by Alan Stern
Rob Staehle (in front) and members of the Pluto Fast Flyby (PFF) design team, ca. 1992, at Jet Propulsion Laboratory (JPL) with a PFF mock-up. (NASA)
The “Pluto Not Yet Explored” stamp issued by the U.S. Postal Service in 1991, which was issued along with a set that showed pictures of the other planets. Rob Staehle and Stacy Weinstein took this stamp as a dare.
New Horizons Mission Systems Engineer Chris Hersman with Alan Stern, taken near APL a few days before the flyby. (Alan Stern)
The new U.S. Postal Service Pluto stamp set issued in 2016 after New Horizons had explored the planet.
The external systems and scientific instruments aboard New Horizons. (NASA)
New Horizons during final assembly at its Florida launch site. (NASA)
The augmented Atlas V rocket for New Horizons, showing all the stages and components described in the text, with a tiny New Horizons seen to scale at upper right. (NASA)
New Horizons mission leader Alan Stern in front of the spacecraft after its nuclear power generator was fueled on Friday, January 13, 2006. This is the last known picture of New Horizons; after this, the open hatch was closed for launch. (NASA)
Some of the New Horizons science team in front of their Atlas launcher about one week before it left our planet. (NASA)
The route of flight of New Horizons across the solar system to Pluto. (JHUAPL)
The container, attached to New Horizons, carrying a portion of Clyde Tombaugh’s ashes. (NASA)
Patsy Tombaugh (Clyde Tombaugh’s widow) points to the sky shortly after seeing the launch of New Horizons on January 19, 2006. (© Michael Soluri/michaelsoluri.com)
New Horizons leaps skyward atop the Atlas V rocket on a pillar of flame, January 19, 2006. (NASA)
Moments after the launch of New Horizons, David Grinspoon (foreground, right) celebrates with members of the science team and their families. (Henry Throop)
New Horizons flight controllers and others watching (and cheering) the launch from inside the Mission Operations Center at APL. (NASA)
A typical working shift at the New Horizons Mission Operations Center at APL during the nine-year flight to Pluto. (NASA)
The route of flight of New Horizons as it sliced through the Pluto system on July 14, 2015. Times shown along the trajectory are Universal Time. (JHUAPL)
The special issue of Science magazine with results from the New Horizons flyby of Jupiter. (Reprinted with permission from AAAS)
The flyby-day countdown to the moment of Pluto flyby at APL on July 14, 2015. The clock in the background shows minus 4 seconds. (NASA/Bill Ingalls)
At the Pluto encounter, Alan, APL Director Ralph Semmel (center), and team members Leslie Young and Will Grundy (with Clyde Tombaugh’s daughter, Annette, and son-in-law, Wilbur) hold up a just-cancelled version of the “Pluto Not Yet Explored” postage stamp. (NASA/Bill Ingalls)
Engineers burning the launch-abort safety plan at the post–launch day party in Florida. (Morgaine McKibben)
Burning the emergency communications plan at the team party the night of closest approach to Pluto. (Henry Throop)
New Horizons science team members react to their first look at the “fail safe” image of Pluto, early morning on July 14, 2015. (© Michael Soluri/michaelsoluri.com)
Team members gathered around John Spencer’s laptop at APL, gazing at the first hi-resolution image of Pluto. Back row: Jeff Moore, Randy Gladstone, Ron Cohen, Andy Chaikin, Bill McKinnon, Maria Stothoff. Front row: Laurie Cantillo, John Spencer, Alan Stern, Will Grundy, Steve Maran. (NASA/Bill Ingalls)
What team members were looking at on John Spencer’s computer: the first hi-resolution image shows water ice mountains and a possible ice volcano on Pluto. (NASA)
Members of the rock band Styx at APL during the flyby with Mark Showalter, discoverer of Pluto’s moon Styx. (NASA/Joel Kowsky)
Scientists and NASA spokesperson Dwayne Brown (in front) celebrate at the moment when the crucial “phone home” signal was received from the spacecraft. (Henry Throop)
Alice Bowman and Alan Stern embrace moments after the “phone home” signal from New Horizons revealed that their spacecraft had survived and succeeded at Pluto. (NASA TV)
Queen guitarist and astrophysicist Brian May with team members at the Pluto encounter. (© Michael Soluri/michaelsoluri.com)
