Dawn was approaching Tuesday morning when NASA's SOFIA aircraft touched down in Christchurch at 6.33 a.m. after a successful 8.5-hour flight to watch the dwarf planet Pluto pass in front of a star. On board was an exhausted but cheerful crew of scientists and engineers. As SOFIA's chief science advisor Eric Becklin remarked: 'This was the most beautiful occultation by Pluto I've ever seen.' Becklin (75), a pioneer of infrared astronomy, didn't want to miss out on attending what he called this 'very special occasion'.

During a stellar occultation, starlight briefly shines through Pluto's tenuous atmosphere. By studying how the starlight fades and reappears (after a mere 90 seconds), scientists can learn about the pressure and temperature at various altitudes in the atmosphere, and about possible haze layers that may have been produced by geyser activity on Pluto's frozen surface.

SOFIA (Stratospheric Observatory For Infrared Astronomy) is a NASA-owned converted Boeing 747SP airliner, outfitted with a German-built 2.5-meter telescope that can observe above the clouds and above most of the atmospheric water vapor that hampers infrared measurements from the ground. During the current series of 15 science flights, including tonight's 223th flight, SOFIA flies out of Christchurch instead of its home base in Palmdale, California, to study objects in the southern sky.

Govert Schilling is a freelance astronomy writer from the Netherlands who participated in the SOFIA flight. Photo / Supplied
Govert Schilling is a freelance astronomy writer from the Netherlands who participated in the SOFIA flight. Photo / Supplied

The stellar occultation was a unique boon to astronomers. Pluto's extremely thin atmosphere changes over the years because of the dwarf planet's slow seasons. But two weeks from now, NASA's New Horizons spacecraft will fly by Pluto and sample its atmosphere at practically the same time as SOFIA did, but with different instruments. The SOFIA and New Horizons observations are very complementary and together they will provide a useful reference for future studies, scientists said.

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'We are just very lucky that this occultation happens so close to the time of the New Horizons flyby,' said principal investigator Ted Dunham of Lowell Observatory, who pioneered airborne astronomy in the late 1970's. 'The only thing we needed to do was to make sure we were at exactly the right place at exactly the right time.'

Which wasn't all that straightforward, as became clear during a stressful period earlier in the flight. The 2,300-kilometer wide ellipsoidal region from which the brief occultation was visible (sometimes called the 'shadow', as if the distant star was as bright as the sun) raced across the surface of the Earth at approximately 24 kilometers per second, its precise path being determined by the exact position of Pluto in space. Around midnight, from new Pluto measurements by telescopes in Arizona and Chile, astronomers at the Massachusetts Institute of Technology (MIT) calculated that the 'central line' of the occultation would lie 332 kilometers further north than had been estimated previously, calling for a change of SOFIA's flight plan.

Being at the central line is important, explained Dunham, because refraction of starlight in Pluto's atmosphere would then create a so-called central flash halfway through the 90-second occultation, providing scientists with additional information on atmospheric hazes.

At 4.53 a.m., SOFIA, flying at some 900 kilometer per hour, succeeded in intercepting Pluto's 'shadow', which was moving a hundred times faster, somewhere above Timaru on the east coast of South Island. 'Amazing! This is so cool!' exclaimed relieved scientists when they saw the starlight disappear. Minutes later, wearing the biggest of smiles, they were poring over SOFIA's detailed measurements.

The occultation has also been observed by many astronomers on the ground, including a team at Mt. John University Observatory, which turned out to be only slightly north of the central line. All observations of the unique event will be combined and analyzed in the days and weeks ahead.

'I'm really pleased it worked so well,' said Dunham. According to his MIT colleague Michael Person, tonight's measurements appear to reveal that Pluto's atmosphere is not very much thinner than it was in 2011, when SOFIA observed another stellar occultation by the tiny, remote dwarf planet. 'I'm sure the New Horizons team is eagerly awaiting our data,' he said.

Govert Schilling is a freelance astronomy writer from the Netherlands who participated in the SOFIA flight. His latest book, 'Deep Space. Beyond the Solar System to the End of the Universe and the Beginning of Time' is available in book stores and on the internet.