Special Issue : 15/03/2004
A scientific publication by SGF and NEODyS

Rosetta's launch

A Comet in the garden of Eden by Ettore Perozzi

What's in a surname? In the case of comets, a little history about the discovery circumstances. With a few notable exceptions (e.g. Halley and Lexell are named after the astronomers who studied their orbits), comets inherit the name of the astronomer(s) or amateur(s) who first spotted their appearance in the night sky. When a comet is lost for some time, the name of the re-discoverer is added to the original one. So we may end up in long and complicated sequences of worldwide surname spelling exercises, such as Honda-Mrkos-Pajdusakova or Tuttle-Giacobini-Kresak. Finally, when the same discoverer finds more than one single comet, a sequential number is added (e.g. Schwassmann-Wachmann 1, 2, 3). This long-standing tradition has been recently revised due to the increasing number of comets found by automatic telescopic surveys (e.g. LINEAR) or orbiting telescopes (e.g. SOHO). Comets are now numbered sequentially, while additional letters indicate their "type": P/ for periodic, C/ for non-periodic, D/ for objects which no longer exist and X/ for those whose orbit has not been determined. Yet Rosetta's target - comet 67P/Churyumov-Gerasimenko according to the new rules - tells a rather peculiar and lucky tale.
First of all it was not discovered in the "usual" way. Back in 1969 comet chasers were spending long nights - often in the cold - scanning the celestial sphere with large field binoculars, knowing by heart entire regions of the sky in order to recognise even the faintest "intruder" quickly enough to be the first ones to comunicate the discovery to the Central Bureau for Astronomical Telegrams.

Back in September 1969 Klim Ivanovich Churyumov and Svetlana Ivanovna Gerasimenko were participating to a photographic survey of known periodic comets, carried out from the Alma-Ata Astronomical Observatory, in Kazakhstan. Mrs Gerasimenko noticed that one of their plates, where comet P/Comas-Solà was supposed to be, was badly damaged. Nevertheless upon closer investigation a cometary-like object was still recognizable and therefore identified with that comet. One month later, while reducing the Alma-Ata observations in Kiev, a 2 degrees discrepancy was measured between the position of P/Comas-Solà on the damaged plate and its predicted position in the sky. Too much to be accounted for by observational errors. The only possibile explanation was the presence of another comet, which was eventually confirmed on other plates coming from the same survey. "So the spoiled plate brought us good luck" concludes Mrs Gerasimenko in her recent interview published on the ESA web site (http://www.esa.int/export/SPECIALS/Rosetta/SEMHLR1PGQD_0.html).

An enduring good luck, we should add, since almost 35 years later, that very same comet has brought its discoverers on the front news by "substituting" yet another target comet: P/Wirtanen, which became unreachable for Rosetta due to the one-year postponement of the launch date. But this is not the only coincidence about comet Churyumov-Gerasimenko. According to local tradition, Alma-Ata is the place where the legendary apple tree of the Garden of Eden was located. Thus a comet discovered from the cradle of humankind will unveil to Rosetta the birth of our Solar System. It could not have shared a different fate.

Fig.1 - Alma-Ata is still nowadays a celebrated source of an endless variety of wild apple trees

Fig.2 - The Alma-Ata Astronomical Observatory in Kazakhstan