Telescopes from Wyoming to Mexico City and from California to central Texas will point at Pluto as the dwarf planet occults a star in the Sagittarius constellation next Sunday.
University of Arizona astronomers will host colleagues from Paris Observatory, the Massachusetts Institute of Technology and Lowell Observatory at UA telescopes for the not-to-be-missed event.
Arizona has a special tie to Pluto: Astronomer Clyde Tombaugh was working for Lowell Observatory in Flagstaff when he discovered the ninth planet in 1930. Tombaugh was the only American ever to discover a planet in our solar system. The International Astronomical Union ignited public and scientific controversy last August when it downgraded Pluto's status to a dwarf planet known as number 134340.
But whatever you call it, the object interests planetary scientists.
"Occultations are the only way we can monitor the atmosphere of Pluto from the Earth," said Professor William B. Hubbard of UA's Lunar and Planetary Laboratory, who is coordinating the UA campaign to observe the Pluto occultation.
Not only are the observations important to scientists studying Pluto's atmosphere, Hubbard said, they're important to NASA's $620 million New Horizons spaceprobe, which just flew by Jupiter and is on target to reach Pluto and the Kuiper belt in 2015. Previous observations of Pluto occultations have yielded surprising findings about Pluto's changing air pressure, for example, Hubbard said, "so it's going to be important to keep track of what Pluto is doing until the spacecraft gets there."
An occultation is like an eclipse. Just as the moon casts its shadow onto Earth when it passes directly in front of the sun, planets cast their shadows onto Earth when they pass directly in front of a star. Hubbard, Bruno Sicardy of the Paris Observatory and Faith Vilas, who is now director of the MMT Observatory, discovered Neptune's rings in the 1980s, before Voyager detected them, from ground-based observations made during a Neptune occultation.
In past decades, astronomers could typically expect a Pluto occultation only every five to 10 years, Sicardy said. But now Pluto is moving in front of the Milky Way, and astronomers may see one or two Pluto occultations a year because of the abundance of background stars.
"But even though there are now more than one of these events per year, we can't count on seeing them all because of cloudy weather, or because Pluto's shadow falls on Earth where there are no observatories," Sicardy said.
"This time, the event is observable by a region of the world populated with great telescopes -- the southwestern United States," Sicardy said. "To observe this in Arizona is like closing a big loop after more than 70 years. It's kind of like celebrating Pluto's discovery," he added.
Pluto has a diameter of 2,775 kilometers, or about 1,400 miles, and is almost 40 times farther from the sun than the Earth is. It will pass in front of the star in Sagittarius at 4 a.m. Arizona time (11 Universal Time) on Sunday, March 18. The occultation will last six minutes -- about 3 times longer than typical Pluto occultations -- giving telescopes as small as 50 centimeters (20 inches) time to record the event.
If the telescope view falls in the exact line of sight with the star when Pluto eclipses the starlight, its lucky astronomers might see the "central flash" phenomenon. They would see a sudden brightening, a flash, while entirely in Pluto's shadow. That could give them important information on the shape of Pluto's atmosphere or its winds, as well as a thrill.
All the visible light cameras are fast readout cameras with good time resolution, said Lunar and Planetary Laboratory scientist Steve Larson. He'll observe with the 61-inch Kuiper Telescope in the Santa Catalina Mountains north of Tucson. "This will help provide accurate timings of ingress, egress and a central flash if we are situated right," he noted.
Participating UA astronomers and telescopes include:
- The UA/Smithsonian Institution's 6.5-meter (260-inch) MMT on Mount Hopkins. Steward Observatory astronomers Donald W. McCarthy and Craig Kulesa will use a wide-field infrared camera called "PISCES" that may spot clouds or haze if they exist in Pluto's atmosphere. At the same time, Susan Kern and Michael Person of MIT will use a "POETS" camera loaned by Lowell Observatory to observe at optical wavelengths. POETS is an acronym for Portable Occultation Eclipse and Transit System. http://www.mmto.org/
- Bruno Sicardy will use a camera from his Paris Observatory on the 90-inch (2.3 meter) Bok Telescope on Kitt Peak. This telescope is the largest operated soley by the UA Steward Observatory. http://james.as.arizona.edu/%7Epsmith/90inch/90inch.html
- Catalina Sky Survey Director Steve Larson of UA's Lunar and Planetary Laboratory and Thomas Widemann of the Paris Observatory will observe with Steward Observatory's 61-inch (1.6 meter) Kuiper Telescope in the Santa Catalina Mountains north of Tucson. http://james.as.arizona.edu/~psmith/61inch/
- Rick Hill of UA's Lunar and Planetary Laboratory and Henry Roe of Lowell Observatory will use UA's 60-inch (1.5 meter) telescope on Steward Observatory's Mount Lemmon site. http://james.as.arizona.edu/~psmith/60inch/