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Comet Grigg-Skjellerup on July 10, 1992

10 de Julio de 1992

This photo shows Comet Grigg-Skjellerup, as imaged by the ESO 3.6 m telescope at the La Silla Observatory in the early morning of July 10, 1992, just 15 hours before the Giotto encounter with this comet. The observation was made by Dr. Klaus Jockers from the Max-Planck-Institut für Aeronomie (Katlenburg-Lindau, Germany), and in the course of a special monitoring programme in support of the Giotto Extended Mission. The digital image was transmitted to the ESO Headquarters in Garching via the permanent satellite link, immediately following the observations. The photo is a composite of four one-minute red-sensitive exposures. The other objects in the field are galactic stars.

The comet was situated very low in the western sky (15º above the horizon) and the observing conditions were mediocre; the moon was quite bright and there were thin clouds (cirrus) in this part of the sky. The image sharpness (seeing) was slightly smaller than 2 arcseconds.

The comet was moving towards the left (East) and the projected direction to the Sun is upper right (West-North-West). The small cometary nucleus (probably 1 km in diameter; not visible on this ground-based photo) is surrounded by an elliptically shaped dust cloud (the 'coma') with apparent dimensions of 28 x 19 arcseconds, i.e. 30,000 x 20,000 kilometres. The nucleus is hidden in the brightest part of the coma, somewhat off the geometrical center of the ellipse. The elongation of the coma towards the lower left (South-East), i.e.~in the direction opposite to the Sun, is the first manifestation of the presence of a short tail of dust particles; it was not visible on earlier ground-based images. The magnitude of the comet was about 15, i.e. ~close to the predicted brightness.

Technical details: Exposure time: 00h 05m UT (02h 05m MEST) on July 10, 1992. The photo covers an area of 50 x 70 arcseconds; North is up and East is to the left (as seen on the sky).  1 pixel = 0.61 arcsecond. CCD (Charged-Coupled Device) detector with broad-band red filter (6500 — 7500 A). Flat-fielded, but not further cleaned. Logarithmic intensity scale

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