Pluto was the star of a rare celestial phenomenon that occurred twice within the past month, the European Southern Observatory reported Wednesday.
As viewed from Earth, Pluto moved in front of two different stars on July 20 and August 21. These events provided observers at various observatories in South America and in the Pacific area with a long awaited opportunity to learn more about the tenuous atmosphere of that cold planet.
In July, a series of very sharp images of a small sky field with Pluto and the star was obtained with the NAOS-CONICA (NACO) adaptive optics (AO) camera mounted on the ESO VLT 8.2-m YEPUN telescope at the Paranal Observatory.
With a diameter of about 1,429 miles (2300 km), Pluto is about six times smaller than the Earth. Like our own planet, it possesses a relatively large moon, Charon, measuring about 746 miles (1200 km) across and circling Pluto at a distance of about 12,180 miles (19,600 km) once every 6.4 days. In fact, because of the similarity of the two bodies, the Pluto-Charon system is often referred to as a double planet.
At the current distance of nearly 2,796 million miles (4,500 million km) from the Earth, Pluto's disk subtends a very small angle in the sky, 0.107 arcsec. It is therefore very seldom that Pluto - during its orbital motion - passes exactly in front of a comparatively bright star. Such events are known as "occultations", and it is difficult to predict exactly when and where on the Earth's surface they are visible.
When Pluto moves in front of a star, it casts a "shadow" on the Earth's surface within which an observer cannot see the star, much like the Earth's Moon hides the Sun during a total solar eclipse. During the occultation event, Pluto's "shadow" also moves across the Earth's surface. The width of this shadow is equal to Pluto's diameter, i.e. about 2300 km.
One such occultation event was observed in 1988, and two others were expected to occur in 2002, according to predictions published in 2000 by American astronomers Steve W. McDonald and James L. Elliot (Massachussetts Institute of Technology [MIT], Cambridge, USA). Further refinements provided by other observers later showed that the first event would be visible from South America on July 20, 2002. A second one on August 21 was expected to be observable in the Pacific basin, from the western coast of North America down to Hawaii and New Zealand.
A stellar occultation provides a useful opportunity to study the planetary atmosphere, by means of accurate photometric measurements of the dimming of the stellar light, as the planet moves in front of the star. The observed variation of the light intensity and colour provides crucial information about the structure (atmospheric layers) and composition of different gases and aerosols.
In order to take advantage of this rare opportunity to learn more about Pluto and its atmosphere, more than 20 scientists and engineers from the Paris Observatory and associated institutions gathered to observe the July 20, 2002, event. This involvied an occultation of a star of visual magnitude 11 (i.e., about 100 times fainter than what can be perceived with then unaided eye), referred to as "P126" in McDonald and Elliot's catalogue.
In May 2002, preparatory observations showed that star to be double, with the brighter component of the system ("P126 A") being likely to be occulted by Pluto, as seen from South America. However, because of the duplicity, the predictions of exactly where the shadow of Pluto would sweep the ground were uncertain by about 0.1 arcsec in the sky, corresponding to more than 2000 km on the ground.
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