LAUREL, Md. — Remember those weird black spots that showed up on Pluto's surface a couple of weeks ago in pictures from NASA's New Horizons spacecraft? Now that the piano-sized probe has closed in to a distance of less than 2.5 million miles (4 million kilometers), the spots look like even weirder splotches.
This view of the dwarf planet, captured on Saturday, may be the "last, best look that anyone will have of Pluto's far side for decades to come," the $728 million mission's principal investigator, Alan Stern of the Southwest Research Institute, said in an image advisory.
That's because the splotches are turning out of the field of vision for New Horizons' Long Range Reconnaissance Imager. When the spacecraft makes its climactic flyby on Tuesday, coming within 8,000 miles (12,500 kilometers) of the surface, a different swath of territory will be turned toward the camera — with a bright heart-shaped region front and center.
Saturday's view provides a closer look at the splotches as well as the polygonal, craterlike shapes and mottled, light-colored terrain to the north. But New Horizons' science team still hasn't cracked the mystery behind those splotches. NASA says the irregularly shaped dark regions span areas roughly as big as the state of Missouri, 300 miles (480 kilometers) across.
"It's weird that they're spaced so regularly," Curt Niebur, the mission's program scientist at NASA Headquarters, said in the advisory. Jeff Moore of NASA's Ames Research Center said the team can't tell "whether they're plateaus or plains, or whether they're brightness variations on a completely smooth surface."
Moore and the rest of the team should get a better sense of how Pluto is put together once they analyze all the imagery they're getting from this month's encounter. "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," Moore said.
New Horizons has traveled nearly 3 billion miles (5 billion kilometers) outward since its launch in 2006, and now the mission is reaching its climax. Tuesday's flyby will represent the first opportunity to get an up-close look at an icy world beyond the orbit of Neptune, and possibly the only opportunity for decades to come. Pluto's biggest moon, Charon, could provide additional thrills as scientists learn more about its "dark pole."
Over the next week, several gigabytes' worth of images and other observations will be captured — so much data that it'll take 16 months to send it all back. Keep track of it all by following @NASANewHorizons and @NewHorizons2015 on Twitter, checking in with the New Horizons websites offered by NASA and Johns Hopkins University's Applied Physics Laboratory here in Maryland, and connecting with DailyPluto.com on Facebook.