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Emily LakdawallaJuly 24, 2015

Looking back at Pluto

I don't think anyone was prepared for the beauty -- or the instant scientific discoveries -- in this "lookback" image of Pluto, captured by New Horizons shortly after it flew by.

Pluto's hazes

NASA / JHUAPL / SwRI

Pluto's hazes
Just seven hours after closest approach, New Horizons looked back and captured this spectacular image of Pluto’s atmosphere, backlit by the sun. The image reveals layers of haze that are several times higher than scientists predicted. The image was taken at 19:06 on July 14, 2015, from a distance of about 370,000 kilometers. Three images have been deconvolved, stacked, and enlarged to make this photo.

The first time I saw it, I literally gasped. I know another scientist who cried. We knew, intellectually, that New Horizons would detect Pluto's atmosphere when it flew past. A large segment of the science being done after closest approach involved a stellar occultation, in which we were going to stare at our own star as it set behind Pluto to probe the structure of the atmosphere. But I certainly didn't expect that the atmosphere would shine so brilliantly to the camera. And I definitely didn't expect to see obvious layers.

As soon as I saw the photo today, I fired an email off to several people on the New Horizons team I knew had special reason to care about seeing Pluto's atmosphere, asking them for their reactions to the photo. All have published work on stellar occultations by Pluto, using much weaker stars to try to understand the atmosphere. Happily, two replied:

First reaction was esthetic. It is all so beautiful.

The second was evocative. We are moving away at 1000 Pluto radii a day.

The third was bafflement. It's a good thing we have such a complementary suite of instruments. Any single measurement would lead us astray.

- Leslie Young

My first reaction was that it was a stunning image.  It was beautiful and stark.

But then soon I started to think about it… how high is that haze?  Am I really seeing haze go up that high? And like so many other images from this encounter, it made me stop and think and reassess.

- Cathy Olkin

The New Horizons team "unwrapped" the image, taking out the curvature of Pluto, and squishing the result horizontally to emphasize subtle variatons in haze altitudes. You can clearly see a distinct haze layer as high as 83 kilometers above the surface; there are hints of more, even higher than that.

Pluto's hazes (annotated)

NASA / JHUAPL / SwRI / modified to metric by Emily Lakdawalla

Pluto's hazes (annotated)
Just seven hours after closest approach, New Horizons looked back and captured this spectacular image of Pluto’s atmosphere, backlit by the sun. The image reveals layers of haze that are several times higher than scientists predicted. Two distinct layers lie at 83 and 50 kilometers above Pluto's surface.

To my surprise, there were more lookback images. According to my "what to expect" post, the image at the top of this article was the only one I expected from the immediate post-flyby period. However, the mission added five more optical navigation photos into the downlink plan after the close-encounter period was over. Here is one of those opnavs (actually, it's two of them, stacked).

Looking back at Pluto

NASA / JHUAPL / SwRI

Looking back at Pluto
This photo was taken at 03:35 on July 16, 2015, UTC, about 39 hours after New Horizons' closest approach to Pluto, from a distance of about 2 million kilometers. It was made from two similar images that have been deconvolved, stacked, and enlarged, and rotated to place north at the top.

This image looked so, so familiar. Here, I've paired it with a photo of Titan from Cassini, taken with almost identical geometry, and scaled the same. Titan, on the left, is a bit more than twice the diameter of Pluto.

All Their Sunrises And Sunsets

NASA / JPL / JHUAPL / SwRI / Emily Lakdawalla

All Their Sunrises And Sunsets
Titan as seen by Cassini on June 22, 2010, and Pluto as seen by New Horizons on July 14, 2015. Both worlds are viewed from a phase angle of 165 degrees -- nearly from behind. Both have atmospheres; particles in the atmospheres scatter sunlight forward toward New Horizons, lighting them up. The observing spacecraft are seeing the skies in all the locations where the Sun is rising or setting on the surface. The Pluto image has been downsized to match the scale of the Titan image.

Kim Ennico told me the basic data for these post-flyby lookback opnavs:

Visit Name              Instrument      Target  N_image  Resn(km/IFOV) Phase(deg)       starttime(UTC)

NAV_C4_L1_NONCRIT_1_01  LORRI/1x1       Hydra   2       9.89    166.70  2015-07-16 03:35:01

NAV_C4_L1_NONCRIT_1_02  LORRI/1x1       Pluto   2       9.88    165.36  2015-07-16 03:39:41

NAV_C4_L1_NONCRIT_1_03  LORRI/1x1       Charon  1       9.95    165.53  2015-07-16 03:44:20

After today's press briefing, they released all the raw images that had been downlinked as of Tuesday. So I can finally complete my montage of the New Horizons Pluto encounter data set....except that now I know I am waiting for those five opnavs! They should be released next Friday.

The New Horizons Pluto flyby LORRI data set

NASA / JPL / JHUAPL / SwRI / Emily Lakdawalla

The New Horizons Pluto flyby LORRI data set
In the two weeks surrounding New Horizons' flyby of Pluto, only 1% of the science data that it acquired were downlinked to Earth.

Since the raw images were released so late in the day, I didn't want to rush to post about what amateurs are doing with them. I am going to give the amateur community all weekend to make terrific visualizations from the data released today, and I will do a roundup post on Monday. Stay tuned for that!

In the meantime, if you can't get enough Pluto, check out all the images from today's press briefing, and look around at all the top science news websites for people's writeups of what was said at the briefing today (I particularly recommend Alex Witze at Nature, Eric Hand at Science, Nadia Drake at National Geographic, and Mika McKinnon at io9; you may also enjoy Alex Parker's words on these images, and Sarah Horst's musings about haze. I need a little more time with these images before I'm ready to analyze!

Read more: trans-neptunian objects, New Horizons, pretty pictures, Pluto, Cassini, Titan, Charon, explaining science, dwarf planets beyond Neptune, Pluto's small moons

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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