Space | 22. June 2011

Chasing Pluto's shadow


Astronomers on board SOFIA fly over most of the water vapour in the atmosphere; it is precisely this that makes the desired infrared observations possible. But the fact that SOFIA flies has another major advantage: it is a mobile observatory. In contrast to ground-based observatories, it can be used at different locations around the globe. This can be very useful, for example, for the Pluto occultation that will take place these days.

Many interesting astronomical events can only be observed from specific locations on Earth. Case in point: a solar eclipse. When the Moon passes between the Sun and Earth, the Moon’s shadow only falls upon certain parts of Earth. Therefore, solar eclipses are only visible from certain places, even on the day- side of Earth.

Eclipses are special cases of occultation, or the pass of an apparently larger celestial body in front of an apparently smaller one. Two such occultations of the dwarf planet Pluto will take place at the end of June 2011; on 23 June over the Pacific and on 27 June over Southeast Asia. Pluto's occultation over the Pacific will not be observed by large ground-based observatories since there are simply none there. But SOFIA can fly there, be in the right place at the predicted time, and allow astronomers to carry out observations. This is exactly what the SOFIA team is preparing for right now. To see the occultation, other observers with small telescopes will go on expeditions to islands in the Pacific.

First, SOFIA has to fly about 2600 kilometers over the Pacific from her home base in Palmdale (California). As Pluto passes in front of the star, it will be illuminated from behind, allowing astronomers to perform an observational analysis of its atmosphere, which is of great interest to scientists. Since occultations are rare, they are regarded as particularly valuable observation opportunities. A similar occultation in 1985 with SOFIA’s predecessor KAO allowed scientists to confirm the presence of Pluto's atmosphere.

Image, right: A mosaic image of Pluto obtained with the Hubble Space Telescope. Image: NASA/ESA.

Image: Visibility of Pluto's shadow on Earth during the Pluto occultation of 23 June 2011. Image: CNRS.

The astronomical scales are large, so Pluto's shadow moves rapidly across the surface, reducing the opportunities of obtaining an optimal shot. SOFIA will be positioned in the middle of Pluto's shadow, and will be acquiring last minute information from MIT via satellite phone. The scientists on board, led by Ted Dunham of the Lowell Observatory in Flagstaff (Arizona) , will measure the density and temperature distribution of Pluto's atmosphere. For this they have installed their device, "HIPO" (High-speed Imaging Photometer for Occultation), on the SOFIA telescope. The relatively easy exchangeability of astronomical instruments is one SOFIA's advantages as an airborne observatory. After FORCAST and GREAT, HIPO is the third instrument to fly on SOFIA.

Image: HIPO instrument (left, black) mounted on the SOFIA telescope (blue). Image: NASA.

SOFIA is the world's only observatory that allows these kinds of measurements. I think it’s a very exciting and flexible programme. It is likely that SOFIA will be increasingly requested for missions carried out within the infrared astronomical community. Let's keep our fingers crossed for Ted Dunham and the entire team, so they are able to obtain high quality data from this flight. SOFIA's ground track and current elevation information can as usual be obtained via FlightAware.

Update, 23 June 2011: The mission was a success.

Update 24 June 2011: NASA just published a press release, including the following statements:

"Occultations give us the ability to measure pressure, density, and temperature profiles of Pluto's atmosphere without leaving the Earth," said Ted Dunham of the Lowell Observatory in Flagstaff, Ariz., who led the team of scientists aboard SOFIA during the Pluto observations. "Because we were able to maneuver SOFIA so close to the center of the occultation we observed an extended, small, but distinct brightening near the middle of the occultation. This change will allow us to probe Pluto's atmosphere at lower altitudes than is usually possible with stellar occultations."

Image: SOFIA's 23 June ground track. Credit: Flightaware.

"This was the first demonstration in practice of one of SOFIA's major design capabilities," said Bob Meyer, SOFIA's program manager. "Pluto's shadow traveled at 53,000 mph across a mostly empty stretch of the Pacific Ocean. SOFIA flew more than 1,800 miles out over the Pacific Ocean from its base in Southern California to position itself in the center of the shadow's path, and was the only observatory capable of doing so."

There were some tense moments for SOFIA's international science team in the minutes leading up to Thursday's occultation. The precise position of Pluto in relation to Earth could not be sufficiently refined until a few hours before the event. That evening, a Lowell astronomer used facilities at the U.S. Naval Observatory in Flagstaff to take multiple photographs of Pluto and the star. Those data were passed to collaborators at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass., who refined their prediction of the exact position and timing of Pluto's shadow track.


About two hours before the occultation, the MIT group contacted SOFIA in-flight with the news that the center of the shadow would cross 125 miles north of the position on which the airborne observatory's flight plan had been based. After recalculating and filing a revised flight plan, SOFIA's flight crew and science team had to wait an anxious 20 minutes before receiving permission from air traffic control to alter the flight path accordingly.

Update 30 June 2011: Meanwhile NASA's Brent Cobleigh also wrote a blog post on this flight.