NASA's New Horizons spacecraft, in near perfect health 10 years after launch and more than one year past its historic July 2015 flyby of Pluto, has now sent back about 80 percent of the data it collected during its close encounter and if all goes well, the rest will cross the solar system and make its way to Earth by the end of October.
While no major surprises are expected -- the highest-priority data and images were beamed back first -- project scientist Hal Weaver said the yet-to-be-seen data will help researchers fill in the blanks about a cold, surprisingly active world marked by hard-as-rock water-ice mountains, moving glaciers of slush-like molecular nitrogen ice and an oversize moon that may have cracked open when a subsurface water layer froze.
"We've been ingesting the data, and now we're digesting the data," Weaver said in an interview. "We're trying to not just show people what we've got, we're trying to explain what we've got."
He said the science team has been "blown away by the incredible diversity of terrain on Pluto and just generally, the collection of phenomena we've observed throughout the Pluto system. It takes a while, scratching our heads, to figure out what's going on, but we're making progress. ... We just feel lucky that Pluto is such a spectacular place to visit."
Launched Jan. 19, 2006, the nuclear-powered New Horizons streaked within about 7,700 miles of Pluto on July 14, 2015, studying one face of the world and its large moon Charon in extraordinary detail with a suite of compact, state-of-the-art instruments and cameras. The data were stored on two 8-gigabyte solid state recorders.
Because of Pluto's enormous distance from Earth -- 2.97 billion miles -- the low power of New Horizon's radio transmitter and the relatively small size of its 83-inch-wide dish antenna, it has taken a full year to get the bulk of that stored data back to Earth at about 2,000 bits per second.
"We started downloading the data from the spacecraft in earnest in September," Weaver said. "We've been systematically working our way through the data that we took during what we call the money load, it covered the period seven days before closest approach and then two days after. That's our core sequence."
The data were prioritized "so the most important stuff would come down first. And generally speaking, the stuff closest to the closest approach data on July 14 was generally the highest resolution, the highest sensitivity, so forth. Right now, we have about 80 percent of all the data we collected during that core load, and we expect to finish about the end of October."
The world that data has revealed exceeds scientists' wildest expectations.
Along with towering ice mountains, Pluto features a heart-shaped region informally dubbed Sputnik Planum that is essentially a huge glacier of molecular nitrogen ice, heated from below by radioactive compounds in the dwarf planet's core that drive lava lamp-like convection.
"How do you make something geologically interesting? Well, I think the real key is molecular nitrogen ice, and the importance of it," Weaver said. "That's the dominant thing you're seeing when you look at that face of Pluto with the really bright, giant glacier."
Weaver speculated that an astronaut, suitably protected, would "float" on the "soft and mushy" ice and "you could actually ski across the glacier." Large polygon-shaped features indicate convection, with slightly warmer material rising from below, cooling and then sinking again.
The ice also sublimates and condenses, contributing to Pluto's tenuous nitrogen atmosphere and distributing the ice across the surface.
"You don't need very much heat to cause this activity to occur," Weaver said. "Even the radiogenic heating from the decay of radioisotopes in the interior is enough ... to cause Sputnik Planum to act almost like a lava lamp, where you're heating stuff from below and stuff flows up but then it comes back down.
"We've actually done the detailed modeling now of these things. There's still some uncertainty about exactly how thick the ice layer is and different people come up with different numbers. But it's probably on the order of five to 10 kilometers, something like that."
As for what created Sputnik Planum in the first place, scientists speculate that a large body crashed into Pluto at some point in the distant past, blasting "a big basin" that later filled with nitrogen ice.
There are even indications Pluto might harbor a subsurface layer of liquid water, with ammonia present to act as a sort of antifreeze.
Pluto's large moon Charon does not feature large deposits of nitrogen ice, but its rugged topography and a huge chasm stretching across the visible hemisphere suggest "evidence of something that happened in the past, the freezing of water ice, that maybe split Charon almost wide open," Weaver said.
Charon is one of five moons orbiting Pluto, and by far the largest. Current thinking is that Pluto collided with a similarly sized body in the early solar system, a glancing blow that ripped off "the outer mantles, which formed an ice-rich debris disk within which the small satellites formed," Weaver said.
Pluto is the most famous member of the Kuiper Belt, a vast zone of small bodies left over from the formation of the solar system 4.6 billion years ago. As of this week, New Horizons was about 287 million miles beyond Pluto, on course for a Jan. 1, 2019, flyby of a second Kuiper Belt body known as KBO-2014 MU69.
Weaver said the spacecraft and its instruments continue to operate in near-flawless fashion and even as data from the Pluto flyby continue to trickle in, the science team is busy planning for the upcoming MU69 encounter. Data collection is expected to begin in about one year.
"It'll be happening before we know it," he said. "So we have to have everything basically planned during the next year to make sure we're ready to hit the ground running in October 2018. That's not that far off! We're going to be spending the next year fleshing out all the details."
He said scientists are particularly interested in MU69, which measures 20 to 30 miles across, because it likely is a pristine remnant of the raw material that acted as building blocks for the sun and its retinue of planets.
"The exciting thing about the new object, we think it's potentially the most primitive object ever studied close up," Weaver said. "It is one of those, what we call, classical Kuiper Belt objects, probably formed in place, hasn't moved around much or been perturbed. It really gives us a window back 4.6 billion years to what the solar system was like then."
With the MU69 flyby, scientists will have "taken stock" of the three families of bodies in the Kuiper Belt, from the dwarf planet Pluto, to a comet being studied by the European Space Agency's Rosetta spacecraft that was knocked out of the belt in the distant past and soon, to a classical KBO that has remained in cold storage since the birth of the solar system.
Engineers are debating just how close New Horizons will fly past MU69. A closer flyby would result in higher resolution, but the high speeds involved complicate spacecraft operations, requiring precise, carefully timed movements to capture data in the same way a race fan might pan with a camera to "freeze" a passing car and prevent blurring.
"This is our last gasp in terms of close flybys, so we'd really like to fly by really close, and so far, initial investigations suggest it's going to be possible," Weaver said. "But we've got to work on the details, and it's happening now."