T–9 days: Past, Present, and Future

Pluto occultation observing teams, past and present, at Lowell Observatory, in front of the Pluto dome.
Standing (L-R):  Ralph Nye (Lowell), Bob Millis (Lowell), Luke Weisenbach (MIT), Peter Collins (Lowell),
Amanda Bosh (MIT/Lowell), Hugh Harris (NOFS), Carlos Zuluaga (MIT),
Stephen Levine (Lowell), and Ted Dunham (Lowell)
Kneeling (L-R):  Thanawuth (Atom) Thanathibodee (MIT) and Steph Sallum (Steward Observatory)
Missing from photo:  Larry Wasserman (Lowell), Len Bright (Lowell, and Trudy Tilleman (NOFS)

On 29 June 2015, observers from MIT, Lowell Observatory, Williams College, SAAO, Steward Observatory, NOFS, and more will observe a stellar occultation by Pluto.  Occurring just two weeks before the New Horizons spacecraft's encounter with Pluto, this event has the promise of providing an important look at Pluto's atmosphere that will be used to compare short-timescale changes by comparison with New Horizons results.

In many ways, preparations for this event remind me of preparations for a similar event 27 years ago.  On 9 June of this year, we passed the 27th anniversary of a Pluto occultation observation that revised our understanding of its atmosphere.  Although an atmosphere had been theorized, inferring the amount of gas through analysis of spectroscopic data was very difficult.  The stellar occultation technique (used previously to measure stellar diameters, probe the atmospheres of Jupiter and Mars, and to discover the rings of Uranus) was an ideal technique to be used to directly detect Pluto's atmosphere.  After years of planning, teams of observers fanned out across Australia and New Zealand, while another team of observers boarded the Kuiper Airborne Observatory in Honolulu and set out across the ocean to observe the occultation of the star dubbed "P8", with a custom-built CCD imager.

The MIT team, who flew on the Kuiper Airborne Observatory.
(L-R):  Amanda Bosh, Stephen Slivan, Leslie Young, Ted Dunham, and Jim Elliot

The KAO and ground-based observations were successful, and we had a direct detection of Pluto's atmosphere.  We found that Pluto's atmospheric pressure was around a microbar (one million times less dense than Earth's atmosphere), it was composed of nitrogen, methane, and other gasses, and was at a temperature of about 100K (–280˚F).

Members of the 1988 Pluto occultation expedition.
(L-R):  Ralph Nye, Bob Millis, Amanda Bosh, and Ted Dunham

Between 1988 and 2015, we have continued to study Pluto's atmosphere.  Jim Elliot, his group, and collaborators at Lowell Observatory, Williams College and elsewhere, observed many Pluto occultations.  These became more frequent after 2002, when Pluto entered the Milky Way.  With the greater density of stars, Pluto passed in front of bright ones more often.  This turned out to be important, because Pluto's atmosphere had been theorized to collapse (or at least dramatically decrease in density) some time during this period.  Instead, the 2002 occultation showed an increase in atmospheric pressure on Pluto.  Since then, the derived pressure has stayed relatively constant, but some degree of thinning is expected soon.

Our understanding of Pluto was aided by work done by Jim Elliot, who passed away in the spring of 2011, but his legacy remains.  Three months after his passing, we observed the first occultation (Pluto, of course) from the Stratospheric Observatory For Infrared Astronomy (SOFIA), the successor to the KAO.

SOFIA at sunset in Christchurch, New Zealand.  This image is from SOFIA's Southern Deployment blog.  SOFIA arrived in Christchurch on 14 June, and will spend six weeks observing the southern skies.  Included in this six-week total is one night observing Pluto as it passes in front of a star.

Some members of the group will remain at the Lowell Observatory and the Naval Observatory's Flagstaff Station to observe Pluto every night through the event, refining our prediction and updating the expected ground track for this occultation.

Others are headed to New Zealand for observations on the ground at the Mt. John Observatory (which also participated in the 1988 observations) and to fly on SOFIA.

A pair of observers is headed to Australia to collaborate with astronomers there in observations of this event.

And yet more team members (from MIT, Williams, the South African Astronomical Observatory, and more) are on their way to New Zealand or Australia from locations other than Flagstaff.

It is now 20 June 2015, and for more than two years we have been preparing for this Pluto occultation.  Thousands of astrometric frames.  Observations of the star at the Keck and Magellan Observatories using adaptive optics to discern if the occultation star has a companion.  Photometry.  Instrument testing.  Instrument surgery to replace sticky shutters, and then more testing.  Contacting colleagues throughout New Zealand and Australia.  Observing strategy meetings.  Prediction updates.  On 29 June, just past 17h UT, we will know if all this past planning has been joined by fortuitous weather to produce results.

Over the next 10 days, as groups scatter to different observatories, we will post some of our experiences here.  Stay tuned!