ESA Science News
28 Sep 1999
Companion to comet Grigg-Skjellerup discovered using Giotto data?
On 13/14 March 1986, the European Space Agency's Giotto spacecraft
obtained the first close-up pictures of a comet nucleus during its close
flyby of Halley's Comet. An historic second comet encounter followed on
10 July 1992 when Giotto flew within 200 km of Comet Grigg-Skjellerup.
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.
This is the first time that the discovery of a comet using energetic
particle data has been claimed.
Prof. Susan McKenna-Lawlor
Space Technology Ireland
National University of Ireland
Maynooth, Co. Kildare, Ireland
Dr. Valeri Afonin
Institute of Space Research
Note for the editors:
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.)