30 Jun 2015

In a special celestial event visible only from the Southern Hemisphere, Pluto passed directly between a distant star and the Earth on the morning of June 30, New Zealand time (June 29 in the U.S.). As the dwarf planet and its atmosphere were backlit by the star, this "occultation" caused a faint shadow of Pluto to move across the surface of Earth at more than 53,000 mph, creating a ripe opportunity to perform scientific analysis – if instruments and observers could be in the right place at the right time. The only observatory capable of positioning itself above terrestrial weather and directly in the center of Pluto's shadow was NASA's SOFIA, a Boeing 747SP jetliner modified to carry a 100-inch (2.5-meter) diameter telescope built by NASA's partner, the German Aerospace Center.
SOFIA, the Stratospheric Observatory for Infrared Astronomy, is operating from a base in Christchurch, New Zealand between June 15 to July 24, 2015. Flying out of New Zealand enables SOFIA to study celestial objects that are more easily observed, or can only be observed, from southern latitudes. The occultation observations were conducted during one flight in the observatory's science plan that includes studies of 40 celestial targets to be observed during a 15-flight program.
"SOFIA observations of Pluto demonstrate a capability to make detailed measurements of Pluto's atmospheric density and structure," said Pamela Marcum, SOFIA Program Scientist at NASA's Ames Research Center. "SOFIA conducted its first occultation observation, also involving Pluto, in July 2011. This flight adds to our understanding of how the atmosphere of Pluto evolves over multiple-year time scales as its elongated orbit takes it farther away from the Sun."
The occultation happened at a unique time, just two weeks before NASA's New Horizons mission will make its nearest approach to Pluto on July 14, 2015. New Horizons will use cameras, spectrometers, and other instruments to perform the first close-up studies of that world. Members of the New Horizons team who are co-investigators on the SOFIA observing proposal will have immediate access to the occultation data. Those data will be available to the general scientific community and the public in July 2016 via SOFIA's scientific data archive.
"New Horizons will give us comprehensive 'snapshot' measurements of Pluto for which ongoing occultation studies provide context. This unique opportunity to connect what SOFIA observes remotely with in-situ measurements from New Horizons will allow future analyses of Pluto from Earth by SOFIA and other observatories to be considerably enhanced," Marcum said.
Ted Dunham, of Lowell Observatory in Flagstaff, Ariz., principal investigator for SOFIA's Pluto occultation observations, said, "This observation was a collaboration between researchers at NASA, the Lowell Observatory's Discovery Channel Telescope, and the Massachusetts Institute of Technology. I'm happy to say the mission was hugely successful; we were right where we wanted to be, and, after preliminary examination, the data quality appears excellent."

Image: Three diagonal lines trending SW to NE respectively indicate the center and boundaries of Pluto's shadow track across Earth's surface.