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The Planetary Society Blog

By Emily Lakdawalla

Pluto occultation coming up March 18

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On March 18, just before 11:00 UTC, tiny Pluto will wander across a background star, an event called an "occultation" by astronomers. The event will be visible and in full darkness from the western half of the United States and from almost all of Mexico. The background star will be blocked from sight for about six minutes. An occultation isn't just a fun coincidence; there is extremely valuable science that can be performed by watching this event with a sensitive telescope. According to Bruno Sicardy of the Observtoire de Paris, amateur instruments as small as 25 centimeters in size could "yield extremely valuable contribution for pinning down the astrometry of the event (combining various occultation chords), by helping to detect a possible central flash, and by giving firmer confirmation on possible changes occurring right now in Pluto's tenous atmosphere." Remember, Pluto does have a thin atmosphere, and it is predicted to be freezing to the surface over time as Pluto wanders farther from the Sun. By watching the star vanish behind Pluto, observers can determine the height and density of the atmosphere by the way the light fades before it goes out.

Sicardy's page has all the details for interested observers. One fact that I find pretty surprising is that Pluto's ephemeris -- the numbers describing its orbital position and velocity -- are uncertain enough to noticeably affect predictions of where and when the event will happen. Uncertainties on the predictions for the position of Pluto mean that where on Earth an observer has to be to see a central flash (where the star sits directly behind Pluto, so that Pluto's spherical atmosphere focuses the star's light toward the observer) is uncertain by about 1,000 kilometers! And the timing of the event is likewise uncertain, by five minutes in either direction.

If you're curious about what kind of science can be derived from occultation observations, here's a news article and a blog entry I wrote about an occultation of a star by Pluto's moon Charon.