On September 27, 2007, around 14.8h Universal Time, Pluto passes extremely close to an 8.7-magnitude star, SAO 160793, in Sagittarius (see the chart on page 60 of the July Sky & Telescope). This is the brightest star approached so closely by Pluto since predictions of occultations by this dwarf planet were first made over 30 years ago.

Earth will actually pass "between the shadows" of Pluto and its moon Charon, with no occultation by either object visible from Earth's surface. But occultations by possible Plutonian rings might be observed from the night areas of most of Asia (from Arabia to Vietnam to western and southern China to the Ural Mountains), the Horn of Africa, western Australia, part of Antarctica, and the Indian Ocean. Observers in those and adjacent areas, such as Japan, eastern Australia, New Zealand, and South Africa, are encouraged to record the star with CCD, video, or photometric equipment for a few hours around the time of closest approach; such records can be analyzed later to find evidence of any fadings of the star's light that might be due to Plutonian rings; these would be of great interest to scientists involved with the New Horizons mission to Pluto.

Walker Vaning of San Rafael, California, claims that observations of past occultations show that Pluto has an extensive ring system, and that it could cause fadings of SAO 160793's light even more than 12 hours from the time of closest approach. So he encourages observers worldwide to monitor the star while it is high enough above the local horizon in a dark sky. He thinks that very distant rings may exist that could pass over the star even 3 days from the time of Pluto's closest approach.

The star is also known as BD –16° 4607, P507, and UCAC2 25587116. It is at right ascension 17h 44m 38.4s, declination –16° 46' 35" (equinox 2000.0), and its spectral type is K5.

This event was mentioned in a previous S&T AstroAlert here, and information about it is also given on the Massachusetts Institute of Technology's website:

Reporting Your Observations

For occultations of stars by solar-system bodies, the International Occultation Timing Association has special report forms (.xls versions preferred, but plain-text forms are available as well) at these URLs:

Once you complete one of these forms, please send it to IOTA's e-mail address for reporting asteroid-occultation observations: [email protected].

David W. Dunham

Contributing Editor

Sky & Telescope

[email protected]


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