New images from the European Southern Observatory 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.

The three nuclei of Comet LINEAR as seen in the early evening of May 16 with the 8.2-m VLT YEPUN (UT4) telescope at Paranal. Photo: ESO

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.

Six weeks ago, it was suddenly observed to brighten. 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.

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 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. In fact, the astronomers suspect that there may be other, smaller pieces.

A false-color rendering of the above image shows more clarity. The cometary fragment "B" (right) has split into "B1" and "B2" while fragment "A" (upper left) is considerably fainter. Photo: ESO

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. 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. 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 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.

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). Quite by chance, a series of research papers based on those and other observations of that comet are being published in the latest issue of the research journal Science.