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Headshot of Emily Lakdawalla

Twinkling worlds in motion: New Horizons' first optical navigation images of Pluto and Charon

Posted by Emily Lakdawalla

07-08-2014 14:31 CDT

Topics: trans-neptunian objects, New Horizons, pretty pictures, Pluto, Charon, mission status, dwarf planets beyond Neptune, animation

What's that in the distance? A binary star? Those are two little round worlds dancing in circles, whirling around a point in space located between the two of them. It's Pluto and Charon, clearly separated by New Horizons' high-powered Long-Range Reconnaissance Imager (LORRI) camera.

New Horizons spies Pluto and Charon, July 2014


New Horizons spies Pluto and Charon, July 2014
This animation of Pluto and its largest moon, Charon, was taken by New Horizons spacecraft as it traveled toward Pluto in July 2014. Covering almost one full rotation of Charon around Pluto, the 12 images that make up the movie were taken July 19-24 with the Long Range Reconnaissance Imager (LORRI) at distances ranging from about 429 million to 422 million kilometers.

What I think is especially cool about this animation is the fact that you can clearly see that neither Pluto nor Charon is still; Charon's mass is a large enough fraction of Pluto's that both are in a mutual orbit around a point in empty space, above Pluto's surface, called the system's barycenter. Here's a zoomed view that makes that even more obvious:

New Horizons spies Pluto and Charon, July 2014 (zoom)


New Horizons spies Pluto and Charon, July 2014 (zoom)

If you are wondering why Charon seems to be getting closer to and farther from Pluto, it's because we're looking at the system from an angle, so the circular motions of Pluto and Charon appear ellitpical. Here's a diagram that may help:

New Horizons spies Pluto and Charon, July 2014 (simulated view)

APL / New Horizons GeoViz

New Horizons spies Pluto and Charon, July 2014 (simulated view)

These photos weren't just taken for their coolness value, but they're not exactly for science, either. They're to help steer New Horizons and to reduce the uncertainty in our estimates of Pluto and its moons' positions in the sky. I described the reasons for these optical navigation photos in a post a few weeks ago.

You may notice that Pluto and Charon seem closer to each other in the LORRI photos than they appear in the simulated view. A key point is that LORRI is just barely resolving Pluto in these images. At a range of 422495617 kilometers and an angular resolution of 4.95 microradians, each pixel spans 2090 kilometers. Pluto is about 2300 kilometers across -- barely more than one pixel. But you can see that its light shows up on several pixels, which is why the two worlds seem closer than they are. That's normal; even point light sources like stars spread out across more than one pixel in digital cameras. How much the light from a point source spreads is described mathematically by something called the point spread function. Every camera is a little different. Point sources in LORRI images appear slightly more spread out than point sources in Cassini images, so when I look at LORRI and Cassini images it feels like LORRI images are slightly blurrier. But the spread of a point light source is very precisely described mathematically, so it's very easy to sharpen LORRI images (as well as Cassini and Hubble and any other space camera images) with image processing software.

Sharpening these photos won't get us any details on Pluto's surface, though. We have to wait many many months -- really until just days before the encounter -- to see what kind of features Pluto has. I'm feeling very impatient! These photos are so thrilling -- it's the first moment that really marks the transition of Pluto from an astronomical object to a geological one. But they've just whetted my appetite -- I can't wait to see more of what this distant system has to show us.

See other posts from August 2014


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


Michael Skidmore: 08/07/2014 04:22 CDT

Where could you find the raw Op-Nav images online? Is there a site similar to the Cassini raw image page, or is it a matter of waiting until the data make it to the NASA PDS?

Don Day: 08/07/2014 05:43 CDT

I caught some of Clyde Tombaugh' enthusiasm for Pluto while a 70's student at New Mexico State University where he taught astronomy. He would have loved seeing this day, and I'm feeling some of that exuberant wonder in your writings, Emily. Between New Horizons and Rosetta and MSL, the coming year or two may be a bit distracting. In a good way.

Jonathan Ursin: 08/07/2014 08:32 CDT

These pictures of Pluto and Charon remind me of the images Curiosity took of Earth and the Moon from Mars.

Ned : 08/08/2014 08:33 CDT

Very cool. Lol, when I first saw the sequence, even before I read the text, I was said the myself "wow you can even make out the rotation around the barycenter". Thanks Emily.

Mark Zambelli: 08/08/2014 03:51 CDT

Wow indeed. The cool bit for me is that the distance between New Horizons and Pluto is now the same as the distance between Rosetta and Earth... kinda marks a milestone of-sorts in my view. Can't wait!

Ted Judah: 08/08/2014 08:36 CDT

I am really looking forward to this flyby. I remember when the Voyagers revealed the gas giants close up for the first time. Exciting time we live in!

amcshane: 08/10/2014 06:02 CDT

Great video, and the movement round the barycentre (as others have said) is very noticeable. As New Horizons gets closer it should be possible to more precisely time the orbits of Charon and the other moons and from that get ultra-precise measurements of Pluto's mass.

goldfires: 08/13/2014 06:51 CDT

Is there any reason LORRI images are more blurry than images from Cassini? Is the optic of lower quality? Or does it have a focus problem, like the long focal length cameras in Deep Impact and Mars Express?

Fred Thurber: 08/13/2014 09:59 CDT

I have to ask the obvious question here: I would think that each of these trans-neptunian objects exerts a strong tidal influence on each other. Is it enough to warm them enough to maintain liquid oceans under a coating of ice? Or are they so tidally locked that no such effect takes place?

Emily Lakdawalla: 08/26/2014 10:47 CDT

@goldfires: It's not exactly "blur." It's just different optics. New Horizons has a miniaturized payload compared to a flagship mission like Cassini. In fact, all of New Horizons' scientific instrument payload together weighs less than Cassini's camera instrument.

Charlie: 11/18/2014 03:39 CST

Wondering what the visual magnitude of Pluto would be from New Horizons as of 11/18/14. I'm thinking Pluto would currently be magnitude of 4ish. Would Pluto appear to be magnitude -2 at the end of June 2015? How close am I?

Jim: 01/18/2015 02:15 CST

What are the other 2 items near the edge of animation? They seem the same size as Charon.

arf: 01/18/2015 04:50 CST

What are the other dots in the magnified image: noise, or other satellites (the apparent motion is indicative)? Trivia: Charon orbits Pluto at a distance of ~19000 km. Neptune has a radius of ~24000km. This means 1. The orbit of Charon would fit inside Neptune, and 2. Neptune would be clearly visible as a disc if NH were approaching it now.

arf: 01/18/2015 04:54 CST

Jim, I'm guessing the other two stationary dots are stars. It would be interesting to get an idea of the stellar parallex shift with Earth - Pluto as a baseline. How many decimal places can we add to the distance to Alpha Centauri?

Emily Lakdawalla: 01/21/2015 09:57 CST

Jim and arf: Yep, they're stars. The animation is aligned on them, which is what allows us to see the motion of Pluto and Charon with respect to their barycenter.

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