History of Asteroidal Occultations and Results from their Observation

History of Asteroidal Occultations and Results from their Observation
New: 2003 June 16
The geometry of an eclipse of a star by an asteroid, called an "asteroidal occultation" by astronomers, is shown in this 900 kilobyte figure; click here for a Word version. As the asteroid moves in its orbit around the Sun, it sometimes passes in front of, or "occults", a star as seen from the Earth. A "shadow" of the asteroid is cast on the Earth's surface by the asteroid, and this shadow moves rapidly, often more than 10 kilometers per second. The view for five observers is illustrated in the lower left, with their locations on the Earth marked. "A" and "E" are outside of the path and have no occultation. Observers "B", "C", and "D" have an occultation of different lengths; if they time it, each one traces out a line, or "chord", behind the asteroid.
Timings of asteroidal occultations are used to measure the sizes and shapes of asteroids. Except for the much rarer spacecraft visits, occultations give the most accurate sizes for the most common main-belt asteroids between Mars and Jupiter. In addition, observations of asteroidal occultations have discovered, and measureed the parameters of, several very close double stars, most unresolvable by other means. Although the observations remain controversial, the International Occultation Timing Association (IOTA) claims that the first evidence for asteroial satellites, or moons of asteroids, came from occultations observed in the 1970's.
Asteroids are small and far away, making it very difficult to predict their eclipses of stars (called "asteroidal occultations" by astronomers). The first one, by the large asteroid (3) Juno, was observed by P. Bjorklund and S. Muller from Malmo, Sweden on 1958 February 19. It was not until the mid-1970's that the paths of the larger asteroids were determined accurately enough to predict some of their occultations. On 1975 January 24th, 8 observers in Connecticut and Massachusetts timed an occultation of the star kappa Geminorum by (433) Eros, allowing the first outline of an asteroid to be determined. Later, on 2000 February 14, the NEAR - Shoemaker spacecraft arrived at Eros and orbited that near-Earth asteroid for a year, then landed on its surface.
Some of these early observations, especially those of the occultation of gamma Ceti by (6) Hebe on 1977 March 5 and of SAO 120774 by (532) Herculina on 1978 June 7, had more than one occultation of the star reported, indicating that some asteroids had small moons, but that inference remained controversial until the Galileo spacecraft imaged Dactyl, the satellite of the asteroid (143) Ida, on its way to Jupiter in 1993; now, about 30 asteroids are known to have satellites.
Over 350 asteroidal occultations have been observed, most of them during the last five years after accurate directions for over 2 million stars were measured from space, without the distortions of the Earth's atmosphere, by the European Space Agency's Hipparcos satellite. Click here for a list of all known asteroidal occultations observed to May 2002. For most of the well-observed events, it includes references to articles where detailed analysis of the observations have been published. Dozens of these occultations were observed, mainly by amateur astronomers, well enough to determine the sizes and best-fit elliptical shapes of the occulting asteroids. David Herald (Kambah, ACT, Australia) lists the results here where the number and name of the asteroid is given, followed by its determined major and minor axes, and their errors, the position angle (direction from north) of the major axis, quality (Q), and the date of the occultation. If a circle was fit to the observations rather than an ellipse, the major and minor axes values are the same and "0" is given for the position angle (PA) of the major axis. The asteroids are listed in order of their number within each quality (Q) group, where Q = 3 is an excellent determination from observations well-distributed across the asteroid; Q = 2 where the observations are reliable but their distribution across the asteroid is not complete; and Q = 1 where the determination is less than ideal. Also included are results from 3 occultations of stars by satellites of Saturn (P6) and Uranus (P7), where the satellite's number in the planetary system is given after "M". The parametes of close double stars determined from some of the occultations are given at the bottom.
The occultation of the star 1 Vulpeculae by (2) Pallas on 1983 May 29 was observed from 130 locations across the southern USA and northwestern Mexico, making it the best-observed asteroidal occultation so far. The observations plotted in the plane of the sky at Pallas are shown 3/4ths of the way down Paul Maley's Occultation Page. More recently, the 2nd-best occultation, of 43 Tauri by (345) Tercidina on 2002 September 17, was observed from over 70 locations in Europe. In both cases, most of the observations were made by amateur astronomers who travelled by automobile into the path. For Tercidina, many observers crossed national borders; for example, more Czech observers timed that occultation near Munich, Germany than German observers. Because the Czechs were mobile, they found locations with larger breaks in the clouds, whereas most of the Germans stayed at their homes and were clouded out. The observations of the Tercidina occultation are here with the interesting outline showing a large bulge on one side near the bottom; links near the top show the occultation path, with one showing a weather satellite map with observer locations plotted. The third-best observed asteroidal occultation was timed on March 23rd this year from over 30 stations in Japan and Hawaii.
On 1991 January 13, the largest mobilization for an asteroidal occultation, of the bright star Alhena (gamma Geminorum) by the asteroid (381) Myrrha, was organized by Sichao Wang at the Purple Mountain Observatory in Nanjing, China. He made a large public effort to obtain observations across the country. In those days, the predictions for occultations of bright stars by the faint asteroids were very poor since the Hipparcos stellar data were not yet available, and the actual path for the Myrrha occultation passed about 700 km north of where it was expected. Of the 5000 Chinese who watched, only 4 in Shandong Province saw the occultation. But the northward shift put the shadow right across Tokyo, Japan, where 20 observers were surprized to see the occultation, but prepared to time it, and essentially stole the show from the Chinese. If the Japanese had thought that the path might really cross Tokyo, they probably could have made a public effort to obtain thousands of observations, but they felt that the prediction was too uncertain to risk that effort. But now, with the help of the Hipparcos observations, we can predict these shadow paths quite well, so we can expect to do much better with Alkeste and Zavijava on June 24th, weather permitting.
Most observed asteroidal occultation now involve stars requiring medium-sized telescopes to observe. Eclipses of stars as bright as Zavijava by asteroids as large as Alkeste, where the path can be predicted rather well and the star can be seen without optical aid, occur only once every two or three years, usually over oceans or lands with few if any potential observers. If the weather cooperates, with your help the June 24th Alkeste eclipse could become the best-observed asteroidal occultation.
Note: "Occultation" is derived from the Latin word "Occult" meaning "to hide"; as used by astronomers, it has nothing to do with the "occult" sciences. The words "eclipse" and "occultation" can be used almost interchangeably, but for astronomers, "eclipse" usually involves the Sun and the casting of visible shadows, while "occultation" is used for everything else, most commonly of occultations of stars by the Moon, where the Moon covers the star as seen by an observer on the Earth, but no shadow visible by a casual (non-telescopic) observer is cast.