Category Dreams of Other Worlds

As we peer through the plane of the Milky Way, the center of our galaxy is completely hidden due to interstellar dust.14 The galactic center is 28,000 light-years away and only one in a trillion optical photons can escape—it’s like trying to look through a closed door. Unless this cold and grainy dust is accounted for, intrinsic stellar properties can be badly misjudged, a point first noted by Robert Trumpler in 1930. Star formation takes place in chaotic, dense, swirling regions of gas and dust where the concentration of dust is so high the action is mostly hidden from view. There’s a dramatic difference, for instance, between the optical and infrared views of the Orion nebula. In the optical view the bright nebula contains a dark region that appears to be devoid of stars, while in the in­frared view that penetrates the dust a dense star cluster shows up in that dark region.15 The youngest stars are invisible even to the most powerful optical telescope. However, with Spitzer’s ability to penetrate dust, the broad sweep of star formation in the Orion Nebula (M42) and the Omega Nebula (M17) and throughout the Milky Way has been revealed (plate 15).

Spitzer’s detectors are so sensitive that the telescope has lifted the veil on some of the earliest star formation in the universe. Black holes that inhabit the centers of all galaxies are generally starved of fuel and are quiet. But it was not always this way. When the universe was 7 percent of its present age, nearly 13 billion years ago, it was seven times hotter and 350 times denser than it is now. There was a furious phase of star formation and galactic construction in the first 3 billion years after the big bang, during which many of the largest galaxies were put together. Spitzer has two unique advantages for studying the distant universe. The first is associated with the redshift caused by expanding space. By the time the light from a galaxy 13 billion light-years away reaches us, its light has been stretched and mostly shifted to the infrared. The second advantage is that light from star formation and black hole growth shrouded in dust in the early universe is absorbed and reradiated at longer wavelengths where Spitzer works best. As a result, Spitzer has provided a more reliable measure of the star for­mation history of the entire universe.16 The most distant, infrared- luminous galaxies have a rate of star formation thousands of times larger than the Milky Way. Such a rate was sustained by the ready availability of gas in the denser, primordial universe. Gas was con­sumed so quickly that the fireworks didn’t last long.

One provocative and tantalizing study claimed to use Spitzer to reveal the first objects produced by gravity in the universe, at the end of the cosmic “dark ages.” Alexander Kashlinsky and his group studied the diffuse infrared background in ultra-deep Spitzer images.17 They first carefully removed light from foreground stars and nearby galaxies, leaving only the most ancient diffuse radia­tion. They then analyzed the fluctuations of the faint infrared emis-

sion that remained and matched them to simulations of clustering in the early universe (figure 9.2). “Imagine trying to see fireworks at night from across a crowded city,” explained Kashlinsky. “If you could turn off the city lights, you might get a glimpse of the fire­works. We have shut down the lights of the city to see the outlines of its first fireworks.”18 It’s unclear whether the faint infrared glow in Spitzer’s ultra-deep images actually comes from a first genera­tion of huge stars hundreds of times the mass of the Sun, or from the first wave of supermassive black holes. Either way, this truly ancient radiation carries information about the universe when it was a fraction of its present age.

Closer to home, Spitzer conducted a sky survey of our own gal­axy dubbed GLIMPSE (Galactic Legacy Infrared Mid-Plane Sur­vey Extraordinaire) by making images of swaths spanning 65 de­grees on either side of the galactic center and one degree on either side of the galactic plane.19 A hundred times the sensitivity and ten times the resolution of any previous survey, the GLIMPSE mosaic represents 110,000 different pointings of Spitzer and 840,000 in­dividual images, with the postage stamps painstakingly stitched together to make a seamless map. Spitzer’s GLIMPSE photos are as scientifically meaningful as they are aesthetically stunning. But since Spitzer takes data at invisible wavelengths, there’s no direct correspondence between its images and the colors of the rainbow. The filters Spitzer used for the GLIMPSE mosaic are sensitive to thermal emission from dust at temperatures from 30 K to 1600 K.

In the maps, blue represents radiation typically coming from older stars. Green colors trace stellar embryos by the emission of complex molecules called polycyclic aromatic hydrocarbons. On Earth, these molecules are found in car exhaust and barbeque grills, or anywhere carbon is incompletely burned. Red colors depict ra­diation from the youngest stars, still in their cool placental dust, as well as diffuse emission from graphite grains similar to tiny pieces of pencil lead. While no simple coding scheme can convey all the subtleties of the astrophysics of star formation, this three-color scheme is useful for researchers and armchair connoisseurs alike. Polycyclic aromatic hydrocarbons are too thinly dispersed for any­one to be able to savor their sooty aroma, but they’re excited by ultraviolet radiation so they map out newborn, massive stars.20 New stars reside in bubbles or on the edge of green ridges carved out by stellar winds. Many of these stars die as supernovae, which further excite the diffuse gas. The young stars appear as yellow or red dots, some in tight knots where they formed together; others are more randomly sprinkled throughout the disk of the galaxy. Older stars register as blue in the combined image and they’re so abundant that they merge into a blue-white haze.

