Occultation of HIP 111398 by a trans-neptunian binary
(66652) 1999 RZ253 on 4 October 2007
(Updated 29 September 2007)
Original Goffin prediction in GIF and PDF format.
The following message was posted by J.Lecacheux (France)
at Pic du Midi to the Yahoo "IOTAOccultations" mailing list.
Dear colleagues,
Denis Denissenko has predicted three years ago a possible
exceptional occultation of a bright V= 6.5 star (HIP 111398)
by the double trans-neptunian 66652 = 1999 RZ253 on next
2007 October 04, 11:25-34 UT.
Simply looking at Denis' original map, or at E.Goffin's similar
one, based upon the old MPC elements MPO48771-,
ftp://ftp.ster.kuleuven.ac.be/dist/vvs/asteroids/2007/PDF/ ...
...
A07_10099.pdf
and keeping in mind the large ephemeris uncertainty (not plotted
on the quoted map), we feel certain that the double event will
take place somewhere across the Pacific Ocean and in some country
westside, like Japan, east China, Indonesia or Australia.
Alas that expectation looks optimistic.
Indeed although it was numbered by the IAU, this mag.22 TNO was
poorly observed by astrometrists past the promissing 1999-2004
initial period (31 measurements) : no new position has been added
in the AstDys and MPC public databases since May 2004, and even
nine only were entered during 2001-2004...
In consequence the occultation prospect has turned exceedingly
uncertain in 2007.
Using the Astorb or MPC (MPO101945) current elements, based as
above explained on the 1999-2004 arc, my 'WinOccult' software
no longer finds this occultation.
So I investigated using the IMCCE and JPL online ephemeris and
completing calculation by hand, and found that the double shadow
should miss the Earth by a big 0".75 amount or 22 000 km, (China
being closest, New-Zealand farthest). Also the closest approach
should occur 45 mn late.
So the occultation now seems cancelled !
However, don't be too impressed by this apparent cancellation.
Indeed according to AstDys, the uncertainty (1 sigma) now has
grown up to 23 000 km perpendicular to the path and 58 000 km
or 50 minutes along the path.
It results that the occultation still keeps 10 % probability of
hitting our planet.
Let us summarize the situation, now assuming a 95 % confidence
level, i.e. error limits at 2 sigmas instead of 1 sigma :
- The TNO should miss the Earth by 22 000 ± 46 000 km northside
(what means that passing south is not excluded -probability 7%-);
- the TNO should by late by 45 ± 100 mn
(what means that happening early is not excluded -probability
20 %-).
It remains that a naked eye star approached so nearly by some
major TNO, (a remarkable binary besides), is a very rare
circumstance.
For this reason I think that any observer within the Pacific zone
should try monitoring the mag. V= 6.5 orange giant in Aquarius
on Oct.4, in spite of the minimal (~ 1/400 ?) chance of individual
success, and the uncomfortable long duration of the survey.
So long that automatic recording followed by methodic play back
seems required for any efficiency.
The main body in the 66652 couple is supposed about 170 km large,
assuming a 0.15 V-albedo, and its satellite about 140 km, if both
albedos are equal.
According to Noll et al. ((2004) Icarus, 172,402), who had studied
the system with the 'Hubble' Space Telescope, the 46 days orbit is
markedly elliptic, with the periastron at 2 500 km and apoastron
at 6 800 km.
So the two shadows of the Hipparcos star should appear separated
by a fraction of terrestrial radius, following parallel paths some
minutes apart.
Even better : from the elements published by Noll et al., I may
infer that at the occultation date the satellite should be trailing,
about 5 minutes late relative to the primary body, and that it
should pass 3 000 or 4 000 km more south.
How any double occultation crossing the Pacific would present
concretely ?
Let us give two examples, chosen among a multitude :
-First an optimal example : the main shadow could fly over Hawaii
at one time, then the secondary over south-east Australia ten
minutes later.
-Other scenario, a degree less favourable : the main shadow could
miss our planet north from Siberia, then the secondary could hit
Japan five minutes later.
However don't forget that the more probable scenario by far, of
probability 75 %, is a double very distant Earth miss to the NW.
Already before this occultation, I guess there are two lessons to
retain :
1- Don't believe that the few numbered TNOs automatically do have
better elements and ephemeris. This will remain true only if
their astrometry is maintened on long-term.
2- Beside the observational comfort they provide, the occultations
of bright stars don't bring any special bonus to the asteroidal
science.
Actually an huge number of faint stars are occulted by trans-
neptunian bodies. Among the large number of these events often
considered as difficult, some actually might be proved more
promissing than this one above described. Choosing TNOs with
fresh astrometry could be more rewarding than focusing on the
brightest target stars.
In any case, very good luck to the Oct.04 observers !
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