Mission Success: Pluto Fever is Contagious

There are 60 busy scientists and support specialists in Christchurch, New Zealand, but everyone paused on Monday evening, June 29. That’s when the mission briefing room was packed to capacity with everyone anticipating the night’s flight. There was a buzz in the air; as the briefing went on, more and more people were caught up in it.

The night’s mission was to observe Pluto as it passed in front of a star. The event, known as an occultation, would cast Pluto’s faint shadow across the Earth’s surface, and the center of that shadow would pass somewhere off the southern coast of New Zealand. Many ground-based observatories and research teams would be observing the occultation as well, but SOFIA, NASA’s largest airborne observatory, would attempt to fly exactly in the center of the shadow to observe the occultation from the ideal vantage point.

Image: NASA/SOFIA/L. Proudfit

Should SOFIA capture the occultation, the research teams would be able to determine the detailed pressure and temperature profile of Pluto’s atmosphere, and verify if there are aerosol particles or dust in the dwarf planet’s atmosphere. If SOFIA were to hit the shadow in the exact mid-point, researchers could even determine whether there are winds in Pluto’s atmosphere and learn how they behave.

As the briefing went on, Mission Director Nancy McKown reminded everyone that this was only one of 14 science missions planned during the deployment, which includes observations of more than 40 celestial objects, each just as important as the next. Her attempt at calming the excitement in the room lasted for a few short minutes as people reflected on SOFIA’s overall mission, but the excitement was soon back in full force.

Research Scientist Michael Person from the Massachusetts Institute of Technology (MIT) also spoke about the night’s mission. Person’s area of research focuses on the techniques needed to observe stellar occultations, eclipses, and transits. He has a special interest in Triton, Pluto, and a number of objects that reside in the Kuiper Belt that stretches from Neptune out past Pluto. As Principal Investigator for this flight’s Pluto occultation observations, Person would be responsible for coordinating the work of all of the instrument teams on board.

A Little Bit of Back Story

An entire room full of science specialists, led by Jeff VanCleve and Ken Bower, had been working on where to fly SOFIA for the night’s observations. They looked at the weather forecast and developed a number of flight plans for a variety of possibilities, depending upon where the shadow would fall. Some predictions called for the center of Pluto’s shadow to fall north of Wellington, New Zealand, while others put it between the island nation and Antarctica. Working with observers at the U.S. Naval Observatory and Lowell Observatory’s Discovery Channel Telescope, both in Flagstaff, Arizona, with additional support from telescopes in Australia and Chile, Pluto observations were made and the data was provided to astronomers at MIT. They in turn made calculations to generate predictions for Pluto’s path that were provided to the SOFIA team, who generated a flight plan that would put the observatory in the shadow’s track.

Once the science flight plan was generated, it was turned over to SOFIA’s navigator for the flight, Jeff Wilson, who then converted it for use by air traffic controllers.

The observations would be made at 39,000 feet, an altitude that did not require maximum thrust from SOFIA’s four 50,000 pound thrust JT-9D-7J engines. The air at 39,000 feet was predicted to be very clear of infrared-blocking water vapor, and flying at the lower observation altitude provided more reserve power to the pilots in case they needed to make up a minute or two on the flight plan to intercept the shadow at the precise point.

Below the cockpit, on SOFIA’s main science deck, teams coordinated by Instrument Scientist Maureen Savage would monitor the three instruments used together to capture the occultation:

  • The First Light Infrared TEst CAMera (FLITECAM), an infrared camera collecting data between wavelengths of 1 and 5.5 microns.
  • The High Speed Imaging Photometer for Occultations, HIPO – Principal Investigator Ted Dunham, a visible light camera that was co-mounted with FLITECAM.
    • Fun fact: When FLITECAM and HIPO are mounted together, they are known as “FLIPO.”
  • The third instrument in use was the Focal Plane Imager-Plus (FPI+), a highly sensitive tracking camera on board SOFIA that can also be used as a photometer collecting data at visible and near-infrared wavelengths between 0.36 and 1.1 microns. FPI+ is managed by a team from the German SOFIA Institute at the University of Stuttgart, with Principal Investigator Jürgen Wolf heading the team.

All three-instrument teams would work together on this flight to ensure the success of their observations both individually and as a group.

SOFIA's FLITECAM and HIPO Instruments, "FLIPO." HIPO is getting cooled with liquid helium before flight.
SOFIA’s FLITECAM and HIPO Instruments, “FLIPO.” HIPO is getting cooled with liquid helium before flight. Photo: NASA/SOFIA/Carla Thomas

The SOFIA team’s Pluto fever was being fed by the anticipated success as well as a keen awareness of NASA’s New Horizons spacecraft, which will pass by the distant celestial body on July 14. Data from SOFIA’s observations will be an important part of a decades-long study of Pluto’s atmosphere that provides a baseline and context for New Horizon’s fly-by observations. Studying a target with as wide a variety of sensors as possible results in the most complete understanding of an object. That’s why NASA will often observe a celestial object with sensors that cover the entire electromagnetic spectrum. In Pluto’s case, New Horizons provides ultraviolet, radio, and visible wavelength coverage while SOFIA collects data in both the visible and infrared bands.

SOFIA is a GO!

As the mission briefing continued, each of the responsible team leads presented the status of their part of the mission.

The command pilot for the flight would be Clayton “Ace” Beale. Ace briefed the weather, fuel load, alternate landing fields, in case Christchurch’s notorious fog rolled in, and discussed flight timing for the mission.

