by Jim Bell
Voyager imaging team liaison Candy Hansen was involved in the planning of the mosaic, and she recalls that even after the NASA HQ directive, there was lingering disdain from some of the project leadership. “Some of the science management was doing the equivalent of holding their noses,” she recounted to me, “and so at our sequence kickoff meeting they made me—essentially acting as a sequence engineer—make the presentation, rather than the usual process of asking a science team lead. But that was my chance to take the podium and sermonize about what a fabulous opportunity this was and how profound the images would be—hah!” Ed’s and Candy’s support and Carl Sagan’s enthusiasm made a difference, and with the blessing of NASA’s top brass, they made it happen.
Even twenty-five years later the resulting mosaic is mesmerizing. Starting out at Neptune and working its way inward in case the cameras got damaged, Voyager was commanded to snap photos of one planet after another. When the camera was pointed toward the sun to try to photograph the inner planets Mercury through Mars—all bunched up close to the blinding glare of our parent star—there was some saturation of the images, but the camera was not fried. “In order to minimize the glare from the sun while trying to image the inner planets,” Randii Wessen told me, “Voyager 1 was commanded in such a way that its high-gain antenna blocked some of the glare for the cameras—like a person at the beach, placing their hand in front of their face to block the sun.” It was Candy Hansen’s job to look through the images as they came in, to make sure that everything had gone OK with the observation. Just like Rich Terrile, by this point in the mission, she knew in great detail what stars look like in Voyager images, and she also knew every little artifact and blemish of the Voyager cameras by heart. So even though the photos were mostly empty black space, she was still able to quickly zoom in and isolate the stars and the blemishes from . . . something else. “Found Neptune—check. Saturn—check. No Mars—as expected, the crescent view would be too dim,” she recollects, remembering her first views of the solar-system portrait images. “I finally got to the photo that was pointed at the Earth. At first I couldn’t find it—there was a lot of scattered light in the image—but then I spotted it, in a ray of that scattered light.” It turned out that one swath of yellowish scattered sunlight had passed right through Voyager’s photo of the Earth.
Our world was “a pale blue dot . . . a mote of dust suspended in a sunbeam,” Carl Sagan poetically intoned. “As I am sitting here recalling that experience,” Candy wrote to me, “there are chills going down my spine, just like that day when I saw our little planet from a vantage point so far away.”
Once again the citizens of Planet Earth, then numbering about 5 billion, bore witness to the next great paradigm-shifting change in perspective. This time it was not just off-world, not from just beyond our backyard, but a vista from out there, looking down from near the edge of the sun’s realm to behold the entire solar system. As was his way, Sagan challenged and inspired us to internalize this new perspective, and to use it to guide our paths forward. “It has been said that astronomy is a humbling and character-building experience,” he wrote. “There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we’ve ever known.”
The powerful message and literal imagery of Voyager’s Pale Blue Dot photo led to a string of selfies by subsequent planetary exploration missions. Many of the people who have gone on to become leaders in NASA’s robotic planetary exploration missions were either trained by, worked with, or became disciples of Carl Sagan, and as such they are the kind of people who understand the enormous symbolic, inspirational, and educational value of photographing our home world from space. Some of my favorite planetary selfies include hypnotizing movies of the Earth spinning gracefully on its axis while the Galileo and MESSENGER spacecraft made their gravity-assist flybys of our planet on their way to Jupiter and Mercury, respectively; a glorious HD movie of a full Earth rising over the horizon of the moon from the Japanese Kaguya lunar orbiter; and absolutely stunning photos of our distant, faint planet nestled against the rings of Saturn, taken while the Cassini orbiter was passing through Saturn’s shadow.
For the most recent one of these spectacular Cassini photos, NASA and the Cassini imaging team asked people to go outside at a particular time of day (or night) on July 19, 2013, look up, and smile and wave as the Cassini camera, from nearly a billion miles away, took an image of the (now) 7 billion of us all crammed into a single pixel. Cassini imaging team leader Carolyn Porco, who worked with Sagan on the Voyager imaging team, wrote on her Facebook page, “After much work, the mosaic that marks that moment the inhabitants of Earth looked up and smiled at the sheer joy of being alive is finally here. In its combination of beauty and meaning, it is perhaps the most unusual image ever taken in the history of the space program.”
