Pluto at Last

Nine years after leaving Earth, New Horizons closes in on the last of the Original Nine planets, and whatever lies beyond.

Artists have imagined Pluto, but mission scientists are loathe to predict what the first close-ups will reveal. Craters? Mountains? A subsurface ocean? All are possible. ( NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)
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For all of New Horizons’ sophistication, Stern and his team had to make compromises with the design to save weight and cost. In particular, the spacecraft lacks a scan platform, so it won’t have Voyager’s ability to point its antenna at Earth while gathering data. In fact, Stern says, “on encounter day, we don’t talk to or hear from the spacecraft once. It’s completely on its own. It doesn’t even check in until it’s all over.” That also prevents the team from knowing if the craft survived passage through the plane of Pluto’s satellites, where the dust hazard is thought to be greatest, until the craft sends a brief update on its health that evening. Still, Stern says, “after 25 years, we can wait another 12 hours to find out how it all went.”

New Horizons will make its closest approach, at a distance of about 6,200 miles, at 7:49 a.m. Eastern time on the 14th. Light from the sun will be nearly a thousand times dimmer than it is on Earth, so long exposures will be necessary; to avoid blurring, the spacecraft will have to rotate to track its target, a technique used when Voyager 2 took close-ups of Neptune’s moon Triton in 1989. New Horizons will be flying four times closer to Pluto, however, and its images will have more than 10 times finer resolution. The best views will show features half an acre in size.

What will we see in those images? “I don’t make predictions,” says Stern, and most of his teammates are just as reticent. By chance, New Horizons will reach Pluto 50 years to the day after Mariner 4 captured the first close-ups of Mars, images that shocked scientists with a moonlike expanse of craters on a world long thought to be relatively Earth-like. Everyone is only too aware of the many surprises from half a century of planetary exploration, from active volcanoes on Jupiter’s moon Io to braided rings at Saturn. Still, people are bound to speculate. One NASA geologist—he’d rather his name not be attached to such guesswork—thinks that if Pluto’s volatile ices really are evaporating away from the sunlit northern hemisphere, the process may have created a spectacular landscape of cliffs and mesas resembling a deep-freeze version of Arizona’s Monument Valley. Others venture that there could be signs of a subsurface ocean like those believed to exist within some of the icy satellites of Jupiter and Saturn. And the team will also be looking for geysers like Triton’s—although spotting them might take some luck, since half of Pluto will be shrouded in darkness.

Speeding away from its encounter, New Horizons will look back to watch Pluto and Charon eclipse the sun, providing a brief opportunity for the instruments to peer through the bodies’ atmospheres and study their structures, temperatures, and pressures. The spacecraft will also glimpse Pluto’s night side in the dim light reflected from Charon, with about the same resolution as a naked-eye view of the moon from Earth—enough to tell whether fresh ice has begun to accumulate at the winter pole. 

If New Horizons does survive its passage through the Pluto system, a pre-selected sample of encounter data will begin arriving at Johns Hopkins University’s Applied Physics Laboratory in Maryland at around 7 a.m. on July 15. This batch, including what are anticipated to be some of the most dramatic images—what the team calls their “New York Times dataset”—will be sent in slightly compressed form to speed the transmission. Even so, because of the enormous distance and the relatively small size of the spacecraft’s antenna, the data will trickle in at just 2,000 bits per second, slower than a 2400-baud modem from the 1980s.

For safety’s sake, New Horizons will spend the following two months sending back all the encounter data in compressed form, making the pace of discovery seem more like that of an orbiter mission than a one-time flyby. Then the craft will re-transmit everything in its full, uncompressed state, a process that won’t be complete until the fall of 2016.

By then, New Horizons will be a little more than two years from another flyby, this one within the Kuiper Belt itself. Finding a post-Pluto target that the craft could reach with its limited fuel budget turned out to be a challenge; several years of searching with ground-based telescopes came up empty. Then, last summer, Hubble came to the rescue: The team was awarded a whopping 202 orbits of time on the spaceborne observatory to search for a target. In October, they announced they had found what they were looking for: two tiny worlds, each less than 35 miles across and roughly a billion miles beyond Pluto. For now, the objects are just called PT1 and PT3. Pending a decision in August, sometime this fall New Horizons could fire its engine to adjust its path for a rendezvous with one of them in 2019.

“We launched going to two Kuiper Belt objects, Pluto and Charon,” says Young. “We’re now planning for sure to fly past six Kuiper Belt objects: Pluto, Charon, and the four small moons. To be able to pick up another one that was in a whole different environment is going to be fabulous.”

For now, the team is focused on the coming exploration of a world that has captivated them for decades. Young says she’s most looking forward to “being confused and then not being confused—having the first set of data come down, and we wonder, ‘Why does it look the way it does?’ And then, as we tease out the rest of the data…learning something fabulous and new about Pluto” and the Kuiper Belt.

For Alan Stern, this is more than the end of a 25-year quest. It’s a chance to excite the public with an experience he says has been missing since the last Voyager encounter. “It’s raw exploration,” he says. He’s looking forward to sharing with the world “how exciting it is to turn a point of light into a planet before their very eyes.” And soon, if all goes well, a student in science class will open a textbook and see a clear picture of Pluto.  

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