The relevance of asteroid occultation measurements

1. The relevance of asteroid occultation measurements Detlef Koschny, GerhardDrolshagen, Nicolas Bobrinsky European Space Agency,Solar System Missions DivisionKeplerlaan 1NL-2201 AZ Noordwijk ZH Detlef.Koschny@esa.int Asteroid occultation primerPredictionsHow are they observed? Results: Size and shapeResults: PositionsConclusions Image courtesy: David A. Hardy

2. Asteroid occultation primer Prediction error because of position uncertainties

3. These observations are relevant because: • This is the only direct method of determining shape and size of an object other than going there (optical interferometry was used and published in Feb 2009 – almost direct but not quite, see Delbo 2009, using the VLT/MIDI) • The main contributing error in the determined astrometric position is the error of the star position in the catalog => milliarcsec accuracy is achievable

4. These observations are relevant because: • This is the only direct method of determining shape and size of an object other than going there (optical interferometry was used and published in Feb 2009 – almost direct but not quite, see Delbo 2009, using the VLT/MIDI) • The main contributing error in the determined astrometric position is the error of the star position in the catalog => milliarcsec accuracy is achievable Is it useful to include occultation work in our SSA systems?

5. Occultation primer - predictions Steve Preston’s predictions Hristo Pavlov’s ‘OccultWatcher’ David Herald’s ‘Occult’ Oliver Klos’ maps

6. How are they observed? • Use telescope with video camera to record a video of the star • Ensure high-accurate timing measurements by inserting GPS-synchronized time into video

7. Resulting video

8. Star intensity versus time

9. Star intensity versus time Occultation

10. Results: Size/shape • Combining several observations, size and possibly even shape of theasteroid (in the planeof observation) can bedetermined directly

11. Occultation observations Shape model derived from photometric observations

12. Results: Positions • D. Herald (Australia) has set up an interface with the Minor Planet Center to allow submission of positions determined by occultations, using the IAU observatory code 244. • Accuracies can be obtained to 0.01” down to 0.0002” relative to the star. Star accuracies are about 0.07” (=> Optical astrometry results in ~0.1”) • When better positions available data can be improved (e.g. using the USNO Robotic Astrometric Telescope, Zacharias et al. 2009; J-MAPS – Gaume + Dorland 2008; Gaia)

13. Conclusions - I • Asteroid occultations (= asteroid occults star) can provide shape and size of asteroids in the plane of observation • They can provide high-accuracy position measurements • Accuracy limited by accuracy of star in catalogue • Observation limit is the magnitude of the star (not the asteroid) • Observations mainly done by amateurs • Pipeline for shape/size: see work by Euraster, E. Frappa • Pipeline for position measurements set up by D. Herald • BUT: For objects <a few km accuracy issues • Timing accuracy => go to higher frame rate • Star positions => get better star catalogue • Path uncertainty => use more observing stations

14. Conclusions - II • Possible improvements • One central system for predictions • One central database for results • Use larger aperture for more events (fainter stars) or just better signal-to-noise • Organise campaigns forshape/centroid determinations • Use better cameras,e.g. EM-CCDs; more dynamics,higher frame rate • Go space-based – Kepler?

15. Conclusions - III • Yes… but: • 0.02 s timing accuracy needs to be ensured – higher accuracy preferred (use high-speed cameras) • Good signal-to-noise needed • Enough stations to cover predicted path and still give reasonable spatial accuracy are needed Is it useful to include occultation work in our SSA systems?

16. Resources • Steve Preston’s predictions • http://www.asteroidoccultation.com/ • Occult • http://www.lunar-occultations.com/iota/occult4.htm • OccultWatcher (prediction software) • http://hristopavlov.net/OccultWatcher/OccultWatcher.html • European Asteroidal Occultation Network • http://www.astrosurf.com/eaon/ • Euraster – results from European occultation observations • http://www.euraster.net/ • Asteroid occultation page by the Czech Astronomical Society (on behalf of the International Occultation Timing Association IOTA • http://mpocc.astro.cz/ • Database of observations by Mike Kretlow • http://sky-lab.net/?Solar_System_-_Occultations • LiMovie – Light Measurement tool for occulations • http://www005.upp.so-net.ne.jp/k_miyash/occ02/limovie_en.html • Bruno Sicardy’s home page • http://www.lesia.obspm.fr/perso/bruno-sicardy/ • Leslie Young’s home page • http://www.boulder.swri.edu/~layoung/occl/ • Archive of occultations in the Planetary Data System (PDS) • http://www.psi.edu/pds/resource/occ.html • Japanese page on occultations • http://uchukan.satsumasendai.jp/data/occult-e/occult-e.html • Australasian page on occultations • http://occsec.wellington.net.nz/ • North America’s page on occultations • http://www.asteroidoccultation.com/observations/Results