Deep snow, high winds, and equipment glitches didn't stop dozens of observers in New Zealand and Tasmania from recording Pluto's occultation of a bright star on June 29th — just two weeks before New Horizons' close-up flyby.

The stars and planets really were aligned on June 29th. On that date, just two weeks before NASA's New Horizons spacecraft was to dash past Pluto, the celebrated dwarf planet passed directly in front of an obscure 12th-magnitude star in north-central Sagittarius. It's the second-brightest star that Pluto has ever been known to occult, and careful records of the star's disappearance and reappearance could tell researchers a great deal about the state of Pluto's atmosphere at a most opportune time.

Path for Pluto occultation on June 29, 2015
Observers in New Zealand and Tasmania had front-row seats when Pluto covered a 12th-magnitude star on June 29, 2015.
Sky & Telescope diagram / Source: Carlos Zuluaga (MIT)

Unfortunately, the predicted path of Pluto's "shadow" across Earth was deep in the Southern Hemisphere, largely falling over open water between Australia and Antarctica. But New Zealand and Tasmania were both squarely inside the path limits, so dozens of observers from those countries and elsewhere did their best to make sure this rare celestial opportunity wasn't wasted.

During a stellar occultation by an airless body, such as an asteroid, the star's disappearance and reappearance are very abrupt. But Pluto's extremely tenuous atmosphere creates a more gradual decline and rise, along with nuances that reveal the pressure and temperature of the gas and the presence (or not) of haze layers. This is especially true when the star is bright enough to record its changing light at high speed with a good signal-to-noise ratio.

The effort involved several teams of observers on the ground and a large contingent aboard the SOFIA flying observatory. All of the teams, contacted by Sky & Telescope, report good results.

An especially detailed record came from Mount John Observatory on New Zealand's South Island, where Jay Pasachoff and Bryce Babcock (Williams College) led a team that included resident observers Alan Gilmore and Pam Kilmartin and Williams undergraduates Christina Seeger and Rebecca Durst.

Mount John in snow
The much-anticipated occultation by Pluto occurred during winter in New Zealand, and a deep blanket of fresh snow surrounded Mount John Observatory on South Island.
Jay Pasachoff

The facility and its 1-meter McLellan Telescope were positioned very near to the occultation's centerline. So the Mount John observers hoped to see a strong central flash, created when the thin atmosphere acts like a lens to refract a concentrated beam of light toward Earth. For this event, it could only occur if the center of Pluto's disk and the star were almost perfectly aligned.

Success was in doubt just one night before, when fierce winds whipped the observatory and the deep snow cover around it at speeds up to 55 miles (85 km) per hour. But the wind abated and the clouds parted when it mattered most. "Yes, we have a central flash! We are oh so pleased," Pasachoff reported afterward. The nearly 2-minute-long event was also captured by Mount John's two 0.6-m telescopes, one operated by Stephen Levine (Lowell Observatory) and the other by Nagoya University observers led by Fumio Abe.

Meanwhile, veteran occultation-chaser Bruno Sicardy (Paris Observatory) had established two observing stations. One was the brand-new, just-opened 1.3-m Harlingten Telescope at Greenhill Observatory near Hobart, Tasmania, where he'd assisted saff astronomers Andrew Cole, Kym Hill, and Barry Giles in recording the event. The other, the robotic 0.6-m Yock-Allen (BOOTES-3) telescope at Lauder, New Zealand, recorded a central flash.

Covering All the Bases

Amatuer observer's recording of Pluto occultation
From Brian Loader's location in Canterbury, New Zealand, the occultation lasted about 90 seconds. There's even a hint of a central flash during the event's midpoint — an indication that Loader's telescope was near the centerline.
John Talbot / RASNZ

Three groups went to great lengths to make sure the occultation would be seen. John Talbot provided coordination for the Royal Astronomical Society of New Zealand, and more than two dozen amateur observers fanned out for the event — some observing an occultation for the first time. "Many observers had cloud, and many had equipment problems of some sort," he tells Sky & Telescope. Despite those issues — and interference from a nearly full Moon only 30° away — Talbot concludes, "I would rate this as a very successful campaign and an excellent demonstration of pro-am cooperation."

