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Headshot of Emily Lakdawalla

Pluto minus one day: Very first New Horizons Pluto encounter science results

Posted by Emily Lakdawalla

13-07-2015 12:16 CDT

Topics: trans-neptunian objects, New Horizons, pretty pictures, Pluto, Charon, explaining science, dwarf planets beyond Neptune

Before I mention the science, here is the very latest of the images from New Horizons:

Pluto and Charon on July 12, 2015


Pluto and Charon on July 12, 2015
These are the final optical navigation photos returned from New Horizons on July 12, 2015. They were taken at 08:45 and 08:50 UT on July 12 from a distance of 2.5 million kilometers, just a bit more than 2 days away from closest approach. The images have been enlarged, stacked, and deconvolved by the New Horizons team.

At a press briefing this morning, New Horizons principal investigator Alan Stern divulged some extremely preliminary first science results from the New Horizons Pluto encounter. Without further ado:

New Horizons images have dramatically reduced uncertainty in Pluto's diameter. Stern reported Pluto's radius to be 1185 +/- 10 kilometers, so its diameter is 2370 +/- 20 km. Implications of the new diameter:

  • Pluto is larger than Eris, and is therefore the largest known object in the Kuiper belt by diameter. Eris lacks an atmosphere so we have very precise measurements of its diameter from stellar occultations: 2336 +/- 12 km.
  • A larger diameter than thought means a lower density than thought, which means it has a higher proportion of ice to rock than thought. At the moment these comparisons are all qualitative; we'll see quantitative ones eventually. It makes the density contrast between Pluto and Eris even greater, hinting at very different histories for the two worlds. (Eris is 27% more massive than Pluto.)
  • A larger diameter than thought also makes its atmosphere thinner than thought; the troposhere is shallower than models have predicted. It's going to be a while before atmospheric scientists determine the implications of that for their models of Pluto's atmosphere.

The diameter will be even better known in a couple of days, once we get two much higher-resolution global images of Pluto down from the spacecraft. Stern also mentioned that they applied the techniques they used to measure Pluto's diameter to Charon (whose diameter is very well constrained by stellar occultations at 602.4 +/- 1.6 kilometers), and the New Horizons measurements "got the right answer."

New Horizons compositional measurements have confirmed that Pluto has a polar cap made of methane and nitrogen ices. A cap was suspected, but measurements taken over the weekend are high-resolution enough to resolve the pole separately from the equatorial regions and determine that ices are present. Stern said: "These first compositional measurements where we could resolve different regions show us that the polar regions are compositionally very different from the dark regions. For example, the methane absorption bands are much weaker in the dark regions."

New Horizons has detected ionized nitrogen escaping from Pluto's atmosphere five days before closest approach. This is significantly earlier (farther away, around 6 million kilometers) than they expected to detect the atmosphere; they expected to see it 24 to 48 hours (1 to 2.5 million kilometers) out. Seeing the atmosphere so much farther from Pluto suggests (Stern said) either that the atmospheric escape rate is higher than thought, or else the mechanism is different than thought. Later in the briefing, he expanded on this: he said "either the source is stronger (the escape rates are stronger), or the conversion from neutrals to ionized species is more efficient, or there is some mechanism that's concentrating the ionized species along the flight path. At Jupiter, we found that the ionosphere was very inhomogeneous. We found that there were regions of enhanced concentration of sulfur and oxygen. Whether Pluto has that is a great question; we don't have the answer and it may take a long time to get the answer."

Here is the latest composite of images returned -- and to be returned -- by New Horizons. The images at the top of this post are the last optical navigation images; at their original resolution they would have showed Pluto and Charon at 189 and 94 pixels across, respectively. I will add them to this composite once the raw images have been posted. Very late tonight, the mission will receive a Charon image about twice as good as this one, and a much, much larger Pluto image.

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.
See other posts from July 2015


Or read more blog entries about: trans-neptunian objects, New Horizons, pretty pictures, Pluto, Charon, explaining science, dwarf planets beyond Neptune


Sergey: 07/13/2015 01:22 CDT

Sorry, Emily, Eris' diameter is 2326 +/- 12 km (radius 1,163+/-6 kilometres) in this article

Rob: 07/13/2015 02:21 CDT

Looking more and more like Triton, as was predicted. It looks like it may even have canteloupe terrain as on Triton.

FrenchSpaceGuy: 07/13/2015 03:11 CDT

Just a comment: 602.4km is the radius of Charon, not its diameter.

rsholmes: 07/13/2015 03:13 CDT

If the numbers given in this post are correct then the difference in the diameter measurements for Pluto and Eris is only about 1.5 sigma. Sounds to me like the claims that this establishes Pluto as larger than Eris are premature; more precise measurements could easily put Eris slightly ahead. Of course whether Pluto is a few km larger or smaller than Eris matters only in the Guinness book. Either way the two are basically the same diameter.

Johan: 07/13/2015 08:23 CDT

Pluto look so much like Triton to me, I'm so excited like most I have been waiting 10 years for this. Go New Horizons and break new ground!

Tony Fisk: 07/13/2015 09:03 CDT

We are starting to see shadows in the terrain. I am intrigued by the presence of a polar ice cap. Pluto has a high tilt to its rotational axis, and NH has been approaching from the sunward side. This hemisphere has been experiencing a long, 'hot' summer, so that polar cap must be very stable. The South(?) Pole on the night side will be just as interesting.

MikeQ: 07/13/2015 09:34 CDT

Any word on when more MVIC data will be released? As I understand it, all the color data we have is several weeks old and is extremely low resolution.

Plutonian: 07/13/2015 10:19 CDT

Does the distinctive polar cap suggest that the weak sunlight reaching Pluto still has a significant effect on atmosphere and terrain?

Alex: 07/14/2015 12:37 CDT

I can almost hear the concept artists feverishly updating their acrylics and oils as the data continues to pour in. I wonder if we’ll have truck stops there one day.

LocalFluff: 07/14/2015 02:28 CDT

I'd like a closer look at that crater-in-a-crater to the right. But there's only one flyby and it is at the side of that huge whitish formation to the left. And that is very lucky. Many craters have been observed in the solar system, but not so many "butterflies".

Deoy: 07/14/2015 05:36 CDT

Thanks Emily for your very interesting updates. After fly by mission, would NASA be casting away New Horizon to deep space or it will remain in kuber space for more pictures and studies

Shalaka: 07/14/2015 05:47 CDT

Great article, To know more about space, science and technology visit and you have another source then please share with me

Mewo: 07/14/2015 06:59 CDT

@Deoy- There are plans to extend the mission post-Pluto and visit a second Kuiper belt object. A potential target has been found by Hubble, which New Horizons would reach in early 2019. It's about 40km across, so similar to Pluto's smaller moons.

ChucK G.: 07/14/2015 10:51 CDT

I have waited 48 years for this day since I read my first book, The Search for Planet X. Any and all the science that is gained from this is wonderful. Thanks for this Blog.

Deoy: 07/14/2015 04:07 CDT

Thanx Mewo. @ Chuck G, glad that these things happened in our times. It may be a small step for Man but a leap for mankind

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