120 seconds in Pluto’s shadow

A nerve-wracking path correction put a 747 on course to measure Pluto’s skies

SOFIA test flight

SCOPE’S ON A PLANE  A 2.5-meter-wide telescope peers through an opening on SOFIA during a 2010 test flight over the Sierra Nevada mountains in California. 


Pluto’s shadow raced across the Pacific Ocean at more than 85,000 kilometers per hour. Planetary scientist Michael Person was on a 747 chasing the shadow, which formed as Pluto passed in front of a distant star. Until last summer, everything researchers knew about Pluto’s atmosphere came from stellar eclipses like this one. But for this June 2015 run, the analysis would get a reality check from the New Horizons spacecraft, which was speeding through the Kuiper belt toward the dwarf planet.

On the night of June 29, Person and his team departed Christchurch, New Zealand, on SOFIA, a jumbo jet fitted with an infrared telescope. Operated by NASA and the German Aerospace Center, SOFIA lofts its telescope, perched within a large hole in the airplane’s side, to an altitude of roughly 12,500 meters, high enough to get above most of the molecules in Earth’s atmosphere that absorb infrared light.

The rendezvous with Pluto’s shadow required precision: The remote world would block the star for just a couple of minutes. And while the entire shadow is as wide as Pluto — 2,374 kilometers — SOFIA was aiming for a 50-kilometer-wide spot at the shadow’s center. At that spot, known as the “central flash,” researchers can get the fullest sense of Pluto’s skies. Partway through the eight-hour flight, plans had to quickly change. Colleagues in Arizona who were monitoring Pluto’s position with a telescope in Chile called to say Person’s team needed to shift the flight plan 250 kilometers north.

“We were talking frantically with the navigator,” says Person, of the Massachusetts Institute of Technology. The plane was headed out to sea and instead needed a course that would send it back over the southeast coast of New Zealand. After some fancy maneuvering, SOFIA soared through the central flash, its instruments recording how Pluto’s atmosphere absorbed the starlight. Person and his team measured properties such as atmospheric pressure and particle sizes in the haze around Pluto.

CHANGE IN THE AIR When SOFIA’s crew got word that Pluto’s shadow would not be where expected, the team drew up a new flight plan, including some zigs, zags and time adjustments, to head back over New Zealand.M. Person

Fifteen days later, New Horizons got an intimate look at Pluto, providing crucial support for SOFIA’s observations (SN: 12/26/15, p. 16). New Horizons scoped out the upper and lower levels of Pluto’s atmosphere while SOFIA checked out the middle altitudes. The combined data, presented January 7 at a meeting of the American Astronomical Society, paint a consistent picture of Pluto’s multilayered, hazy atmosphere.

Flying on SOFIA is “kind of mind-bending,” says Person, who has made four flights on the jet. Looking toward the back of the plane, the telescope appears to be quivering, but in truth, the instrument is steady. “It’s the thing you’re standing on that’s wobbling.” Next trip: 2017, when Pluto passes in front of another bright star.

Editor’s note: This story was updated on Feb. 18, 2016, to note that SOFIA flew at an altitude of roughly 12,500 meters.

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