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Alert Notice 460: Possible occultation by Pluto from US East Coast

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10 June 2012: We have been asked to help disseminate the news of a possible occultation by Pluto visible to observers on the US East coast. Although the AAVSO does not ordinarily issue announcements of upcoming occultations, in this case the object is Pluto and the NASA New Horizons mission ( will be visiting Pluto in 2015.

The information below has been supplied by Dr. Leslie Young (Southwest Research Institute), who is coordinating this observing campaign on Pluto. Dr. Young is also Deputy Project Scientist for the New Horizons mission.

If your location and observing resources permit, your assistance with observing this important and interesting occultion would be very much appreciated.

ALERT: Possible Pluto occultation Wednesday night (2012/06/14 03:28 UT) from US East coast.

CONTACT: Leslie Young (; work: 303-546-6057; home: 303-747-9161; skype: drpluto). Also see our planning pages in progress at

SCIENCE MOTIVATION: Pluto's thin, nitrogen atmosphere is in vapor-pressure equilibrium with the surface ice, and changes seasonally. We've seen it double since 1988, and now we measure its pressure once or twice a year. The technique we use is stellar occultation, when a star passes behind Pluto's atmosphere. The atmosphere defocuses the starlight. By the timing of the fading of the star, we measure the pressure and temperature in Pluto's atmosphere at ~10 km resolution.

DETAILS OF TIMING: Topocentric predictions for the midtime depend on the combination of Pluto and star positions used, and vary by about 50 seconds for any location. The midtime also depends on the location, affecting the midtime by about 1 minute over the portion of the US East coast that can observe the event. Adding pad for Pluto's extended atmosphere and the chance of systematic errors in the star's right ascension, the recommended time is an uninterrupted run no smaller than 03:20:00 to 03:37:00 UT.

DETAILS OF LOCATIONS OF OBSERVABILITY:  Predictions vary. Pluto's shadow is predicted to be observable anywhere North of approximately South Carolina in all predictions, but any location on the entire east coast of North America has a good chance of being in the shadow. Pluto only subtends 0.1 arcsec, so small errors in the star position and Pluto ephemeris can move the predicted shadow path.  Predictions tend to be good to about 500 miles, barring systematic errors.  Multiple chords are critical for reconstructing, after the fact, the geometry of Pluto's passage. A mix of central and more grazing chords are most useful for most tightly constraining the geometric solution.

NOTES ON EQUIPMENT: Observe with 1-second readout time if you can, and choose a readout mode (region of interest, binning) to keep readout time less than 0.25 second, preferably less than 0.05 second. Observe open, or in I band.

Accurate timing is VERY important. Our goal is 50 ms.
 - Accurate GPS times are ideal if you can do that.
 - Network Timing Protocol (NTP) can help if you don't have GPS times.
 - For other timing ideas, you can call Leslie Young. We have a few specialized occultation cameras and experienced occultation observers. We may ask some of you if we can bring our own camera to your site.


We (Dr. Young and colleagues, not the AAVSO) will be posting finder charts, etc.

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen.


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