I'm involved with a group that is making a serious effort to observe occultations TNO
asteroids. A successful observation will directly determine the diameter of the asteroid.
Additionally, the light curve has the potential to indicate the presence of any atmosphere
(as in the case of Pluto).
The challenges in observing such an event are huge. Because of their slow motion and large
distance, there are very few occultations by any one TNO. Also, with a parallax of only
0.2" or so, a positional uncertainty of 0.2" equates to an uncertainty in the location of
the occultation path of around the radius of the Earth. And the typical ephemeris accuracy
of a TNO is somewhat greater than 0.2"!
On December 7 we are confidently predicting an occultation of a mag 14.7 star by the TNO
20000 Varuna. The occultation path is expected to pass across South America and southern
Africa. Brazil and Namibia would appear to be the most likely locations.
The text below sets out the activities that have taken place in the last few weeks to try
to get an accurate prediction for this event. It also include links that give the latest
predictions of this event. Note that the occultation is predicted to last about 43 secs,
based on an assumed diameter of 900km.
We encourage anyone located in South America or southern Africa to monitor this event -
noting that the actual path location is still uncertain by perhaps 2000km.
Anyone in those regions that are interested in monitoring this event should contact
myself, J.Lecacheux, or Bruno Sicardy (see his page below) for any guidance they might
----- Original Message -----
Sent: Monday, December 01, 2008 1:17 AM
Subject: [PLANOCCULT] News about 20000 VARUNA : an update.
> >From J.Lecacheux.
> A tremendous activity followed my private message to Bruno Sicardy (not
> sent to [planoccult] as written in French) about the possible occultation
> of a faint V= 14.7 UCAC star in Gemini by the big TNO (20000) Varuna on
> Dec.07 morning.
> Preliminary calculation using "Occult 4" first put the nominal track from
> the Arabic peninsula to Morocco and Florida (USA) with a very large "one
> sigma" uncertainty of 1.2 Re (earth radius), later reduced by S.Preston
> to 0.7 Re. Whole Europe then was concerned by an event of almost uniform
> small probability : for instance between 6 and 11 % from Granada (Spain),
> or between 5 and 9 % from Berlin (Germany).
> Then around the last New Moon several observers made "last minute" CCD
> exposures of the V= 18.9 body, namely B.Tregon and F.Colas from Pic-du-Midi
> Observatory (60-cm and 105-cm telescopes), F.Manzini from nw.Italy (40-cm),
> B.Christophe from nw. France (60-cm).
> Then the Rio-de Janeiro team led by M.Assafin (Brazil), and R.Behrend from
> Geneva (Switzerland), made independent astrometric reductions of these data,
> finally obtaining good mutual agreement.
> The updated nominal path is shifted by 0".2 or 6000 km south -unfortunately
> for all the European observers-, and now is predicted from the south tip
> of South-Africa (near 02:10 UT in early dawn) to south Brazil (near 02:15
> UT at low elevation).
> In the same time the "one sigma" uncertainty was lowered to 0.12 Re (i.e.
> ten times better than before the new astrometry), whilst in consequence
> the probability of positive detection was boosted greatly.
> Now it could reach up to 44 % from Porto Alegre (south Brazil), 40 % from
> Sao Paulo (Brazil), 37 % from Capetown (South-Africa), 14 % from Windhoek
> (Namibia), 8 % from Recife (nordeste, Brazil), 1.5 % from Belem (Brazil)
> at the Amazon mouth..., and will be fully negligible from any place in the
> northern hemisphere.
> Never any occultation by a Kuiper body, Pluto excepted, reached a likehood
> level so high.
> More details may be found at the adresses of Bruno Sicardy (Meudon)
> and IOTA-ES
> VARUNA is a reddish "cubewano" of 280 years period, whose size lies in the
> same class as the biggest classical asteroids, Ceres or Pallas. Its
> rotation period is 6h 21mn. See a preliminary lightcurve on "CdR&CdL" at
> Note that the occultation time will coincide with a minimum of brightness.