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SYFY WIRE Bad Astronomy

Pluto Grows in Size While Its Moon Sports a Dark Cap

By Phil Plait

New Horizons is approaching Pluto and is now just over 20 days from closest approach. Every day the tiny world looks a wee bit bigger to the space probe, which is screaming toward Pluto, getting closer to it by 14 kilometers each and every second.

New images just released are showing more detail, as you can see in the images above. These have been carefully processed to enlarge and enhance those details. Note that Pluto is almost certainly very round, but it looks misshapen due to darker features near the edge blending into the black background.

Pluto is severely tilted with respect to its orbit around the Sun; while the Earth is tilted by about 24°, Pluto is flipped over at about 120°. Because of this and its approach angle, New Horizons doesn’t see the entire surface of Pluto, but it does see more than half as Pluto rotates. Projecting forward, scientists know which part of Pluto will be under the probe’s cameras when it passes, and they can see that part of the surface is richly diverse, sporting lots of different features. This promises some pretty exciting pictures come July …

But we’re learning more all the time. Charon is Pluto’s big moon, more than half the diameter of Pluto itself. Yet it’s very different; as you can see in the video above it’s far darker than Pluto, and therefore likely to have less ice on its surface to reflect sunlight. Also, weirdly, its north pole is darker than the average of the rest of its surface! That’s very interesting. On Earth, water and land are dark, but ice is bright, and the icy poles are brighter than average. Pluto too has a bright north pole (it’s not certain what it is, but it could be frozen nitrogen, not water). Why is Charon’s pole so dark? What different processes are at work there than on its larger partner?

We’ll find out soon enough. Right now, New Horizons is less than 25 million kilometers from Pluto. At closest approach it’ll zoom over the surface at distance of less than 14,000 km! And every day, every hour, between now and then this new world grows under the spacecraft’s eye, getting larger and revealing more detail, more secrets invisible from Earth. 

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