File:An unusual view of comet Austin - Eso9007a.jpg

English: An unusual view of Comet Austin. (ESO Press Photo eso9007; BW) Most of the dust that is ejected from a comet's nucleus (i.e. the "dirty snowball" at its centre) assembles in a thin “sheet near the orbital plane in which the comet moves around the sun. This sheet is very thin and is difficult to observe unless it is viewed directly from the side. On June 6, 1990, the Earth crossed the orbital plane of Comet Austin, allowing such a unique, side-on view.

A 10 minute exposure in blue light of Comet Austin was obtained by Guido Pizarro with the 1 m ESO Schmidt telescope at 09:20 hours UT in the morning of June 5, 1990. The image shown here is a photographically enhanced, "negative" reproduction of the original plate by Hans-Hermann Heyer at the ESO Headquarters in Garching. On this date, the comet was seen in front of a rich star field with many dark nebulae, near the galactic centre in the southern constellation of Sagittarius. The stars are trailed since the telescope was set to follow the comet's relatively rapid motion. At this time, Comet Austin was about 51 million km from the Earth and 203 million km from the Sun.

The photo shows clearly the so-called "neck-line" structure, a narrow and very straight feature that stretches at least 2.6 deg (to the plate border) within a broader, diffuse and rather faint envelope. This is sunlight reflected in the thin dust sheet in the comet's orbital plane, here seen almost exactly from the side.

There is also a much weaker sunward spike which can be followed in the opposite direction to about 30 arcmin distance from the nucleus. This unusual feature also represents sunlight reflected in dust particles ejected from the comet, most probably shortly before the perihelion passage in early April.

This ESO photo provides observational confirmation of a theoretical prediction about the brightness and structure of these dust features, made by Italian astronomers M.Fulle (Trieste) and L.Pansecchi (Bologna) in April 1990 and published on IAU Circular 4991. Similar features were seen in Comet Bennett in 1970 and Comet Halley in 1986.