SOFIA Captures a Speeding Shadow

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By Brent Cobleigh

SOFIA Platform Project Manager
On June 23, the SOFIA returned from a mission that the principal investigator, Ted Dunham, called “gutsy.” As a star passed behind Pluto, a faint shadow passed over the Earth at a speed of 51,000 mph. The 100km-wide shadow passed over remote areas of the Pacific Ocean, out of the view of most ground-based observatories. The SOFIA’s unique ability to carry our 17-ton telescope to an altitude three times higher than the world’s best ground-based observatories is one of the reasons this program exists. Here is a quote from Ted:

“Occultations give us the ability to measure pressure, density and temperature profiles of Pluto’s atmosphere without leaving the Earth, which is 3 billion miles away from Pluto. 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 will allow us to probe Pluto’s atmosphere at altitudes lower than usually possible with stellar occultations.”

The Pluto Occultation Mission was performed with the High-Speed Imaging Photometer for Occultation instrument, which is specifically designed to maximize science collection during an occultation. The photometer is the third science instrument integrated onto the aircraft this year. And we also started flying the water-vapor monitoring system back in March.

The Pluto success comes on top of finishing the first competed flight phase, Basic Science 1 – or BS1 – a few weeks ago. I checked a schedule that we made back in November, to see how close we were to finishing the BS2 flight phase as planned, and we finished one day early! Developing and testing the SOFIA has been a huge challenge, but the hard work is paying off.

And the year is not over. We expect to start the BS2 flight phase in July. After that, we will start testing the liquid nitrogen pre-cooling system that will chill the telescope mirrors prior to takeoff so that we don’t have to waste valuable flight time waiting for temperatures to stabilize. We also have enhancements to test that will improve the telescope pointing accuracy, and a goodwill deployment to Germany in September. The team is developing systems for the segment 3 downtime scheduled to start in November. Completing, installing and testing all the new and upgraded systems will be another challenge that will require the diverse skills of the Platform Project team.

On June 24, I was notified that SOFIA was selected to receive a NASA-wide group achievement award for the Initial Science Flight that we successfully completed last November. Congratulations to the whole team.

My job satisfaction is always based on two things: achieving an ambitious goal, and working with an excellent team. So I am glad to have this job because it allows me to achieve both. Though I’ll be the first to admit that there are ups and downs from day to day, the bottom line is that we are executing what we planned and fulfilling our promises to the science community and to the public. Like many ambitious projects, there are 10 hectic days for every day available to reflect on our success (sometimes it feels like 100 to 1). From time to time, step back and realize the progress we are making.  

Many thanks to SOFIA team members for their hard work and dedication.

One thought on “SOFIA Captures a Speeding Shadow

  1. Kevin Rohrer Post author

    On Jun 30, 2011 07:41:19 PM Laurel Kornfeld wrote:
    Could you explain the process by which occultations enable scientists to measure the pressure, density, composition, and temperature of Pluto’s atmosphere? I run a blog about Pluto and would love to explain this to readers.

    A response from the SOFIA team:

    The short answer is, the light from the background star is changed as it passes through Pluto atmosphere.

    Before and after the occultation, we have measurements of the brightness and color of the star’s light.

    During the occultation, the star’s brightness decreases (obviously) and the color changes. This is the same physics as the Sun’s brightness diminishing and color changing as its light passes through more of Earth’s atmosphere at sunrise or sunset than when it is overhead.

    The pressure, density and temperature of Pluto’s atmosphere all contribute to the changes in the light. The effects of those parameters overlap, so lots of data (and difficult calculations) are needed for them to be untangled.

    We already know the composition of the atmosphere from spectroscopy measurements (although we don’t necessarily know how, or whether, the composition changes with altitude in Pluto’s atmosphere, and that potentially can be untangled with the rest of the parameters).

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