Abstract

The Promise and Limitations of Hipparcos Astrometry
for Observations of Asteroidal Occultations

David Dunham

Die Vorhersagen für die Beobachtung von Dutzenden von Asteroiden wurden in der letzten Zeit durch den Hipparcos-Sternkatalog und ein Programm am U.S. Naval Observatory so verbessert, daß man auf der berechneten Zentrallinie mit 50 % Wahrscheinlichkeit auch eine Bedeckung beobachten kann.
Nun steigt die Zahl erfolgreicher Beobachtungen stark an.
Aber erst die Beobachtung durch mehrere Stationen erlaubt auch die Bestimmung von genäherten Profilen von Asteroiden. Zur weiteren Information werden drei Internetadressen angegeben.

On 1997 June 10, Patrick O'Connor travelled to a location north of Winnepeg in the updated path of an occultation of the 6.2-magnitude Hipparcos star SAO 164249 by the small asteroid 170 Maria (angular diameter less than 0.02") and successfully timed the event.

This stunning first success of Hipparcos astrometry applied to an asteroidal occultation, based on astrometric observations obtained at Table Mountain Observatory in California, gave confidence in the accuracy of similar observations that were made of 253 Mathilde to support the successful flyby observations of that asteroid 16 days later by the Near Earth Asteroid Rendezvous spacecraft.

Since then, thanks also to the extensive program to observe asteroids relative to Hipparcos and ACT stars at the U. S. Naval Observatory in Flagstaff, Arizona, and by a few other observatories, we have refined the predictions for dozens of asteroidal occultation paths. As a result, more observations of asteroidal occultations were made during 1998 than in any previous year, and the rate is even better this year. However, many of the asteroids are quite small and the Earth-based observations of the asteroids to link them into the Hipparcos/ACT system are not as accurate as the Hipparcos data. As a result, when we update a path, we can not guarantee that an observer on the central line will see an occultation (weather permitting), but this does happen now almost 50% of the time. Those within a path-width of an updated prediction have about a 30% chance for an occultation, meaning that if they try three times, they will probably succeed at least once. However, still too many of the events are observed from only one station, whereas at least two chords are needed to determine a circular diameter, and at least three are necessary for measuring an elliptical outline. The detail of the profile of the asteroid that we can determine is proportional to the number of observations from separate locations.

More information about these events is at http://www.anomalies.com/iotaweb.htm with the latest updates generally at http://www.lunar-occultations.com/iota; a site giving all known worldwide observations of asteroidal occultations is at http://sorry.vse.cz/~ludek/mp/world.

Dr. David Dunham, President of the International Occultation Timing Association & Johns Hopkins Univ. Applied Physics Lab.
Address: 7006 Megan Ln., Greenbelt, MD 20770-3012, U.S.A. E-Mail: dunham@erols.com Internet: http://iota.jhuapl.edu
Telephone: +1 301 474 4722 Fax: +1 240 228 1093

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