When Edwin Goffin dispatched in 2003 May his set of occultation predictions
and for the year 2004, immediately the event by 121 HERMIONE of February
identified by us as a top level priority at several years scale.
First it gathered properties favourable for occultation observers :
- this was a very slow event (of 41 s duration) involving a big asteroid (>
- concerning a bright red star of magnitude V= 9.2 or R ~7.5 ,
- occurring very high in the sky (in Gemini) with no moon,
- at end of evening and during university vacations,
- and crossing countries with dense population.
Then HERMIONE was an asteroid of very high physical interest :
- a member of the Cybele family, probably born from collisional disruption
of a large
C-type precursor in remote past,
- a body probably elongated and quickly spinning, but with rotation period
pole still unsure in spite of a lot of studies since 1976,
- a BINARY body, possessing a tiny satellite discovered on 2002 Sep.28 with
Keck telescope (cf. W.J.Merline et al., IAUC 7980),
- a body observed since this discovery during four more telescope runs at
the 8-m VLT
and 10-m Keck by a french and american team led by F.Marchis at Berkeley
The adaptative optics systems of these giant telescopes not only showed the
satellite of magnitude 17-18 at 0".3-0".4 of the primary, so permitting
astrometry of it, but also it partially resolved the elongated central
displayed variable rotation phases on successive nights.
It resulted that 121 Hermione had become month after month one of the best
binary asteroids on record !
Near the end of 2003, more and more favourable elements coalesced to build an
exceptional occultation opportunity.
- First I know very well the french team of the IMCCE, Observatoire de
observed Hermione and reduced the data : J.Berthier, P.Descamps et
We had collaborations with them since some years, especially in the
They kindly agreed to communicate to me unpublished data and some
conclusions from their image reductions in progress.
- Also at the approach of a new Hermione opposition of 2004 January, a new
F.Marchis et al. had been succesful at Keck telescope on Dec. 06-07, and
were arriving at IMCCE.
- And fortunately a run with the VLT was planned also, which should end up
only 41 days
before the coming occultation (!).
For all these reasons we got serious hope of applying a reliable
extrapolation of the
orbital motion up to the occultation date, permiting usable occultation
track to be
In 2004 January-February our comprehension of the Hermione system accelerated.
- The VLT observation of January 4-6 was successful, yielding more satellite
- The AUDE amateur group led by R.Behrend at Geneve Observatory discovered
from new CCD
light curves the true rotation period : this was 5.551 h, i.e. 3/5 of the
best candidate period. So Hermione is really a fast spinning body, not
very far from
centrifuge splitting, or if we prefer, not very far from a situation of
- Freshly reduced Keck images indeed showed a bilobated silhouette (Marchis et al., 2003), that the
team modelised as "a snowman" : "two connected components of radius 90 and
60 km (a
'snowman' shape) separated by a center-to-center distance of 115 km". They
this new result together with AUDE people in the IAUC 8264 on Jan.09, then
displayed their interpretation of the body rotation as an animated .GIF on
personal web page of F.Marchis.
- Soon R.Behrend pointed out that Hermione should reach a minimum of its
at the future occultation time, leaving to us limited hope to retrieve
occultation 'the snowman sihouette' that the Keck images of 2003 Dec. did
Indeed the "snowman" should be tilted with his head in our direction.
At this stage a major problem remained : the available observations were a
from where no clear solution emerged, neither for the rotation pole of the
265 x 180 x
180 km "snowman" (soon a "rockman", I guess...), nor for the satellite
and orbital period.
Only one thing was sure : as the orbital radius of the satellite was
to 794 km with small uncertainty, and as the Hermione main occultation
follow a centerline from Athens (Greece) to Napoli (Italy), then north
Pyrenees and Bayonne (France), the satellite occultation had to follow a
somewhere between a northern limit from central Romania to south England,
and a southern
limit from north of Egypt to south of Portugal. No important mobilization
was to be expected upon such a fuzzy basis.
