A variety of complex features is visible in this view of Pluto, composed from images taken on July 14. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Sep 13, 2015 Get set for a feast of pictures from Pluto!

Sen—There was much excitement in July when NASA’s New Horizons probe reached the climax of its mission by flying past Pluto. After a nine-year journey to its distant destination, the space probe sent home a number of astonishing images of the dwarf planet and its largest moon, Charon.

Then, by the end of July, all seemed to have gone quiet, and people have asked me why we are not seeing more results from this exciting mission. Had something gone wrong, some wondered.

In fact everything is going to plan. Even before the encounter, which saw New Horizons zip through the Pluto system at 50,000 km per hour, the mission team warned that there would be a hiatus lasting throughout August when no new processed images would be released. Instead the spacecraft has focused on sending other data and telemetry before resuming the return of pictures.

But of course it is the images that excite the general public most, allowing us to set eyes on a world—or rather worlds—about which very little was known before. Well the good news is that since September 5, just a week ago, new close-ups have begun to arrive. They show not just the fascinating and varied landscape of Pluto itself but new views of Charon and two of the smaller satellites, Nix and Hydra.

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A 350km-wide view of Pluto demonstrates the diverse range of surface features including dark, ancient craters, bright, smooth, young terrain, mountainsand dune-like ridges. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

These new images are just the first in a stream of downloads that will take around a year to complete, meaning that there should be a feast of new views to enjoy for a long time yet. Most were grabbed during that short but hectic window when New Horizons was close to Pluto, because its speed and trajectory soon sent it racing to ever-growing distances from the dwarf planet. The spacecraft is now more than five billion km from Earth and every bit of data takes more than 4.5 hours to reach us.

New Horizons principal investigator Alan Stern, of the Southwest Research Institute (SwRI), Boulder, Colorado, is clearly delighted with the quality of the results seen so far. He said in a statement this week: “Pluto is showing us a diversity of landforms and complexity of processes that rival anything we’ve seen in the Solar System. If an artist had painted this Pluto before our flyby, I probably would have called it over the top—but that’s what is actually there.”

Of the incoming stream, Stern said: “What’s coming is not just the remaining 95 per cent of the data that’s still aboard the spacecraft—it’s the best datasets, the highest-resolution images and spectra, the most important atmospheric datasets, and more. It’s a treasure trove.”

Much of this treasure’s complexity is proving challenging for planetary scientists to explain. They include what look like ice flows of nitrogen flowing from mountainous areas onto flat plains, valleys that appear carved by material moving across Pluto’s surface, and chaotic ranges of mountains rather like what has been seen on Jupiter’s frozen moon Europa.

Another oddity is what resembles a field of dark, wind-blown dunes. If that is what they are, then experts will be scratching their heads because Pluto’s atmosphere is currently too thin to produce strong enough winds to form such dunes. Could its atmosphere therefore have been thicker in the past?

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The image of Pluto’s hazy atmosphere, left, has been processed by the mission team to bring out the many layers within it. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

The hazy atmosphere is also proving interesting because is it more layered than was thought, creating gentle illumination of features around Pluto’s terminator—the dividing line between its dark and sunlit sides.

Incidentally, it has only been known that Pluto has an atmosphere since 1985 when astronomers in Israel watched the gradual fade and reappearance of a star that the dwarf world briefly passed in front of. Subsequent similar events—termed occultations—confirmed the atmosphere’s existence. If there had been no atmosphere, the stars would have disappeared in an instant and reappeared in the same way.

While I was attending a science festival in the UK last weekend, at the East Sussex site that once housed the Royal Greenwich Observatory, an astronomer colleague remarked that with Pluto’s surface now laid bare, we had now reached the final chapter in our survey of the classical Solar System as it had been understood since our childhood. The former planet had been the last great mystery waiting to be unveiled. The Hubble Space Telescope’s long-range attempt to decipher some clues about the surface had only hinted at what was to come.

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A new, detailed image of the remarkable surface of Charon, including fractured plains, mountains and craters.Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

But of course we are nowhere near closing the book on our understanding of even the principal players in the cast of worlds that makes up the Sun’s family. While the mapping of innermost planet Mercury was only fully completed in 2013, inner neighbour Venus still holds surface secrets. Spacecraft have mapped its terrain using radar, but the planet’s permanent veil of cloud prevents astronomers from seeing it directly and discovering how many of its volcanoes are active.

Mars is the planet studied in the greatest detail, and yet new and fascinating features are being revealed all the time. Petrified sand dunes are the latest to be pictured by NASA’s Curiosity rover around the base of Mount Sharp. A little further out, Dawn has surveyed asteroid Vesta and is now studying dwarf planet Ceres—but many thousands of smaller asteroids remain unexamined.

There is much still to learn about the gas and ice giants beyond Mars—Jupiter, Saturn, Uranus and Neptune. And their retinues of moons wait to be explored further by future space missions. Meanwhile, with Rosetta having observed a comet up close over an extended period for the first time, scientists will be keen to send probes to similar objects. In fact NASA’s Jet Propulsion Laboratory announced this month that it is looking into a “comet hitchhiker” concept to tour these minor bodies and asteroids.

It is clear that Solar System exploration is still very much in its infancy!

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Separate new images of tiny moons Nix, left, and Hydra, released on Friday. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

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