Queen guitarist and astrophysicist Brian May and science team member Leslie Young look at a brand-new stereogram of Pluto, which May put together at the flyby. (Henry Throop)
Senator Barbara Mikulski, whose support was pivotal in making New Horizons a reality, speaks at APL during the encounter. (NASA/Bill Ingalls)
The victorious New Horizons core mission team gather at APL two weeks after flyby. (© Michael Soluri/michaelsoluri.com)
Press conference at APL on July 15, 2017. From left: Dwayne Brown (NASA), Alan Stern, Hal Weaver, Will Grundy, Cathy Olkin, John Spencer. (NASA/Bill Ingalls)
The special issue of Science magazine, with first results from the New Horizons flyby of the Pluto system, published just ninety-four days after flyby. (Reprinted with permission from AAAS)
(true color): The “fail safe” image of Pluto, the last and most detailed image taken on July 13, 2015, the day before closest approach. (NASA)
Fifteen minutes after closest approach, New Horizons shot this dramatic image showing a complex surface with icy mountains and smooth plains, and layers of haze over a crescent Pluto. (NASA)
This image shows the wide range of surface ages and terrain types on Pluto, from dark, ancient heavily cratered terrain at the bottom to bright, young, crater fields of nitrogen ice at the top. (NASA)
A look back toward Pluto as New Horizons receded outward from the Sun revealed a stunning ring of blue sky surrounding the planet, just like Earth. (NASA)
New Horizons captured this high-resolution enhanced color view of Pluto’s largest moon Charon just before closest approach on July 14, 2015. (NASA)
The mountainous shoreline of Sputnik Planitia shows peaks of water ice towering above flat plains of nitrogen ice with a surface texture that may indicate dune fields created in epochs of higher atmospheric pressure on Pluto. (NASA)
(color enhanced): The first set of feature names on Pluto proposed by the New Horizons Team and formally adopted. (NASA)
(color enhanced): The young surface of Sputnik Planum (the left side of the heart) shows a geological pattern of cells caused by the churning motion of convection in the nitrogen ice. (NASA)
(color enhanced): Mountains capped with bright methane snow in the Cthulhu region of Pluto. (NASA/Alex Parker)
(color enhanced): This enhanced color image of the area near Pluto’s north pole shows a region cut by deep canyons and valleys. (NASA)
Patterns on the edge of Sputnik Planum closely resemble large glaciers on Earth, indicating flowing nitrogen ice. (NASA)
The four small moons of Pluto, shown to scale with the crescent (below) of Pluto’s giant moon Charon. (NASA)
(color enhanced): A detailed New Horizons view of Sputnik Planum shows a wide variety of terrains, including smooth plains surrounded by sharp mountains, deep canyons, and dark, cratered areas. (NASA)
Single frame of a double planet. The highest resolution image from New Horizons, which captures both Charon (left) and Pluto (right) in enhanced color. Here the two bodies are realistically shown to scale with one another and also to scale with their true separation distance. (NASA)
This feature, about 30 kilometers (20 miles) long, is suspected to be a frozen lake of liquid nitrogen, hinting at a time in the past when surface pressure may have been much higher. (NASA)
(color enhanced): This high resolution image shows the Southeastern edge of Sputnik Planitia where it meets the very rugged darker terrain of Krun Macula (the westernmost of the “brass knuckles”), which rises a mile and a half above the plains. The bright surface of Sputnik is dotted with kilometer-scale pits probably caused by sublimation of nitrogen ice. (NASA/John Spencer, Paul Schenk, Pam Engebret
son)
(color enhanced): This strange “bladed terrain” stretching hundreds of miles on Pluto is seen nowhere else in the solar system. It is composed of jagged spires of methane ice an average of twelve hundred feet high. (NASA)
(color enhanced): Part of a high-resolution color mosaic of Pluto taken just before closest approach shows features as small as 270 yards across, from craters to faulted mountain blocks to the textured surface of Sputnik Planum. (NASA)