SOFIA's flight crew during the Pluto occultation flight.
SOFIA’s flight crew during the Pluto occultation flight, Ace Beale, Dean Neeley, and Tom Speer. Photo: NASA/SOFIA/Carla Thomas

Navigator Jeff Wilson, call sign “Elvis,” presented the night’s flight plan with the caveat that it might change depending upon the information received during the flight on Pluto’s estimated track. In flight, Wilson would collaborate with Jeff VanCleve and co-Mission Director Karina Leppik as updated Pluto shadow position forecasts were received from MIT.

Aircraft Operations Engineer Andrew Fischer said, “All aircraft systems are ready.” Fischer’s words captured the work of Aircraft Crew Chiefs Jerry Dobbins and Sal Ramirez and their crew of more than 20 technicians and engineers who worked in often cold and wet conditions to prepare the aircraft and its airborne systems for the flight.

Oliver Ziele, representing the telescope team from the German SOFIA Institute, reported: “The telescope is balanced and the telescope bearing temperature is within limits.”

Matt Enga, whose group is responsible for all of the mission communication systems on board the observatory – computers that interface between the telescope to the instruments – reported all systems were ready for the night’s mission.

Shawn Granen from SOFIA’s information technology group talked about computer protocols on the flight and then announced: “The celestial map overlays have been uploaded.”

SOFIA is a GO!

With all systems reporting as ready for the flight, Mission Director McKown reviewed the mission timing – when the crew had to board, when the door would close, taxi and takeoff times. With that, the excited crew grabbed their gear and headed for the plane.

Once on board, last minute details were attended to and the main cabin door was closed. On-board, in-flight safety technicians Mike Moore and Steve Laney gave the safety briefing and everyone took their seats. The engines were brought to life, and with clearance from the tower SOFIA headed out to the runway.

Having received permission from the control tower to take the runway and take off, Ace Beale positioned SOFIA facing south on Christchurch International Airport’s Runway 20. Beale, co-pilot Dean Neeley, and flight engineer Tom Speer completed their checklists and with the brakes holding SOFIA back, all four engine throttles were pushed forward as the flying observatory began to shake, building up thrust. At 10:07 p.m., local time, SOFIA thundered down the runway lifting off into the cold New Zealand night sky.

SOFIA taking off from Christchurch International Airport.
SOFIA taking off from Christchurch International Airport. Photo; NASA/SOFIA/Greg Perryman.

SOFIA flew to the south to set-up and test all systems. There were a couple of rough patches as systems were synchronized, but nothing the experienced group on board could not handle.

As the instrument teams were watching Pluto’s movements across the night sky, an updated prediction of the shadow’s path was received on board. Jeff Van Cleve’s pencil was processing the data and converting the prediction into a flight path. He then went upstairs to the cockpit to confirm his numbers, calling MIT on the aircraft’s satellite phone.

Having replanned the flight with navigator Wilson to intercept the shadow everything was set. Then another further updated position prediction came in. This one required the aircraft to be positioned 227 km (141 miles) north of the current flight path.

The race was on. SOFIA was trying to reposition itself before the Pluto’s shadow passed in front of it.

On the screen, everyone on board could see Pluto begin to approach the occulting star. Then it got closer. Then closer. The excitement was building on the science deck.

Memers of the German SOFIA Institute (DSI) monitoring the Pluto occultation. Photo: NASA/SOFIA/Carla Thomas

At a point approximately -44.8 latitude by 171.0 longitude SOFIA’s instruments captured Pluto’s shadow. As SOFIA reached the precise center of Pluto’s shadow, viewing Pluto exactly centered in front of the star, the star’s light brightened, which was reflected on the light curve as an obvious spike in the center. That momentary “central flash” held a wealth of data about Pluto’s atmosphere. As Pluto completed its pass in front of the star, there was loud shouting, clapping, handshaking, and congratulating all around!

Members of SOFIA’s science team react to the successful observations. Photo; NASA/SOFIA/Carla Thomas.

“In this single flight we were able to obtain observations covering four different wavelengths as close to the predicted center of the occultation as possible, which shows the power and flexibility of SOFIA to go wherever is required for events like these,” said Ryan Hamilton of SOFIA’s FLITECAM team. “We faced challenges at every point and overcame them all, and we’re all excited as can be for the next part of these observations: the science and interpretation!”

Shortly after the mission, occultation principal investigator Michael Person said, “All of the instrument teams worked together very well to get us the best possible result. The HIPO team, based at Lowell Observatory, the UCLA-based FLITECAM team, and the German FPI team all came together to help each other overcome challenges during the mission, resulting in successful observations of the event from all cameras. I couldn’t be more pleased with the cooperation from the instrument teams, the aircraft, and observatory personnel with our efforts to observe this historic occultation.

SOFIA's flight path for June 30, 2015 indicating where the Pluto occultation observations were made.  Image:NASA/SOFIA/L. Proudfit
SOFIA’s flight path for June 29, 2015 indicating where the Pluto occultation observations were made. Image:NASA/SOFIA/L. Proudfit

Two reporters, Nadia Drake and Govert Schilling were on board this flight and documented the experience. Their articles can be found below.

National Geographic- Scientists in Flying Telescope Race to Intercept Pluto’s Shadow by Nadia Drake

National Geographic Phenomena- I chased Pluto’s Shadow Over Across the Southern Pacific Ocean by Nadia Drake

Sky and Telescope- SOFIA Dashes Into Pluto’s Shadow by Govert Schilling

New Zealand Herald- Chasing Pluto’s Shadow Over Timaru by Govert Schilling

Additional information can be found on SOFIA’s social media channels: Facebook, Twitter, Instagram, YouTube

— Nicholas A. Veronico