Pluto has been slowly revealing itself to NASA's New Horizons probe, which is speeding towards a July 14 close encounter with the dwarf planet. Here, Nature looks at how this distant world has come into view.

Ready for a close-up
By late May and early June, New Horizons was still roughly 50 million kilometres from Pluto—but it was capturing better images of the dwarf planet (below; center) and its large moon Charon (far right) than any telescope on the ground or in space ever had.

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Colour view
This July 8 picture, taken as New Horizons closed in at a distance of 6 million kilometres, shows Pluto (below; right) and Charon (left). The difference between the two is striking: Pluto has sharply differing patches of bright and dark, whereas Charon's most noticeable feature is its dark pole, seen near its top. Pluto is also reddish in colour, probably because ultraviolet radiation pummelling its surface turns ices into complex hydrocarbon compounds called tholins.

Charon, by contrast, may have a chemically simpler surface. “They look as if they are completely different worlds,” says Alan Stern, the mission’s principal investigator and a planetary scientist at the Southwest Research Institute in Boulder, Colorado. “They could as well have been raised billions of miles apart.”

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Dark side
This July 11 image (below), taken when New Horizons was 4 million kilometres away from Pluto, is the last best look at the hemisphere that will not be seen during the July 14 close encounter. Most puzzling is the belt of dark spots that girdles Pluto's equator (visible near the bottom of this image). Each spot is roughly 500 kilometres across, with patchy, complex borders. Above the spots are linear features that hint at polygonal shapes, like the face of a honeycomb. Mission scientists do not know what these might be, but New Horizons has collected data—information on the chemical composition of Pluto’s surface—that could help them to unravel the mystery. 

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

The whale and the heart
On July 7, at 8 million kilometres, the probe's long-range telescope photographed the hemisphere of Pluto that will come into great detail during the fly-by. Again, patches of bright and dark stand out. A dark area at the bottom left has been dubbed 'the whale'. A bright area at the bottom right (measuring about 2,000 kilometres across) is 'the heart'. And the polar region, above them both, is neither bright nor dark. The closer New Horizons gets, “the more you can confirm or deny what you are speculating about”, says project scientist Hal Weaver at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “You see it dividing into more things, becoming more complex.”

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Charon revealed
Charon's once-featureless surface emerged in this July 11 image as a geologically active world. Impact craters appear to pockmark its surface (below; bottom center and right center), and huge chasms (lower right) slash across its southern hemisphere. The largest such canyon is longer and deeper than the Grand Canyon in Arizona. Charon is about 1,200 kilometres across.


Questions about Pluto or the New Horizons mission? Tune in to our live blog on July 14. Our panel of experts will be taking reader questions as we report from mission control.

This article is reproduced with permission and was first published on July 13, 2015.