The GLIMPSE survey has contributed to new understanding of the morphology of our Milky Way galaxy. Teams working on the GLIMPSE images cataloged over 100 million stars, and used them to trace out the Milky Way’s spiral arms and stellar bar. Spitzer project scientist Michael Werner reports that the survey “strongly confirmed previous evidence that the Milky Way is a barred spi­ral galaxy.”21 Prior to this survey and that of Hipparcos, it was believed that our galaxy had four spiral arms: Norma, Perseus, Sagittarius, and Scutum-Centaurus. Adding its data to the Hip – parcos survey, GLIMPSE confirms that the Milky Way has only two major arms, Scutum-Centaurus and Perseus that trail from the ends of the stellar bar at the galactic core. Additionally, as a result of GLIMPSE, astronomers have determined a new obscuration relationship for the galaxy, allowing more accurate estimates of distance to stars even in the presence of dust. Most fundamentally, they’ve measured the “pulse” of the galaxy by counting how fast new stars are forming. Our galaxy generates, on average, one star like the Sun per year, using mostly recycled gas from the death of older stars. That doesn’t sound like much for a system with several hundred billion stars, but as the Milky Way settles into the steady rhythm of middle age its wild times of forming thousands of stars a year are long over.

Given the volume of GLIMPSE data, the public has been re­cruited via the website Zooniverse. org to assist astronomers in analyzing Spitzer’s views of our galaxy. Launched in 2007, Zooni – verse developed the Milky Way Project, by which citizen scientists scour Spitzer’s GLIMPSE images to identify gaseous nebula and newly forming stars.22 Zooniverse offers a variety of mission data sets to volunteers who assist in identifying new solar system for­mation, track ancient weather patterns, report solar storms, and hunt for massive explosions of stars called supernovae. For the Milky Way Project, volunteers use simple online tools to circle huge bubbles of gas and dust in which stars are currently form­ing. The GLIMPSE survey imaged thousands of these globe-l ike bubbles where high-energy radiation from newborn stars collides with surrounding dust clouds. As the Milky Way Project site ex­plains: “Right now our best understanding is that these are regions around young massive stars that are so bright that their light has caused a shock wave to affect the cloud around them and blown a bubble which we can see in infrared light.”23 Identifying mas­sive bubbles in Spitzer images assists scientists in determining the mechanisms occurring in star birth.

Of course, it was always understood that the day would arrive when the last drop of Spitzer’s 350 liters of liquid helium would evaporate away. This occurred on May 15, 2009. Without liquid helium, the telescope’s long wavelength instruments have lost all their sensitivity. However, the two shortest wavelength channels of one of the cameras can operate at their design sensitivity, so Spitzer was approved for a “warm mission” phase where it is con­tinuing to do important science. Even in its “diminished” state, Spitzer is more sensitive than any ground-based telescope in the infrared. Also, “warm” is a relative term; touching any part of the spacecraft would give you a very serious cold-burn. At the balmier operating temperature of -242°C, Spitzer is continuing work with two channels of one instrument as part of its “warm mission.”24 Given that its near-infrared channels are able to do science from comets to cosmology, more than 10,000 hours of programs were approved in the first two years of the “warm mission.” Exoplanets are “hot” so some of the largest programs were to characterize exoplanets where our line of sight is aligned with the orbital plane of the exoplanet. As the planet passes behind its parent star, the combined infrared emission from the star plus the exoplanet dips, and measurements made at different wavelengths can give the size and temperature of the exoplanet. By the end of the mission, the number of exoplanets characterized will have increased from a dozen to 50 or 60 and we’ll have a much better understanding of the extraordinary diversity of other worlds in space. To set the stage for this exciting Spitzer science, we take a detour into the role of light and darkness in the evolution of life. We can only imagine the flora and fauna that might possibly exist on planets orbiting other stars, but life’s adaption to light here on Earth might offer some scant clues.

