Seven years later, continuing analysis of data from Giotto's Energetic Particle Detector (EPONA) has led to the conclusion that a second comet, possibly a fragment of the main nucleus, may have been accompanying Grigg-Skjellerup.

The new results have been obtained by Professor Susan McKenna-Lawlor, the Irish Principal Scientific Investigator for the EPONA instrument, and Russian scientist Dr. Valeri Afonin. Their discovery is based on fluctuations in the energetic particle data recorded by EPONA.

One of the most important aspects of the Grigg-Skjellerup encounter was that it enabled scientists to use the same instruments to compare the fairly inactive Grigg-Skjellerup with Comet Halley, its much larger, more active cousin. A number of experiments on board Giotto were functioning during both encounters.

One of these was EPONA, which has the capability to record charged particles -- protons and heavier ions -- with energies ranging from several tens of keV to several tens of MeV. (One electron volt or eV is the amount of energy gained by an electron when it is accelerated through a potential difference of one volt.)

Characteristic fluctuations in the energetic particle records allowed EPONA to detect the same cometary boundaries at Halley and Grigg-Skjellerup as Giotto's other particles and fields experiments.

Recent, detailed analysis of EPONA data by McKenna-Lawlor and Afonin, (described in the journal Planetary and Space Science Vol. 47, p. 557-576 and Circular No. 7243 Central Bureau for Astronomical Telegrams IAU, 1999 August 25), has revealed a complex particle enhancement in the energy range 60-100 keV. This increase was recorded by EPONA some 90,000 km beyond Grigg-Skjellerup.

Several possible explanations for this flux enhancement were considered, but the overall conclusion was that it constituted the signature of a 'companion' comet, three to four times smaller than Grigg-Skjellerup and with a correspondingly lower gas production rate.

It is unlikely that these two objects have existed side by side from the beginning of their existence. A more probable explanation is that the smaller object broke away from Grigg-Skjellerup shortly before the Giotto encounter. Splitting of cometary nuclei is a well known phenomenon that can occur even at large distances from the Sun.

Caption Above: Energetic particle signature recorded by the EPONA instrument on board Giotto in the energy range ~ 60-100 keV. The data are believed to represent the signature of a companion to Comet Grigg-Skjellerup. Particle count rate is shown on the vertical axis. Time and distance from the comet are shown on the bottom scale. (Courtesy Susan McKenna-Lawlor)

This is the first time that the discovery of a comet using energetic particle data has been claimed.

Splitting of cometary nuclei is a well known phenomenon that can occur even at large distances from the Sun. In the case of Grigg-Skjellerup, evidence for small-scale fragmentation of the nucleus was also identified from Giotto's Optical Probe Experiment (OPE).

The Optical Probe detected a number of striking 'spike' features or 'events' during the Grigg-Skjellerup flyby. While scattering of sunlight caused by dust impacts on the spacecraft body may account for some of these, the complex data profiles can also be explained by activity from dust jets in the comet's inner-most coma.

One particular event occurring at least 1000 km from the comet may have been caused by the presence of a small nucleus fragment. The evidence indicates that a chunk, 10-100 m in radius, had broken away from the comet and passed around 50 km from the spacecraft. Despite its small size, it seemed to be producing a small dust coma of its own. Unfortunately, the particular event detected by EPONA was not seen in the OPE data.

(ref. McBride, N., S.F. Green, A.C. Levasseur-Regourd, B. Goidet-Devel and J.-B. Renard, The inner dust coma of comet 26P/Grigg-Skjllerup: multiple jets and nucleus fragments? MNRAS 289, 535--553, 1997.)

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