Last month, Pluto passed in front of a star and cast a small shadow on the Earth - astronomers from Lowell Observatory were among the scientists and crew who observed the rare occultation event from NASA's newest airborne observatory, SOFIA (Stratospheric Observatory for Infrared Astronomy).  

SOFIA has a 100-inch (2.5-meter) telescope aboard a modified 747 SP aircraft, and can fly at an altitude of 45,000 ft., above most of the cloud cover and water vapor in the Earth's atmosphere.

The June 23rd Pluto occultation was observed using HIPO, the High-speed Imaging Photometer for Occultations, and its dual high-speed cameras that record light at two wavelengths simultaneously.    

Pluto holds onto a very thin atmosphere of nitrogen, methane, and a few other gases and was first measured directly by astronomers from MIT and Lowell Observatory in 1988 using the stellar occultation technique:  monitoring the light from a star as Pluto and its atmosphere pass in front of it. The way in which the starlight dims lets astronomers determine the temperature and density of this tenuous atmosphere.  In 1988, astronomers watched from a variety of telescopes in Australia and New Zealand, as well as from the Kuiper Airborne Observatory (KAO), a 36-inch telescope aboard a C-141A transport aircraft, while flying over the Pacific Ocean.

Credit: NASA/Carla Thomas

Pluto's tenuous atmosphere exists because of vapor-pressure equilibrium, where a small amount of gas resides above any solid ice surface.  The atmosphere in this state is very sensitive to changes in surface temperature, and since 1989 Pluto has been receding from the Sun.  Because the Sun is Pluto's prime source of heat, astronomers expect Pluto's surface to become colder and for its atmosphere to become less dense and to finally freeze out onto the surface. However, Pluto's atmosphere is instead becoming more dense, as has been shown through several occultation events observed since 1988.  

One goal of continued study is to understand the timescale of the global changes in Pluto's atmosphere; to this end, astronomers have been tracking Pluto occultations across the globe every year from March through October, during Pluto's observing season.

The ability to precisely position SOFIA within the shadow called for a massive prediction effort in the weeks before the event.  Telescope time for astrometry (the measurement of precise positions of celestial objects) was secured at Lowell Observatory, the U.S. Naval Observatory--Flagstaff Station, and the Cerro Tololo Inter-American Observatory at La Serena, Chile.  Lowell staff members Stephen Levine and Len Bright obtained the observations with the USNO and Lowell telescopes and immediately sent these data to MIT to be analyzed by the astrometry group there.

Dr. Amanda Bosh (MIT,visiting astronomer at Lowell Obs.) reports that astrometric images were appearing at MIT mere minutes after they were acquired.  They were processed and Pluto's path toward the star was analyzed by Bosh and colleague Carlos Zuluaga (MIT) to determine where the shadow path would fall on the Earth. Researchers planned a last-minute flight path update, via Iridium satellite phone call between SOFIA and the researchers at MIT.  The flightpath was modified by about 125 miles; after the revision, SOFIA flew northwest for 30 minutes, capturing the occultation with both channels of HIPO as well as with the Fast Diagnostic Camera (Jurgen Wolf, Deutsches SOFIA Institut and SOFIA Science Center).  The final update put SOFIA within about 65 miles of the exact center of the shadow.

Many ground-based observers also captured the event at locations in Hawaii, southern California,and Flagstaff, AZ.  Other teams from the Southwest Research Institute(SwRI) and l'Observatoire de Paris were observing the same event throughout the Pacific basin; the SwRI team included Lowell astronomer Larry Wasserman, who observed from the island nation of Nauru.  Data analysis is underway and results will be presented at the Division for Planetary Sciences of the American Astronomical Society fall meeting in October 2011 in Nantes, France.

This Pluto occultation observation continues a long history of occultation science at Lowell Observatory that includes many astronomers, past and present, who have used the technique to study bodies as diverse as asteroids, planets, planetary rings,and stars.  Key results include the discovery of the rings around Uranus in 1977, as well as Pluto's atmosphere in 1988.

Funding for this research was provided by grants from USRA for SOFIA and NASA's Planetary Astronomy Program. 

Lowell Observatory is a private, non-profit research institution founded in 1894 by Percival Lowell. The Observatory has been the site of many important findings including the discovery of the large recessional velocities (redshift) of galaxies by Vesto Slipher in 1912-1914 (a result that led ultimately to the realization the universe is expanding), and the discovery of Pluto by Clyde Tombaugh in 1930. Today, Lowell's 19 astronomers use ground-based telescopes around the world, telescopes in space, and NASA planetary spacecraft to conduct research in diverse areas of astronomy and planetary science. The Observatory welcomes about 80,000 visitors each year to its Mars Hill campus in Flagstaff, Arizona for a variety of tours, telescope viewing, and special programs. Lowell Observatory currently has four research telescopes at its Anderson Mesa dark sky site east of Flagstaff, and is building a 4-meter class research telescope, the Discovery Channel Telescope.