There are many more spacecraft self-portraits of our home world—it seems like we can’t stop wanting to look at ourselves from new perspectives. My favorite is one that I played a role in helping to take (I guess selfies are like that). I count myself among Carl Sagan’s disciples, having been influenced at an early age by his Cosmos TV show, his books and magazine articles, and by the immense good fortune of getting to be a colleague of his for a short time at Cornell University. When I had the chance to help lead a robotic planetary-imaging investigation of my own as the lead scientist for the Pancam color stereo cameras on the Mars exploration rovers Spirit and Opportunity, I was looking for chances to take photographs that would capture some of the same aesthetic, artistic, and inspirational appeal as the Pale Blue Dot. Happily, my friend and rover team leader Steve Squyres, another Sagan disciple, was a kindred spirit.
One such chance came in March 2004. Both Spirit and Opportunity are solar-powered, meaning that the power available to drive or take pictures is often dictated by the amount of dust in the atmosphere or by whether the solar panels are dusty or clean. Rover team colleagues Mark Lemmon, Mike Wolff, and I were all originally trained as astronomers, and so we’d been looking for ways to take some astronomical photographs—that is, of stars or other celestial objects, with the rover cameras. About 63 Martian days or “sols” into Spirit’s mission in Gusev Crater, we found ourselves with an abundance of solar power, and thus able to power the rover and camera heaters that would let us take images in the extra-frigid twilight hours before sunrise or after sunset. We knew that Earth was a “morning star” as viewed from Mars at that time (just like Venus and Mercury are sometimes visible as “stars” during twilight from Earth), and we had the power—could we spot ourselves in the Martian sky? Twilight is bright because of all the high-altitude dust in Mars’s atmosphere, so we weren’t sure if we would be able to see the Earth against the five a.m. predawn sky. But the next day when the images were beamed back—voilà, there we were! We’d taken the first photo of our home world from the surface of another planet.
We one-upped our feat by taking the first Earthrise movie from the surface of another planet in late 2005, using the cameras on the Opportunity rover to snap the Earth and Jupiter rising gracefully in the predawn sky above the dunes of Meridiani Planum. We’d worked up valid scientific justifications for taking all these Earthrise photos—for example, the need to measure the thickness of dust or water ice clouds/fog in the early-morning Mars atmosphere. But in the end, for me, it was just the sheer thrill of being able to look up, to glance back—like the Voyagers—to take a historic and introspective photo from a rare perspective, and to ponder what it means to explore a world where we are the aliens, experiencing it vicariously through the eyes of a robot.
9
The Edge of Interstellar Space
WHERE DOES THE solar system end? When I was in grade school, there were nine planets, and once you got out to Pluto, that was it. For science fair one year I spray-painted th
e inside of a box black, speckled it with white paint for stars, tipped it on its side, taped a drawing of the sun on one end, poked nine holes in the top, and hung cutouts of the planets from pieces of yarn. Solar system in a box! Scientists back then didn’t know a whole lot more.
The Voyagers and other robotic missions since have revealed the incredible diversity of worlds within our solar system, including what are essentially mini solar systems around Jupiter and Saturn. Some moons, like Ganymede and Titan, are larger than the planet Mercury. Those moons, plus Europa and Enceladus and others, may harbor subsurface oceans. Earth is not the most volcanically active place in the solar system—Io is. Some large asteroids, such as Vesta and Ceres, appear to have had geologic histories as active as any planet’s. And—among the most surprising discoveries—the solar system does not end at Pluto. Starting in 1992, astronomers have been discovering more and more relatively large, Pluto-sized planetary bodies lurking in the Kuiper Belt beyond Neptune. Almost 1,300 KBOs are now known, and the census is far from complete. The largest one yet found is called Eris, and it’s a planetary body almost 1,500 miles wide (larger than Pluto, and about one-quarter of the mass of the Earth) with a moon of its own, called Dysnomia. Eris orbits, on average, almost twice as far from the sun as Pluto—the solar system doubled in size again when Eris was discovered in 2005. Astronomers estimate that there could be 100,000 or more KBOs larger than a hundred miles or so across, and perhaps hundreds of millions of them that are more comet-sized, only a few miles across.
