Summary

New images from the VLT show that one of the two nuclei of Comet LINEAR
(C/2001 A2), now about 100 million km from the Earth, has just split into
at least two pieces. The three fragments are now moving through space in
nearly parallel orbits while they slowly drift apart.

This comet will pass through its perihelion (nearest point to the Sun)
on May 25, 2001, at a distance of about 116 million kilometres. It has
brightened considerably due to the splitting of its “dirty snowball”
nucleus and can now be seen with the unaided eye by observers in the
southern hemisphere as a faint object in the southern constellation of
Lepus (The Hare).

Comet LINEAR was discovered on January 3, 2001, and designated by the
International Astronomical Union (IAU) as C/2001 A2 (see IAU Circular 7564
[1]).

Six weeks ago, it was suddenly observed to brighten (IAUC 7605 [1]).
Amateurs all over the world saw the comparatively faint comet reaching
naked-eye magnitude and soon thereafter, observations with professional
telescopes indicated the reason for this strange behaviour: the comet’s
“dirty snowball” nucleus had split into two pieces (IAUC 7616 [1]).

During the splitting of the nucleus, fresh material from the interior of
this frozen body is suddenly exposed to the sunlight, causing a rapid
increase in the evaporation process. More cometary material is released and
the overall brightness increases, as more sunlight is reflected off the dust
around the nucleus.

The VLT observes three fragments

But Comet LINEAR has just shown that it is good for another surprise. When
astronomers at ESO’s Paranal Observatory [2] turned the 8.2-m VLT MELIPAL
telescope (UT3) towards that object in the evening of May 14, they noted
that one of the two pieces of the nucleus appeared somewhat elongated. The
comet is rapidly approaching the Sun — it will pass through its perihelion
(the point closest to the Sun) on May 25, and it was quite low in the sky
(about 20 deg above the western horizon). Accordingly, the image quality
was not perfect, but there was no doubt that something was going on with
the fragment that was closest to the Sun (denoted “B”).

And indeed, when the 8.2-m VLT YEPUN telescope (UT4) obtained another image
of the comet in the evening of May 16, it was obvious that fragment “B” had
split into two, see PR Photos 18a-b/01. In fact, the astronomers suspect
that there may be other, smaller pieces.

The distance between the two pieces of nucleus “B” of Comet LINEAR (now
denoted “B1” and “B2”) was only about 1 arcsec, or approximately 500 km
(projected) at the present distance of the comet from the Earth (about
100 million km). The distance between these and the other nucleus (“A”)
increased from about 6000 km (May 14) to 7000 km (May 16).

The ESO astronomers have reported their detailed findings in IAU Circular
7627 [1]. They also note that the shape of the bright cloud (the “coma”)
around components “B1” and “B2” is quite unsual — this is well visible
on the false-colour PR Photo 18b/01. They interpret this as the likely
presence of a large amount of gas in addition to the dust around these
fragments.

Material from the formation of the solar system

Comet LINEAR moves in an exceedingly elongated orbit and it is making one of
its first approaches to the Sun, perhaps even the first one. It is therefore
a “new” comet in which unaltered material from the formation of the solar
system some 4.5 billion years ago may still be present. For this reason, the
splitting of its nucleus is of particular interest to the astronomers: by
spectroscopic observations, they may be able to observe directly such
material and hence to learn more about the processes that took place at
the time of the formation of the solar system.

Interested observers may find the location of this comet in the sky (the
“ephemeris”) at the May 2001 Events webpage at Sky & Telescope.

Results about the disintegration of another comet just published

Last year, the nucleus of another Comet LINEAR (designated C/1999 S4)
disintegrated completely. It was observed extensively with the ESO VLT and
the Hubble Space Telescope (HST), cf. PR Photo 20/00. Quite by chance, a
series of research papers based on those and other observations of that
comet are being published in today’s issue of the research journal Science.

Notes

[1]: Information about this comet has been published by the IAU Central
Bureau for Astronomical Telegrams (CBAT) on several IAU Circulars : IAUC
7564 (discovery on January 3, 2001 and provisional orbit); IAUC 7600
(ephemerides and brightness estimates; March 20, 2001); IAUC 7605
(observations of a rapid brightening; March 30, 2001); IAUC 7611 (brightness
estimates; April 19, 2001); IAUC 7616 (first observations of splitting into
two fragments on April 30, 2001); IAUC 7620 (brightness estimates; May 5,
2001); IAUC 7625 (some brightening of component “A” on May 9-10, 2001); IAUC
7627 (detailed report on the ESO VLT observations described in this Press
Release; May 17, 2001).

[2]: These observations were made by Emmanuel Jehin, Andreas Jaunsen,
Hermann Boehnhardt, Mario Kiekebusch, Hernan Nunez, Rodrigo Amestica,
Christian Herrera, Francisco Delgado, Julio Navarrete (ESO VLT, Chile) and
Richard West (ESO Garching, Germany).

Technical information about the photos

PR Photos 18a-b/01 are based on a 1-min exposure on May 16, 2001 at 23:10
hrs UT through an R-filtre with the VLT Test Camera at the Cassegrain focus
(under the main mirror) of the 8.2-m VLT YEPUN (UT4) telescope on Paranal.
Although the comet was low in the western sky, the atmospheric conditions
were good and the seeing was excellent, 0.6 arcsec. The telescope was set to
follow the motion of the comet in the sky. The fields shown measure 17 x 18
arcsec2 and 27 x 21 arcsec2, respectively; 1 pixel = 0.0455 arcsec. North is
up and East is left.