The Biggest Dust Tail of All Times.

Composition of two photos of Comet McNaught 2006P1 taken on 20 January 2007 (at left, S. Deiries, ESO) from the southern hemisphere (Cerro Paranal, Chile) and on 19 Januray 2007 (at right, M. Fulle, APOD 24 January) from the northern hemisphere (Carnic Alps, Italy): the tail was so long to need observations from both hemispheres to be photographed in all its extension. The image covers 65 degrees of declination, from -50 deg in Grus (left) to +15 deg in Pegasus (right), equivalent to about 150 million km, close to the Sun-Earth distance. Positions of the set Sun, of the one-day-old Moon, and of planet Uranus are plotted. The comet moves on an almost precisely parabolic orbit, projected on the sky as a dotted line. A dust tail can be modeled in terms of synchrones (dashed lines) and syndynes (continuous lines), lines marking the size and the age of the dust grains composing it. In particular, the synchrones are labeled by the date of ejection from the comet nucleus (q is the perihelion day, on 12 January 2007, followed by other seven days b.p., before perihelion); the syndynes are labeled by the size in micron of the dust grains (assumed to be spheres of bulk density 1.5 grams per cubic cm). These lines help us to understand the complex nature of this tail. The nucleus ejects grains of all sizes, up to several microns: shortly before the observations, the tail is crossed by all the plotted syndynes. After ejection, the grains (probably fluffy aggregates of silicates and ices) fragment due to solar heat, creating subcomets which are blown away from the solar radiation pressure. These subtails, lacking of a comet head, spread away from the parent tail, which becomes more and more depleted of large dust. In fact, the older the synchrone, the more external the respective syndyne best fitting the inner boundary of the observed dust tail.