Someone who “missed” the late part of the twentieth century, perhaps by being in a coma or a deep sleep, or by being marooned on a desert island, would have many adjustments to make upon re­joining civilization. The largest would probably be the galloping progress in computers and telecommunications and information technology. But if their attention turned to astronomy, they would also be amazed by what had been learned in the interim. In the last third of the century, Mars turned from a pale red disk as seen through a telescope to a planet with ancient lake beds and subter­ranean glaciers. The outer Solar System went from being frigid and uninteresting real estate to being a place with as many as a dozen potentially habitable worlds. They would be greeted by a caval­cade of exoplanets, projecting to billions across the Milky Way galaxy. Their familiar view of the sky would now be augmented by images spanning the entire electromagnetic spectrum, revealing brown dwarfs and black holes and other exotic worlds. Finally, they would encounter a cosmic horizon, or limit to their vision, that had been pushed back to within an iota of the big bang, and they would be faced with the prospect that the visible universe might be one among many universes.

This book is a story of those discoveries, made by planetary probes and space missions over the past forty years. The word “world” means “age of man” in the old Germanic languages, and that proximate perspective took centuries to expand into a uni­verse filled with galaxies and stars and their attendant planets. The

missions at the heart of this narrative have not only transformed our view of the physical universe, they’ve also become embedded in culture and inspired the imagination—this book is also a story about that relationship. But people were dreaming of other worlds long before the space program and modern astronomy.

Almost nothing written by Anaxagoras has survived, so we can only imagine his dreams. He was born around 500 BC in Clazom – enae in Ionia, a bustling port city on the coast of present-day Tur­key. Before he moved to Athens and helped to make it the intellec­tual center of the ancient world, and long before he was sentenced to death for his heretical ideas, we can visualize him as an intense and austere young man. Anecdotes suggest someone who was far removed from the concerns of everyday life. He believed that the opportunity to understand the universe was the fundamental rea­son why it was better to be born than to not exist.1

Anaxagoras’ mind was crowded with ideas. Philosophy is based on abstraction—the power to manipulate concepts and retain as­pects of the physical world in your head. He believed that the Sun was a mass of fiery metal, that the Moon was made of rock like the Earth and did not emit its own light, and that the stars were fiery stones. He offered physical explanations for eclipses, for the solstices and the motions of the stars, and for the formation of comets. He thought the Milky Way represented the combined light of countless stars.2 We imagine him standing on the rocky Ionian shore at night, with starlight glittering on dark water, gazing up into the sky and sensing the vastness of the celestial vault. The dreams of such a powerful and original thinker were probably suf­fused with the imagery of other worlds.

This is speculation. As with most of the Greek philosophers, and especially the pre-Socratics, very little of their writing has come down to us unaltered. Typically, there are only isolated fragments and commentaries, sometimes by contemporaries and often writ­ten centuries later. Each historical interpreter added their own pre­dilections and biases; the result is a view of the original ideas seen through a gauzy veil.3 Modern scholars pore over the shards and often come up with strikingly different interpretations. Anaxagoras thought that the original state of the cosmos was undifferentiated, but contained all of its eventual constituents. The cosmos was not limited in extent and it was set in motion by the action of “mind.” Out of this swirling, rotating mixture the ingredients for material objects like the Earth, Sun, Moon, and planets separated. Although the nature of the animating agent is not clear from his writings, Anaxagoras was the first person to devise a purely mechanical and natural explanation for the cosmos, without any reference to gods or divine intervention. His theory sets no limit to the scale of this process, so there can be worlds within worlds, without end, either large or small.4 A case can be made that he believed that our world system is not unique, but is one of many formed out of the initial and limitless mass of ingredients.5

Radical ideas often come with a price. For daring to suggest that the Sun was larger than the Peloponnese peninsula, Anaxago­ras was charged with impiety.6 He avoided the death penalty by going into exile in Asia Minor, where he spent the remainder of his life. Pluralism—the idea of a multiplicity of worlds, including the possibility that some of them harbor life—had antecedents in work by Anaximander and Anaximenes, and in speculation passed down by the Pythagorean School. But Anaxagoras was the first to embed the idea in a sophisticated and fully fledged cosmology. By the time of the early atomists Leucippus and Democritus, plurality of worlds was a natural and inevitable consequence of their phys­ics. There were not just other worlds in space, but infinite worlds, some like this world and some utterly unlike it.7 It was a startling conjecture.

The next two thousand years saw the idea of the plurality of worlds ebb and flow, as different philosophical arguments were presented and were molded to accommodate Christian theology.8 The pluralist position was countered by the arguments of Plato, and particularly Aristotle, who held that the Earth was unique and so there could be no other system of worlds. European cultures were not alone in developing the idea of plurality of worlds. