Just as the Moon passes in front of background stars, so too do planets and minor planets (also called asteroids). However planetary occultations occur less frequently than lunar occultations because the planets appear so much smaller in our sky than does the Moon. Nevertheless, observing occultations of stars by planets has yielded some stunning discoveries - for example, the rings of Uranus, and the atmosphere around Pluto.
While occultations of bright stars by major planets are very rare, occultations by asteroids are a little less so. This is not because any one asteroid has a greater chance of passing in front of a star. Rather, it is because there are so many more asteroids to choose from!
Typically, the occultation shadows of perhaps 100 asteroids will sweep across parts of Australia and New Zealand annually. Each shadow is usually about the same width as the minor planet (typically 100-200 km), although the shadow itself does sweep out a strip many thousands of kilometres long across the Earth. Thus the same asteroid occultation can potentially be observed in New Zealand, and on both the east and west coasts of Australia.
WHY OBSERVE ASTEROID OCCULTATIONS?
The value in observing minor planet occultations is rather similar to that in observing grazing occultations. If the occultation shadow band is perhaps a few tens to a few hundred kilometres wide, then observers situated within the band and perpendicular to the direction of motion of the shadow will each see the star occulted by a different part of the asteroid. If enough observations are obtained, one can essentially "join the dots" to build up a picture of the shape of the asteroid. Even if only two observers see an event, so long as they are separated by a reasonable distance an average diameter for the asteroid can be deduced - the only direct way in which this information can be obtained. Determining the diameter of a minor planet is important because it can provide clues to the asteroid's density, which in turn tells us something about its bulk composition and thus its origin.
Because the chances of a specific asteroidal occultation track passing across an established observatory are not high, the contribution that amateur astronomers with portable telescopes can make is considerably enhanced. Indeed, it is the ability of amateurs to relocate themselves and their telescopes at short notice which is frequently crucial to the success of an observation.
Observers in Australia and New Zealand have achieved several excellent successes in doing just this, most notably in deriving the diameters of minor planets (9) Metis, (44) Gyptis and (94) Aurora.
PREDICTING ASTEROIDAL OCCULTATIONS
Predicting exactly where an asteroid's shadow will pass as it moves across the Earth is quite difficult. To do so the relative positions of the star and moving asteroid need to be measured very accurately, which is a difficult task given that both appear in a telescope as a point source of light. Until recently the best we have been able to do is to predict an occultation track with an error of perhaps one hundred to a few hundred kilometres. This has meant that the chances of seeing an actual occultation have not been high, and persistence in observing many events has been the name of the game. Fortunately though, through the use of "last-minute astrometry" (in which the positions of both the asteroid and star are measured on the night before the occultation, when they are very close together), together with recently-released high-precision star catalogues, the accuracy of the predictions is being enormously improved.
Planetary occultation predictions are prepared annually by Edwin Goffin in Belgium and distributed through the International Occultation Timing Association (IOTA) and the RASNZ Occultation Section.
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