SOFIA is the world鈥檚 largest airborne observatory (Image: NASA)
The race is on. In a few hours, Pluto鈥檚 shadow will tear across Earth鈥檚 surface at 25 times the speed of a bullet. Our task: to manoeuvre a jumbo jet through the night sky so that it coincides with the shadow鈥檚 path at the point that will gives us the best view of an exceptional astronomical event. And the latest calculations suggest we are going to fall 332 kilometres short.
of the Lowell Observatory in Flagstaff, Arizona, appears worried by the news. 鈥淚t鈥檚 an unexpectedly large shift.鈥�
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We are in , a converted Boeing 747SP that is the largest airborne observatory in the world. The re-fitters have clearly been busy: gone are the familiar rows of airline seats and overhead bins, ripped out to make room for a multitude of computer monitors 鈥� and a German-built 2.5-metre telescope. There are no flight attendants, no movies, no free whisky. I even had to bring my own food.
SOFIA is flying for a special reason. Tonight 鈥� 29 June 鈥� Pluto will pass in front of a star in the constellation Sagittarius. As it does so, the weak starlight that normally reaches Earth will fade and temporarily disappear. The event is called an 鈥�occultation鈥�.
Astronomers can use this change in starlight to learn about the pressure and temperature at various altitudes in Pluto鈥檚 extremely thin atmosphere. 鈥淭hese measurements nicely complement the observations that NASA鈥檚 space probe New Horizons will carry out when it flies past Pluto on 14 July,鈥� says SOFIA deputy programme scientist , who sits next to me at one of the consoles.
High speed shadow
Making the measurements isn鈥檛 easy, though. Earth鈥檚 orbital motion around the sun will take us through Pluto鈥檚 shadow at high speed 鈥� somewhere in the order of 85,000 kilometres per hour. The occultation will be over in just 90 seconds.
But SOFIA鈥檚 total flight time will be almost 8.5 hours. We took off from Christchurch International Airport in New Zealand late in the evening, . So far, most of the 30-plus scientists and engineers on board have spent the flight testing and calibrating their instruments 鈥� photometers that will measure the changing brightness of the star every second or so. I walk around and take pictures, although the rear of the cabin, where the most sensitive instruments are located, is forbidden territory.
The instruments of SOFIA鈥檚 telescope (Image: Govert Schilling)
Now, as midnight ticks by and SOFIA flies above the ocean somewhere between New Zealand and Antarctica, comes the news that has unsettled Dunham. Instrument scientist has been in contact with astronomer at the Massachusetts Institute of Technology to get the latest information on the precise location of the 鈥渃entral line鈥� of the occultation, where astronomers hope to observe a flash of refracted starlight at mid-eclipse.
From ground-based measurements of Pluto鈥檚 position made just hours ago in Arizona and Chile, Bosh calculated that the central line lies 332 kilometres further north than had been thought. 鈥淚 hope Amanda is right,鈥� says Dunham.
Mission Director 2 Karina Leppik works out a new flight plan. Despite network problems, a lost memory stick and a printer that has run out of paper, it reaches Jeff Wilson, the navigator on the flight deck, in time. SOFIA changes course.
Rapid light show
Next comes what Roellig calls 鈥渢he calm before the storm鈥�, as SOFIA works its way towards the new interception point, eating up the night sky at 900 kilometres per hour. With little to do but wait, some people take a nap. Leppik plays an arcade game on her iPad.
A few hours pass, and the tension increases again. Then, as we approach 4:53 am, it鈥檚 all eyes on the computer screens. Pluto and the star already appear as one blob of light. Then, at the predicted time, the star starts to dim. 鈥淎mazing! This is so cool! 鈥� exclaim scientists all around me.
杏吧原创s on SOFIA await their first measurements (Image: Govert Schilling)
The show lasts 90 seconds 鈥� and straight away, the analysis begins. A plot of brightness measurements made by the guiding camera of the telescope shows a beautiful central flash, indicating that SOFIA had been smack in the right place at the right time. The plot also reveals two intriguing short-duration blips. 鈥淚n principle, they could indicate the presence of ring arcs around Pluto,鈥� says Dunham 鈥� back in 1977, as a graduate student, Dunham had been part of the team that discovered the rings of Uranus during a stellar occultation. On this occasion, though, he thinks the blips are probably a spurious artefact.
For 15 minutes or so, I secretly hold the hope of having witnessed the discovery of a Plutonian ring system. But then data from one of SOFIA鈥檚 main instruments comes in, confirming Dunham鈥檚 hunch: there are no rings. There is another surprise, though: an indication of a possible sudden temperature change at a certain altitude in Pluto鈥檚 atmosphere. It鈥檚 too early to know what it might mean, says MIT astronomer .
One hundred minutes after the occultation, just before dawn, SOFIA touches back down at Christchurch airport. 鈥淟et鈥檚 make some history,鈥� pilot Ace Beall had said as we took off. And we have done. But there鈥檚 more to follow on 14 July. Man, would I like to be on board New Horizons during its Pluto flyby.
