Pluto Ho! Close Encounter Just Two Years Away


Hurtling toward the Pluto system at 33,000 mph, NASA’s New Horizons probe was briefly awakened from electronic hibernation earlier this month to go through a full dress rehearsal for the July 2015 flyby.

With eight cameras and four spectrometers running on an anemic 30 watts of power, New Horizons is “the most sophisticated payload ever sent on a first reconnaissance mission,” said principal investigator Alan Stern of the Southwest Research Institute during a presentation at the Johns Hopkins Applied Physics Laboratory in Laurel, Md., on July 23.

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Stern emphasizes that this is a mission of superlatives. New Horizons is the fastest manmade object ever built. It is not only reaching the farthest classical planet in the solar system, but also surveying a new class of binary world.

There is little doubt that Pluto could have fascinating weather and geology, and serve as a Rosetta stone for the history of the solar system’s vast outer rim. This region, called the Kupier belt, contains countless icy bodies — perhaps 900 others the size of Pluto.

The marathon flight will complete our initial reconnaissance of the solar system that began over 50 years ago at the dawn of the Space Age. There will never be another time like this in the history of mankind.

Yes, we’ve sent orbiters and landers to follow in the track of the trailblazing probes like Voyager and Pioneer. But to people under age 30 today, the Pluto mission will be their first — and maybe last — experience at the thrill of seeing a new world close-up for the first time. The last planetary flyby was of Neptune in the summer of 1989 by Voyager 2.

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Untold wonders await our robotic ambassador from Earth. Hubble Space Telescope images show that Pluto has a markedly variegate, mottled surface of bright and molasses-colored dark regions. In the early 1990s Pluto became significantly redder. This could result from surface ices sublimating on the sunward-facing north pole and then refreezing on the shadowed south pole. (Like Uranus, Pluto’s spin axis is tilted nearly parallel to the plane of its orbit.)

During the fast flyby New Horizons may tie these albedo feature to surface geology. At 3 billion miles from the sun, where temperatures are a few tens of degrees above absolute zero, water, ammonia, and methane may replace the role of lava on the terrestrial planets. (Such volcanic processes have been seen on icy moons orbiting Saturn.) This cryo-lava could remain molten on Pluto’s surface at temperatures above -348 degrees Fahrenheit. This cryovolcanism could  create smooth plains, calderas, active vents, and ridges.

Volcanism would be evidence that Pluto has thin shell of ice with a source of heat beneath it, not unlike what has been found on Saturn’s moon Enceladus. This could be very likely because Pluto may have undergone tidal heating after the collision with another Kuiper Belt object billions of years ago that formed Charon. Orbital and rotational energy would have been dissipated as heat in the crust. Pluto would have melted and differentiated with heavier elements sinking to the core, and a water-ammonia ocean forming under the crust.

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New Horizons will also inventory the number of impact craters on Pluto and Charon. The crater survey will offer clues to the population of icy objects in the Kuiper Belt. This could be made even more complicated by the fact that Pluto may have “daughter” craters formed from material ejected by impacts on Charon. This is also compounded by the fact that if Pluto’s surface is comparatively young and geologically active as Neptune’s giant moon Triton, geological processes may overlay a lot of ancient craters.

The spacecraft will scrutinize the Pluto-Charon equatorial regions to look for remnants of a ring that might have collapsed onto the bodies. Saturn’s moon Iapetus (shown above) has a globe circling equatorial ridge that may have come from a fossil ring. Astronomers have used Hubble to search for any evidence of a faint set of ringlets existing now.

New Horizons will map the smaller satellites discovered by Hubble: Nix, Hydra, Kerberos, and Styx. Ground-based observations of light variations on these worlds suggest that they are chaotically tumbling, kind of like a Tilt-a-Whirl ride. They are torqued by a gravitational “eggbeater” effect caused by the swirling of Pluto and Charon around a common center of gravity that is between the binary pair. There is nothing else dynamically quite like this in the solar system.

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The New Horizons probe overcame seemingly insurmountable barriers (five cancellations) to get where it is today. The National Academy of Science’s 2001 Decadal Survey for Astronomy deemed it a top priority mission. “Persistence pays,” says Stern.

Thankfully the probe was fully government funded and had already reached Jupiter before Pluto got belittled and demoted in a raucous debate at the International Astronomical Union assembly in 2006. Stern is nonplussed, “Pluto has all the hallmarks of planethood.”

Now the grand piano-sized probe has covered 82 percent of its sprint to the Pluto system. One year from now it will be crossing Neptune’s orbit. Pluto is already within range for New Horizons’ cameras to resolve its binary companion, Charon.

“The long cruise is almost over … we feel a lot more momentum now,” says Stern. “There is a lot more to do than we ever thought.”

Image credit: JHUAPL/SwRI, NASA

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