At January end, as the time began to slip away dangerously, and as the
in development at IMCCE, although in progress, didn't yield coherent
I resolved trying to treat the problem by hand...
After 3 days of paintful calculations, sometimes using semi-graphic
methods, I could
conclude that two very different solutions were permitted by the data for
orbit, but alas with equal chances 50-50 % : one with a P ~1.6 day period,
with inversed apparent motion on the sky and a P ~ 2.5 day period.
The first one provided a satellite occultation track across Greece, central
south of France. The other one pushed the track very far to the south,
across south of
Spain and south of Portugal, in rough neighbourhood of the above described
On February 07, as I was bewailing of the apparent impossibility to prefer
(though maybe the first one yielded a smaller scatter...?), I realized that
7980 where Merline et al. had claimed their discovery in fact contained the
information that I needed to reject the P= 2.5 j solution.
So finally we had to consider ONLY the Greek-Italo-French occultation path.
After that, it was easy to derive accurate P= 1.6273 day period, between
2002 Sep. 28,
the discovery epoch, and 2004 Jan. 06, the last known observation.
The following stage (the last one) was to estimate the slow precession of
plane of the satellite around the rotation pole of the central body. Such a
so-called "nodal precession", is a classical feature of any satellite
orbiting a non-
spherical primary, by example a primary body with polar flattening. So
satellites of Earth on low orbits, with i.e. 1.5 - 2 h sideral period,
precession at typically 6 deg./day of angular speed. A classical property
precession is constant inclination of orbital plane about the rotation
equator of the
primary. Here we expected a constant inclination of the orbital plane of
upon the rotation equator of Hermione.
But just a small detail remained : the true rotation axis of Hermione
However T.Michalowski of Poznan Observatory, who was working simultaneously
on new light
curves of Hermione from the AUDE group and also from Pic du Midi, and who
them with older data collected in the litterature, had proposed to us some
days ago a
new possible pole orientation. In theory asteroidal photometry can provide
rotation axis, when done from several heliocentric directions and properly
analyzed. T.Michalowski is an expert in this matter.
In spite of that, I used as a first approximation the old pole orientation
G.De Angelis in 1995, that the IMCCE considered previously as the most
the general fit of the satellite positions appeared degraded when I forced
to keep a constant inclination with De Angelis' equator between 2003
January and 2004
Then I remarked that another pole, wich was distant by about 50 degrees on
sphere could provide an excellent fit. Then I realized that it almost
concided with the
new one by Michalowki !
After that, all things fitted nicely. The nodal precession went in inverse
sense of the
satellite motion, as it is required by celestial mechanics. The precession
found ~40 deg./year, a very interessant value that specialists must
interpret, as it
should contain information about the actual mass distribution within the
With a reliable precession model, the final geometry at occultation time
was easy to
predict. The occultation track derived at last stage was by chance almost
the first and provisional one that I had announced some days early, i.e. it
through central Italy between Roma and Firenze, then through Provence and
France in direction of the big city of Bordeaux.
We issued a new IAUC 8285 on Feb.11 to publish the prediction.
The IMCCE team said yesterday that the new rotation pole by Michalowski
with all the available imaging of the elongated primary.
Now we have to observe the occultation to detect the satellite, though
Please remark that my claimed ± 75 km lateral uncertainty of the computed
on the ground is somewhat... uncertain itself.
In fact relative to Hermione the uncertainty of the satellite position is
less than the
occultation uncertainties claimed by Jan Manek or by Steve Preston. In fact
we get the
instantaneous radius vector of the satellite with a better accuracy than we
Hermione with respect to the star. This paradox... is not a true paradox,
as no star is
intervening when differential measurements of Hermione and its satellite
Finally the ± 75 km ground uncertainty that I have proposed in the new IAUC
reflects a Manek + Preston combined accuracy. A central question is how
is improved when we use a mix of Manek's and Preston's results, rather than
arbitrarily one of them.