The glut of Pluto-sized bodies being recently discovered beyond Neptune is what gave Pluto itself all that trouble, of course. Rather than accepting the fact that there are indeed many hundreds of newly discovered planets out there, and countless more still to be found, some astronomers chose to be “splitters” instead of “lumpers.” In 2006, after some contentious debate, the International Astronomical Union (IAU)—the world’s governing body tasked with giving planets and moons and asteroids and comets (as well as craters and mountains on those worlds) their names—decided to strip Pluto, and other places like it, of their “planet” status. Instead, such worlds were demoted to “dwarf planet” status, and the number of true planets in the solar system was decreased to eight, throwing textbooks and elementary school science-fair projects into chaos and disarray.
I’m a card-carrying member of the IAU and generally proud and supportive of the work that my colleagues in that organization do on behalf of astronomy and planetary science worldwide. But this time, I think they got it wrong. Personally, I judge a planet (like a person) on what’s on the inside, rather than what it looks like or where it’s been. Mercury is a planet because it has had a complex geologic history, including formation of a core, mantle, and crust, and the eruption of volcanoes on its surface, all fueled by substantial internal heat. It happens to be in orbit around the sun. Io, comparable in size, has had a similarly complex surface and interior geologic history. It happens to be in orbit around Jupiter, but it is still the same kind of object. So I call Io a planet. As well as Europa, and Ganymede, and Callisto. Plus Titan, Triton, Enceladus, Dione, Rhea, Tethys, Ariel, Ceres, Vesta, Eris, our own moon, and lots more. And Pluto—for God’s sake, it’s got an atmosphere and five moons of its own. If that’s not a planet, I don’t know what is. By my reckoning (and I’m a bit of a weirdo among my astronomy friends for this), our solar system has about thirty-five known planets so far, and it’s likely that dozens more will be discovered over the coming decades. Let’s celebrate those numbers and the diversity of planetary characteristics within our cosmic neighborhood rather than splitting them up into categories implying substandard status, such as “moon” or “dwarf planet.” I’m a lumper rather than a splitter.
SOLAR WIND
Although astronomers and planetary scientists don’t yet know exactly how far toward the nearest stars the sun’s gravitational influence extends (it’s probably somewhere near a half to two-thirds of the way), they have been expecting over the past decade or so that far-flung spacecraft like Voyager should soon be able to find the edge of the sun’s nongravitational influence on the solar system. The sun produces energy by the conversion of four hydrogen atoms into one helium atom deep in its interior, at super-high pressures and at temperatures of millions of degrees. The conversion releases a tiny bit of energy, in the form of photons and other subatomic particles like protons and electrons, that bounce around inside the sun and eventually make their way out. The sunlight—photons—that warms our faces on a sunny afternoon was created, on average, deep inside the sun, maybe 50,000 years ago or more. The stream of protons and electrons coming off the sun every second creates a flow of charged particles called the solar wind. The solar wind creates a giant spherical “bubble” around the sun in interstellar space, known as the heliosphere. The heliosphere extends far beyond the orbit of Neptune, until it becomes so diffuse and weak that it merges into the background of rarefied hydrogen and helium gas that permeates the space between the stars—the interstellar medium. The sun and every other star reside inside their own such cocoons, blowing bubbles in the interstellar medium from their own solar, or stellar, winds. Like all bubbles, there must be an edge, a boundary between inside and outside of that bubble. Inside the bubble is the solar wind, outside the bubble is the interstellar wind. Finding that edge, then, and going beyond it, provides a way to explore truly interstellar space.