NOTES
1. DREAMS OF A GRAND TOUR
1. Clyde Tombaugh and Patrick Moore, Out of the Darkness (Harrisburg, PA: Stackpole Books, 1980).
2. Clyde Tombaugh and Patrick Moore, Out of the Darkness (Harrisburg, PA: Stackpole Books, 1980).
3. Tombaugh came after William Herschel, who discovered Uranus in 1781, and Johann Galle, who found Neptune in 1846. Galle often shares credit with Urbain Le Verrier, who predicted Neptune’s position exactly based on calculations made from irregularities in the orbit of Uranus, and so told Galle where to look.
2. THE PLUTO UNDERGROUND
1. The classic paper reporting on this discovery and making other clever inferences about size and reflectivity was “Pluto: Evidence for Methane Frost,” written by Dale Cruikshank, Carl Pilcher, and David Morrison, published in Science on November 19, 1976.
2. We should point out that the first analysis of the discovery observations was done by Robert Harrington and James Christy. It was Christy who first noticed the “bumps” and inferred the existence of the moon, but in the scientific literature Christy and Harrington rightfully share credit for the discovery.
4. THE UNDEAD
1. Sublimation is like evaporation, which occurs when a liquid is heated and becomes a gas; when an ice becomes a gas the same way, with no liquid phase in between, scientists term it sublimation.
13 ON APPROACH
1. The video of this event can be seen at https://www.youtube.com/watch?v=DaUhaVUN3Yc
2. The lyrics are easy to find online and if you read the full song you’ll see what we mean.
APPENDIX
THE TOP TEN SCIENCE DISCOVERIES FROM THE NEW HORIZONS EXPLORATION OF THE PLUTO SYSTEM
Research papers describing and analyzing scientific results from the New Horizons flyby of Pluto, Charon, and Pluto’s other moons already fill volumes in peer-reviewed journals of planetary science. And there is sure to be much more scientific yield to come over the years, as understanding of the results grows and is integrated into our overall understanding of the origin and evolution of planets. What follows here (not in any ranked order) is a brief recapitulation of the ten most important discoveries made by the New Horizons mission, as named by NASA and the New Horizons team in 2016. For each of these “top ten” discoveries, we provide a little elaboration.
THE SHEER COMPLEXITY OF PLUTO
The diversity of phenomena seen on Pluto was far beyond what anyone, even New Horizons team members, expected to find on such a small planet so cold and far from the Sun. Ground fogs, high-altitude hazes, possible clouds, canyons, towering mountains, faults, polar caps, apparent dune fields, suspected ice volcanoes, glaciers, evidence for flowing (and even standing) liquids in the past, and more. This little red planet perched 3 billion miles away in the Kuiper Belt packed more punch than any other known small world explored, and indeed more punch than many much larger worlds. The variety of terrains, its complex interactions between the surface and the atmosphere, and the wide range of surface ages even prompted the New Horizons team to adopt the slogan “Pluto is the new Mars.”
THE STUNNING DEGREE OF LONG-TERM AND CONTINUING ACTIVITY ACROSS PLUTO’S SURFACE
There were many reasons that some expected Pluto to be a relatively dead world, geologically speaking. After all, it is such a small planet, and it lacks the tidal heat sources that being in a giant planet’s satellite system would provide. Also, it is far from the Sun and solar heating is weak. So, by the conventional wisdom born of exploring the rest of the solar system, Pluto should have been largely or even completely geologically inactive for eons. But the conventional wisdom was seriously wrong. New Horizons found a wide range of surface ages, ranging from ancient and heavily cratered to completely fresh-looking areas with no craters at all—meaning that Pluto has been geologically active throughout its 4-billion-year history. In fact, Pluto has been alive and kicking throughout history, and is even today. Why that is so is the subject of intense scientific debate and modeling, and it portends that we can expect more surprises when other small planets in the Kuiper Belt are explored.
