Two Days Prior to Closest Approach New Horizons Images Pluto's Charon-Facing Hemisphere for the Last Time

With just two days remaining before New Horizons zips through the Pluto system at a breakneck speed of 49,600 kilometers per hour, the spacecraft is now entering the encounter phase of its mission which will culminate with its close fly by of the dwarf planet from a distance of approximately 13,000 km away on the evening hours of July 14. While preparing for this historic event and as part of its daily observations of the distant dwarf planet, New Horizons took one last look at the hemisphere of Pluto that always faces its largest moon Charon, presenting scientists with their final best-ever views of the side of the planet that will be opposite to the one the spacecraft will be seeing during the day of closest approach.

Pluto’s different hemispheres had also been glimpsed by New Horizons on late June, while the spacecraft was more than 15 million km away from its ultimate destination. Even from this greater distance, one of the things that really stood out in these previous images were a series of surprisingly evenly spaced dark spots along the planet’s equatorial regions, which have amazed scientists and space enthusiasts alike. Now, this intriguing region on Pluto has been imaged again by New Horizons from less than one-third the distance, providing scientists with a much more detailed look of the enigmatic dark spots. In the newly-released image, the latter appear to have a much more complex structure than previously thought, while the boundaries between the brighter and darker patches of the surrounding terrain seem to be irregular and more sharply defined, leaving scientists baffled about their origins. “We can’t tell whether they’re plateaus or plains, or whether they’re brightness variations on a completely smooth surface,” says Jeff Moore, a planetary geologist at NASA’s Ames Research Center, in California.

“It’s just amazing what we’re seeing now,” comments Dr. John Spencer, a New Horizons science team member at the Southwest Research Institute, in Boulder, Co. “It was a gradual approach and every picture was just a little bit better [at first]. Now, everyday we’re just seeing a whole new view of Pluto that’s telling us things we never knew before. And we’re seeing these crazy black-and-white patterns [on the surface] and we have no idea what those mean. And we’re seeing a lot of circular things and we’re wondering ‘are those craters or are they something else?’ We also saw circular features on Neptune’s moon Triton that are not craters, so we should know in a few days, but right now we’re having an awful lot of fun by just speaculating”.

Now, just as New Horizons begins the Pluto near-encounter phase of its mission on July 12, it will transmit most of the already collected data back to Earth, which mainly include optical navigation images and other science instrument data. The purpose for this data downlink will be two-fold, on one hand ensuring that most of the images and data on Pluto and Charon that will have been already collected will make it back to Earth, while also allowing the spacecraft to empty its data recorders on the other hand for the intense brief period of closest approach two days later.

During its close fly by of Pluto, New Horizons will be seeing just one side of the planet, the one that faces away from Charon, which the spacecraft will image in unprecedented detail with a resolution of up to 0.4km/pixel. For this reason, the image of the planet’s Charon-facing hemisphere that the mission’s science team released today is invaluable to astronomers and planetary scientists, providing them with their only detailed view of over half of the faraway planet for a long time to come. “This is the last, best look that anyone will have of Pluto’s far side for decades to come,” says Dr. Alan Stern, principal investigator for New Horizons, at the Southwest Research Institute. Nevertheless, by combining the images that New Horizons will gather of both of Pluto’s hemispheres, scientists are hopeful that they will be able to gain great insights to the planet’s overall geologic history and evolution. “When we combine images like this of the far side with composition and color data the spacecraft has already acquired but not yet sent to Earth, we expect to be able to read the history of this face of Pluto,” adds Moore.

Meanwhile, the best is yet to come. New Horizons’ science teams are already enthusiastically pouring over the data and all the detailed images that the spacecraft sends daily back to Earth. “We just can’t wait,” says Alice Bowman, mission operations manager for New Horizons, at the Johns Hopkins University Applied Physics Laboratory, in Laurel, MD. “We were used to seeing numbers on our screens at the mission’s operation center, so it’s just fantastic to see those numbers turn into images and spectra”.

The long wait is finally over. Two days from now, the all-mysterious Pluto and its biggest moon Charon, will be mysterious no more.

Stay with AmericaSpace for regular updates and LIVE COVERAGE of New Horizons’ approach and flyby of the Pluto system.

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