Cathy Olkin

Determine size and shape This requires

multiple chords

With only one chord you only get lower limit on size2009/11/19 Occultation

by Terpsichore (124x112 km), Dunham, Garrett and Desmarais IOTA website

If we could get a Pluto occultation and a Nix or Hydra occultation by the same star, that would greatly reduced our orbit uncertainty

Sicardy et al. 2009

Multiple chords in small area with large uncertainties

Prediction on July 18 Occultation on July 31, 2007

How well can we predict Pluto occultations? Only to about 0.5 radius or ~600 km (25 mas) Usually limited by Pluto position not star position

Assume diameter = 80 km Space observers 40 km

Can’t get large telescopes with this spacing => need bright star

To cover 600 km, you would need 15 observers Only go to dry weather places

Good news: To get shape you can use cheaper equipment

How uncertain are the Nix and Hydra ephemerides now?

This is on top of the uncertainty in the star relative to the Pluto system

From Tholen et al., 2008

Title: Attempted Observations of the 2009 Occultation of a Star by Nix

Authors: Pasachoff, Jay M.; Widemann, T.; Sicardy, B.; Lister, T.; Tholen, D. J.; Gulbis, A. A. S.; Adams, E. R.

Publication: American Astronomical Society, AAS Meeting #214, #606.01

Abstract We attempted to observe the predicted 4 March 2009, 13:56 UT, approximately 6-second-long occultation of an 11th magnitude star by Pluto's small moon Nix, which is fainter than 23rd magnitude. In parallel starlight, the path was 88 km in width with a one-sigma range from 44 km to 110 km. The Tycho star is at 18 12 09.86 17 42 03.3 (J2000.0), and has R=11.8 and K=10.4. The occultation path was predicted to cross the Hawaiian islands, with a southern boundary cutting through Maui and missing the telescopes on Mauna Kea, but the uncertainty in the prediction (1 sigma) was about 500 km (though only 5 minutes in time). The path's speed was 15 km/s. In the event, we obtained data only from the 2-m Faulkes telescope on Maui, with uneven skies. Our trailed images did not show evidence of the occultation, which would have dropped the observed intensity at the merged star/Nix position by about 12 magnitudes or, more likely, if Nix and Pluto were merged by about 3 magnitudes. Weather prevented observations with MegaCam on the CFHT, with a MIT POETS on the IRTF, with PanSTARRS on Maui, as well as with an 0.4-m telescope at Windward Community College on Oahu. For administrative reasons, we did not succeed in obtaining data with the 3.7-m AEOS telescope on Maui. In any case, the event was at low altitude, only 21°, so pointing was at the limits for several of the telescopes.

Lots of Prospects… Bruno Sicardy lists 202 Nix occultations in

2010 and 185 Hydra events http://www.lesia.obspm.fr/perso/bruno-

sicardy/predic_occn_10/Hydra_2010/g3_occ_data_hydra_2010_table

Good weather Somewhat bright star (V<15), allowing lots of

small telescopes Already deployed for Pluto or Charon event to

minimize cost