Update on March 18th’s Occultation of a Star by Pluto

If you have access to a 10-inch or larger telescope (the larger, the better) and a sensitive video camera or CCD and recording system, and if you are in central or western North America, please help us find the central flash for Pluto’s occultation of a 15th-magnitude star in Sagittarius. This widely anticipated event will occur early on Sunday morning, March 18, 2007, and is described in my earlier AstroAlert:

Any observations of the central flash will provide valuable information about the atmosphere of Pluto. This information will help in studying the time history of Pluto's atmosphere, which is expected to freeze out and collapse sometime during the next several years as Pluto's distance from the Sun increases. It will also be important for planning observations by the New Horizons spacecraft that is en route to the Pluto system. The prediction uncertainties are large enough that the central flash could occur anywhere from the north-central USA to northern Mexico and southern Baja California. Since a good central flash is expected to occur only within a path about 100 kilometers wide, many observatories must try to record the occultation so there will be a reasonable chance of anyone observing the central flash.

Even if you don't observe the central flash, your observation will still help to determine accurate astrometric parameters of the occultation. This will aid in determining the Plutocentric latitudes of the recordings of the star's fluctuations in the atmosphere that are expected to be made from major observatories. Your observation could also capture some unique aspect of Pluto's atmosphere; we are interested in how the atmosphere varies with different latitudes around Pluto.

Much information about this event was given in my earlier message. More is on my website for the event at http://iota.jhuapl.edu/pluto.htm, where a link is also given to Bruno Sicardy’s comprehensive site.

I've checked Accuweather predictions for Texas; it shows that there will be only scattered cirrus over most of that state, with slightly thicker cirrus only in the far south.

Station List

A few observers are mobile and will want to avoid "chords" that will be recorded from the several fixed observatories attempting this event. Therefore, please let us know if you will be at one of them, especially if it is not marked as a confirmed attempt in the station list that I am maintaining on my site.

If you have sent me your station plans and don’t see them listed, send an email message to me and to Derek at breit_ideas@hotmail.com. Dave Herald provided the Occult input file for this event several months ago. I have modified it so that the central line is about halfway between the "Swiss" and "Brazilian" predictions with Pluto offset as described on Bruno Sicardy's website. But, as noted above, the actual location of the center of the occultation path (from which the central flash can be observed) is poorly known, since it is not certain that the Pluto ephemeris offset will be the same this month as it was for the previous event. Sicardy believes that the error could be 1,000 km or even a little more. There's only a small chance that we might get some recent astrometry to improve the prediction, now that the two objects are in the same CCD field for long-focal-length astrometric telescopes. So for now, the central line defined by my prediction should be considered only as a reference line from which different observatories can be compared.

The station list is also useful because you can use it to tell the predicted time of closest approach of Pluto's center to the star. Note that, for all observers, the event will occur late during 10h UT, which is 5 am CDT, 4 am MDT, and 3 am PDT (= MST for Arizona) on Sunday morning.

I'll post updated versions of the station list each day up to Saturday morning at 14h UT. The predicted central occultation by Pluto's surface is 333 seconds, but Pluto atmospheric variations will extend this at least to 400 seconds. It is recommended that observers record for at least a 10-minute period centered on the predicted time of closest approach (that is, from 5 minutes before to 5 minutes after).

Please check the online version of this AstroAlert on Sky & Telescope's website for possible minor updates:

Any major updates will be announced via subsequent AstroAlert messages. Good luck, and clear skies!

David W. Dunham
Contributing Editor
Sky & Telescope
work: david.dunham@jhuapl.edu

Note added March 20th: I know of 19 observatories that recorded or otherwise observed the occultation by Pluto last Sunday morning; information about them, and preliminary results, are at http://iota.jhuapl.edu/pluto.htm. I'll update that website as I learn more.

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