On the professional side, a collaboration among Williams College, MIT, and Lowell Observatory established a widespread network within the Pluto occultation zone. "We had people at 12 telescopes in nine locations," notes MIT's Amanda Bosh, who became the go-to person for last-minute prediction updates. (The all-critical astrometry used to calculate Pluto's path came from a 1.1-m telescope at Lowell, a 1.55-m scope at the U.S. Naval Observatory's Flagstaff Station, and the 0.6-m SARA telescope at Cerro Tololo in Chile.)

Some of the collaboration's observers, like those at Mount John, witnessed the occultation, while others did not (due to clouds or mechanical problems). Chris Morley was one of the few to see the event from Australia. He established a mobile station 70 km north of Bairnsdale, Victoria.

Not shy about mounting a big observing effort for such an important occultation, Eliot Young (Southwest Research Institute) dispatched seven teams from the United States to New Zealand, Tasmania, and southeastern Australia. Most of these paired a professional observer with an undergraduate student. Some got to use an existing telescope, but others lugged "portable" 14-inch Schmidt-Cassegrain telescopes to desirable locations. And often the original plan didn't work out as anticipated.

For example, Amherst College student Jason Mackie and Lowell Observatory's Larry Wasserman got as far as a cattle farm in Napier, New Zealand, only to find that their scope wouldn't track (stripped gears). So they scrambled to join local amateur John Drummond at his observatory in Gisborne, a 3-hour drive up the coast. "We'd gotten up at 2 a.m. for the event, but it was cold and completely overcast," recalls Mackie. It cleared just in time for the event, which lasted 105 seconds, and they were very lucky: "It completely clouded over just a few minutes later."

watching the Pluto occultation
Under cold but mostly clear skies in Timaru, New Zealand, Matt Nelson and Aaron Resnick use a 14-inch Meade telescope to record Pluto's passage in front of a 12th-magnitude star on June 29, 2015.

Carol Carriazo, also from Amherst, headed with Anne Verbischer (University of Virginia) to Greenhill Observatory outside of Hobart. They piggybacked on the visual observations that Sicardy and the facility's staff had planned by adding a dichroic beamsplitter. This allowed them to obtain a near-infrared light curve simultaneously. It was the first trip that Carriazo, just 19, had ever taken outside the U.S., and it proved eventful.

Because of the use of a special filter, the observatory's CCD required repeated machining to mate properly with the telescope and reach focus. But during one focus check, the camera fell roughly 7 feet and "crunched" onto the observatory floor. After that it wouldn't turn on. Luckily, two of the staff astronomers discovered a dislodged wire inside and soldered it back in place, which got the camera working again in time to record the occultation.

Young, who observed from the Anglo-Australian Telescope but saw no cover-up, thinks a careful analysis of all these light curves will show that Pluto's tenuous atmosphere has no haze, despite earlier occultation results to the contrary. And he thinks the starlight probings at various locations around Pluto's disk will ultimately reveal a fairly uniform global temperature. In any case, the rich results from June 29th's event will get a reality check very soon, since New Horizons will monitor occultations of both the Sun and Earth just after it zips past Pluto.




Image of Graham-Wolf


July 12, 2015 at 11:11 pm

Great to hear that SOFIA has recently stayed-over in NZ a few days ago, on a Pluto Occultation mission. It's predecessor the KAO, was also here during the 1980's for p/1 Halley and SN 1987A. At that stage in history, a methane atmosphere around Pluto was indeed discovered via an occulting star, as were the rings of Uranus. The RASNZ Occultation Section, through it's late founder:- Graham L. Blow (a senior workmate of mine, at Carter National Observatory), was heavily involved in NZ ground based observations at that time.

How apt these latest efforts, with the "New Horizons" probe barely 1 1/2 days from closest encounter with the elusive planet (11:50 UT July 14th). AND... it's even carrying some of dear old Clyde's ashes! The latest photo looks great... the "whale" artifact, I'm tempted to re-name "Kaikoura Planitia" (IAU:- please ignore my remark). Anyway... good luck and best wishes to Alan Stern's hard working team.

Memo to Pluto:- Please behave yourself, and be nice to Alan's little probe. It's taken 9 1/2 years and over 700 million dollars to get to your neighbourhood. Remember to smile for the cameras!
Claer skies and best wishes:-
Graham W. Wolf, BGO, Lower Hutt, New Zealand

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