>From J.Lecacheux.

First a recall : 
Good record of a positive occultation of a V= 17.1 star by the big 
TNO 136199 ERIS was obtained in Chile almost one month ago. Belgian, 
Spanish and French astronomers used 60-cm, 40-cm and 50-cm telescopes 
respectively, all equipped with thinned back-illuminated CCDs. Two 
telescopes, the 60 and 40-cm, were remote-controlled. 

As occultations of mag. 17 stars are more frequent by a factor 30
than those of mag. 13, and as the TNOs are desperately slow-moving 
on the sky, there is no doubt that we have to accomodate to such 
faint stars to export in the Kuiper world the collective successes 
we got in the Main Belt since two decades.

Now the incoming event : 
Another occultation by a large TNO, 19521 CHAOS, could arise on the 
next December 04-05 Saturday night. The magnitude of the star, lying 
in the Taurus constellation high in the sky, is V= 17.2.

I obtained the here attached map 10z04CHa_xl.gif by taking the star 
position from the new PPMxl catalog, which is containing fainter 
stars than the UCAC*, with perhaps more accurate proper motions.

You can see that the nominal track computed with the PPMxl and the 
astorb.dat (JPL) elements is from Madagascar to Brazil.

Actually such a plotting precision of the path is misleading, as 
systematic astrometry of CHAOS seems practically discontinued since 
2008. In consequence the position uncertainty is large, and was 
growing along 2008, 2009 and 2010 continuously. 

Taking the current ellipse of uncertainties from the AstDys database, 
I found that the real chance that CHAOS' shadow will intercept our 
planet is only 37 % (with probabilities 42 % for south miss and 21 % 
for north miss).


Now what is the probability of positive occultation from a given 
observatory ? 

To estimate that, I first have to estimate CHAOS' diameter. 
OCCULT assumes 740 km. We only know that CHAOS belongs to the 
class of dark reddish TNOs. Its real size is unknown (as we are 
just searching for it !), but we easily deduce that the 740 km 
value would correspond to the very low albedo 0.035. 

Considering 570 km (or albedo 0.06) as slightly more likely, 
I can finally compute the following probabilities :
- 1.7 % on the nominal track, for example in Rio-de-Janeiro;
- 1.5 % at any place in Europe.

The current magnitude of CHAOS is V= 21.1, not out of reach with 
60-cm telescopes and long CCD exposures. However because of the 
lack of time, it is dubious that last minute astrometry of CHAOS 
and related star will be achieved before next Saturday.

So little time remaining is a pity [Sorry, I discovered this 
occultation only yesterday !], as I guess that one or two sets 
of good astrometric exposures would bring immediate improvement 
of the prediction accuracy by a factor over 5. 
For example it would result a revised probability of success 
just under 10 % for any single station in Namibia.


Nevertheless I hope that some European people accessing large 
telescopes (remote-controlled or not) will try this occultation, 
in spite of the tiny 1.5 % probability.

Then one should well note that following the AstDys model of 
CHAOS uncertainties, the 2 sigmas time uncertainty is +/-30 mn 
(i.e. twice the amount directly read across the large circle 
on 10z04CHa_xl.gif).
So please plan one full hour record, from 00:02 to 01:02 UT.


I wish worldwide clear sky and good luck to everyone !

J. Lecacheux,
IMCCE