ESA ESO Elst Pizarro: Strangest Comet

On August 7, 1996, Eric W. Elst (Royal Observatory, Uccle, Belgium) reported his discovery of a cometary image on mid-July exposures by Guido Pizarro with the 1.0-m ESO Schmidt telescope at the La Silla Observatory.

Further ESO Schmidt plates were then obtained, and on August 19, with the help of orbital computations by Brian Marsden (IAU Central Bureau for Astronomical Telegrams, Cambridge, Mass., USA), Elst was able to identify the object on them.

Even though the orbit (Period = 5.6 years; inclination = 1.4 deg; eccentricity = 0.17) is entirely characteristic of that of a main-belt minor planet with the implied long-term orbital stability, the continued presence of a tail seemingly confirms the object as a 'comet'.

The object now carries the designation 'Comet P/1996 N2 (Elst-Pizarro)'.

Guido Pizarro and his brother Oscar have worked as nights assistants at the ESO Schmidt telescope since 1973. It is the first comet which carries their name.

Zdenek Sekanina (Jet Propulsion Laboratory, Pasadena, California, USA) believes that the comet's narrow, straight and structureless tail is likely to be a signature of a past dust-emission episode, probably in late May - early July 1996.

At this moment, it is not known, whether it was caused by an outburst from the surface of the object (dust being pushed into space by the gas pressure of evaporating ice), or perhaps a collision with another orbiting object.

It is therefore not entirely excluded that the object is in fact a minor planet (a kilometre-size piece of solid rock), and not a comet with a comparatively large content of icy materials. Further observations are needed to decide this question.

Eso9637a is also available in a larger version. It is reproduced from a 10-min R-filter exposure obtained on Augus t 23, 1996 with the 1.5-m Danish telescope at La Silla and the DFOSC multi-mode instrument. The observers were visiting astronomers Heike Rauer (Paris Observatory, Meudon France) and Hermann Boehnhardt (Munich Observatory, Germany).

The field of view here shown is 8.1 x 6.6 arcmin with North up and East to the left. At the time of the observation, the comet was 1.68 AU from Earth and 2.68 AU from the Sun.

The comet can easily be identified in the frame. No coma is seen, only the pronounced, extremely narrow dust tail which points towards position angle p.a. = 252 deg (about 2 deg away from the direction towards the Sun).

The overall length of the tail in the frame is about 7.6 arcmin (= 555,000 km at the comet), but actually it is longer than 8.5 arcmin, since it extends beyond the edge of the field of view of the original image.