In some grand and philosophical way, as Voyager plowed on toward the boundary of this bubble, it became important to be able to determine the precise moment in time when we could say without question that we had left the confines of our solar system and we were now “outside,” in interstellar space, the space between the stars.
While the vast distances traversed by Voyager to date are nearly incomprehensible on any human scale, it is perhaps even more difficult to grasp the lonely future of our spacecraft as it travels through interstellar space, heading off into infinity. Yet despite the cosmic emptiness that we are facing, the dream of the Golden Record remains alive in our hearts and minds. We can imagine a time in this incomprehensible future when some vastly superior beings, traveling these distances with the ease of today’s intercontinental airline flights, would receive an earnest message from an Earth long gone but preserved in small part aboard our timeless Voyager emissaries.
But before it could be declared that Voyager had crossed into interstellar space, particles and fields emanating from our sun as well as from outside of our solar bubble would have to be tracked so we could witness the changes directly. Solar astronomers have discovered that, like the winds of our own planet, solar wind streams are in constant motion, acting out their own solar weather systems. Sometimes the solar wind is gentle and flows smoothly, like a breezy day. The slow solar wind (where “slow” is only 900,000 miles per hour) is an extension of particles that were accelerated through the sun’s upper atmosphere—the expanding “corona” of the sun. And the fast solar wind (at more than 1.7 million miles per hour) appears to stream off the sun’s visible surface (known as the photosphere). A few billion pounds of material streams off the sun every second, but the mass lost over time has still been only a minuscule fraction of the overall mass of the sun. Although invisible to our eyes, we can see evidence for the solar wind in the beautiful ion tails of comets, which always point downstream in the solar wind, away from the sun. These somewhat steady breezes are interrupted by occasional gale-force storms of particles called coronal mass ejections—the giant, looping arcs of hot plasma gas that launch off the surface of the sun and send electromagnetic shock waves and sprays of ionizing radiation outward toward the planets. Sometimes these waves and radiation produce glorious auroral displays in Earth’s polar regions, and sometimes they also wreak havoc with electronics in orbiting satellites and surface power grids. The sun has weather, and it’s important for our modern, electronic civilization to pay attention to it.
That’s where space physicists like Ed Stone come
in. Ed has spent his career working to understand high-energy particles from the sun and other cosmic sources, how they interact with the magnetic fields of the sun and the planets, and what they can tell us about how the planets, the sun, and other stars work. Ed’s Cosmic Ray Subsystem (CRS) instrument is designed specifically to measure the energies and intensities of high-energy particles from the sun and other sources in the galaxy, in order to map out the sun’s magnetic field as a function of distance, and to understand the effects of that field on the planets. It’s an example of “squiggly line science” (you know, like the medical devices they use to monitor your vital functions, or the seismometer plots you can see monitoring for earthquakes in some science museums). Ed’s instrument generates streams of data that most often appear on plots and graphs, rather than images. But scientists like Ed read them with ease. If you want to study the geological diversity of a planet’s surface, pictures are the way to go. But if your aim is to understand the energies and densities of subatomic particles in space, a picture simply won’t do. Other sorts of measurements are necessary. And it’s a little-known fact that some of the most important discoveries from missions like Voyager, the Mars rovers, and many other missions come not necessarily from the pictures but from the investigations that produce squiggly line science.
After the Neptune flyby, and the success of the Pale Blue Dot photograph, Voyager’s focus was shifted almost entirely toward the fields and particles investigations that helped to characterize the interactions of the solar wind with the magnetic fields of the giant planets. With our beloved planets fading into the distance, these became the only instruments with something left to measure. And not just something, but something profound. Where does the solar wind stop blowing? Where does the sun’s influence give way to the different kinds of fields and particles that infuse the spaces between the stars? Finding this edge, the edge of the heliosphere’s bubble known as the heliopause—and studying for the first time the nature of interstellar space—became Voyager’s prime directive.