THE VAST, 1,000-KILOMETER-WIDE SPUTNIK PLANITIA NITROGEN GLACIER
Among the varied and active terrains discovered on Pluto, probably the most remarkable of all is the vast ice field of Sputnik Planitia, which revealed itself to be actively convecting, churning like a slow-motion pan of sauce cooking on a stove. Sputnik Planitia is now understood to be a vast and deep layer of nitrogen ice, spiked with methane and carbon monoxide, resting within a huge bowl-shaped depression, which is most likely an ancient impact basin. Nitrogen glaciers feed into it from the mountainous areas around its periphery. Giant water-ice icebergs float along its margins. In a way, Sputnik Planitia is like a frozen ocean of nitrogen on Pluto’s surface. Sputnik Planitia—the western lobe of Pluto’s heart—dominates every view of the flyby hemisphere. But it also seems to play a deeper role within the strangely coupled geology and meteorology of Pluto. It is a huge reservoir of nitrogen, the primary substance that is transported through the atmosphere and onto and across Pluto’s icy, cold surface. Depending on the season and climate phase that Pluto is in, the amount of nitrogen fluctuates dramatically between atmosphere and surface. So Sputnik Planitia may at times be deeply eroded and at other times be recharged with even more nitrogen—and there is evidence for this in the currently empty glacial channels in the surrounding highlands. Nothing like Sputnik Planitia exists anywhere else in the solar system.
THE DISCOVERY OF EXTENSIVE, WELL-ORGANIZED ATMOSPHERIC HAZES
The spectacular “look back” images of Pluto, illuminated by the Sun from behind the planet as New Horizons was racing away after the flyby, vividly show Pluto’s beautiful blue atmosphere. But these dramatic images also reveal dozens of delicate layers of atmospheric haze hanging in Pluto’s cold nitrogen air. These extensive hazes extend up at least 300 miles above Pluto’s surface and are organized in concentric layers that stretch across many hundreds of miles of varied surface terrains. These hazes result from complex chemistry in Pluto’s atmosphere, somewhat like the organic hazes seen in the atmosphere of Saturn’s moon Titan. As on Titan, the methane in the air reacts with sunlight to make complex organic molecules which eventually rain out onto Pluto’s surface, giving it its ruddy color.
A DRAMATICALLY LOWER ATMOSPHERIC ESCAPE RATE COMPARED TO EXPECTATIONS
All planets with atmospheres are constantly losing some amount of gas into space. Because Pluto is such a small planet, with low gravity and a low escape velocity, it was expected to be losing its methane and nitrogen atmosphere at a high rate. But one unanticipated and highly surprising result of New Horizons’ measurements is that the loss rate of nitrogen was much slower than models had predicted—in fact, more than ten thousand times slower! The reason seems to be that the upper atmosphere is much colder than was expected. This means that the molecules are moving with lower velocities, so fewer of them are traveling fast enough to escape. The reason why the atmosphere is so cold is a mystery that has not been solved.
EVIDENCE FOR STRONG CHANGES IN ATMOSPHERIC PRESSURE AND THE PAST PRESENCE OF RUNNING OR STANDING LIQUID VOLATILES ON PLUTO’S SURFACE
We know that Pluto’s atmospheric pressure varies exponentially with surface temperature. We also know that its atmospheric pressure should also change dramatically over time as it goes through climate cycles of millions of years and its orbit and spin slowly wobble, changing the angles and amounts of sunlight heating different parts of its surface. This
was expected before the flyby, but New Horizons found several kinds of forensic evidence for much higher surface pressures in the past. These include areas of “washboard terrain,” dunes, channels that might have been cut by flowing liquids, and even one distinct feature that looks like a frozen lake suspended in a mountain valley.