unknown companion.
Dr. Yanga R. Fernández, graduate student Scott S. Sheppard, and Dr.
David C. Jewitt found a trail of fragments strung out in a line extending almost
thirty minutes of arc away from the comet itself. (As a comparison, the diameter
of the full Moon also covers thirty minutes of arc.) At the distance of the
comet, this trail is about one million kilometers long (about 620,000 miles).
The IfA team identified the fragments by taking successive images of the field
and detecting the motion of the pieces against the background stars. So far,
they have confirmed the existence of eighteen fragments in addition to the primary
comet and the first fragment. Their findings have been announced by the Central
Bureau for Astronomical Telegrams, the internationally recognized official clearinghouse
for reporting astronomical discoveries.
Commenting on the size of the pieces, Dr. Fernández said, "We cannot
be sure of the sizes of the fragments, but the brightest ones are probably less
than a few hundred meters (a few hundred yards) across. The smallest fragments
are probably no more than a few tens of meters across, roughly the size of a
house. We expect that most of the fragments will fade to the point of invisibility,
but we don't know how long that will take. A few might last for years." The
comet and its fragments are no brighter than 15th magnitude, much too faint
to be seen with the naked eye, or even with binoculars.
Comets are huge dirty snowballs, conglomerates of water ice and rocky material
formed in the early days of the solar system. When a comet comes within 400
million kilometers (250 million miles) of the Sun (a little bit beyond the orbit
of Mars), the sunlight is strong enough to start evaporating the ice in large
quantities. 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,
which is the small solid body of the comet. Under normal circumstances, vapor
and tiny dust grains are all that fly off the surface of the nucleus. When that
happens, we see a comet with a long tail, such as those of Hyakutake and Hale-Bopp
in the late 1990s.
Occasionally, however, the thermal stresses caused by solar warming become
so great that entire chunks of the nucleus are ejected. While scientists think
they understand the basic principles involved here, they still do not understand
the details, because they do not know many of the fundamental structural properties
of cometary nuclei. In the case of this comet, they cannot yet determine even
when the fragmentation took place. The IfA team plans more observations to attempt
to answer this and other questions about the basic structure of comets.
Comet 57P/du Toit-Neujmin-Delporte is named for the three people who discovered
it in 1941. The "57P" means it is the fifty-seventh comet in the list of comets
that have been seen on at least two of their passages around the Sun. (The first
comet in this list, "1P," is the famous Halley's Comet.)
For more information, see http://www.ifa.hawaii.edu/~yan/57p.html.
Close-ups of each of the eighteen new fragments discovered by IfA astronomers.
There is great diversity of physical characteristics among the fragments. Image by Yan Fernández, University of Hawaii.