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Emily Lakdawalla • Jul 03, 2015

Pluto's progression: Third-to-last Pluto day before encounter

Only two days remain until New Horizons' historic encounter with Pluto....two Pluto days, that is. Pluto and Charon rotate together once every 6.4 days, so as New Horizons has approached the pair over the last week, we've been treated to one stately progression of all of their longitudes. Here, Björn Jónsson has collected and stacked a week's worth of images of Pluto and Charon:

Pluto's progression: June 22 to July 1, 2015 (third-to-last Pluto day before New Horizons encounter)
Pluto's progression: June 22 to July 1, 2015 (third-to-last Pluto day before New Horizons encounter) Pluto nearly doubled in apparent size in New Horizons' forward view between June 22 and July 1, 2015. This period covered more than one Pluto day.Image: NASA / JHUAPL / SwRI / Björn Jónsson
Charon's progression: June 22 to July 1, 2015 (third-to-last Pluto day before New Horizons encounter)
Charon's progression: June 22 to July 1, 2015 (third-to-last Pluto day before New Horizons encounter) Pluto nearly doubled in apparent size in New Horizons' forward view between June 22 and July 1, 2015. This period covered more than one Pluto day.Image: NASA / JHUAPL / SwRI / Björn Jónsson

There's enough detail now that my eye wants to make coherent shapes of the splotches on the surfaces. Anything round or ring shaped, my eye wants to turn into a crater; I have to remind myself that with Pluto and Charon lit at nearly full phase, there's no hope of seeing shape from shading anywhere on these dots just yet. But if I tell my mind there's no craters here, my eye starts catching faces and animal shapes.

It's slightly better to look at what the New Horizons team has made from recent Ralph MVIC color in combination with high-quality, grayscale LORRI data: two distinctly different faces of the two round worlds we'll be passing in two weeks. What is that maddening series of dots on the Charon-facing hemisphere? Pluto looks like a bear's paw.

Pluto and Charon in color: LORRI + MVIC, June 25 & 27, 2015
Pluto and Charon in color: LORRI + MVIC, June 25 & 27, 2015 Pluto shows two remarkably different sides in these color images of the planet and its largest moon Charon taken by New Horizons on June 25 and June 27. The images were made from black-and-white images combined with lower-resolution color data. The left image shows the side of Pluto that will be seen at highest resolution when New Horizons makes its close approach on July 14. The hemisphere is dominated by a very dark region that extends along the equator. The right image is of the side that faces Charon; the most dramatic feature on this side of Pluto is a row of dark spots arranged along the equator. (The equator appears near the bottom of the images of both Pluto and Charon, as New Horizons' view is mostly of their northern hemispheres.)Image: NASA / JHUAPL / SwRI

Among other worlds we've seen in the solar system so far, Pluto is reminding me most of Triton, with its brighter and darker patches. But we know already that Triton's is not as contrasty a surface as Pluto's. What makes Pluto's darks darker than Triton's? We'll have to wait and see!

Voyager 2 approach image of Triton
Voyager 2 approach image of Triton Taken on August 21, 1989, four days before Voyager 2's closest approach to Neptune, this was one of the first images to show surface features on Neptune's largest moon, Triton.Image: NASA / JPL / color composite by Ted Stryk

I tuned in from vacation to watch last week's televised mission update. The most important news it contained was that after deep searches, the mission has discovered no new moons or rings. That means it's safe (as far as we can tell) for New Horizons to continue down its originally planned course past Pluto; it will traverse Pluto's potentially hazardous equatorial plane at a position opposite that of Charon. The mission is now commited to that course for good.

The next televised mission update is on Tuesday, July 7 at 8:30 PT / 11:30 ET / 15:30 UT; from that day forward, the mission updates will come daily. I'll be traveling to Maryland on July 8 in order to cover the mission from the Applied Physics Laboratory, the center for all things Pluto. Check my What to Expect post for a schedule of, well, what to expect for the week of encounter.

Two more Pluto days left!

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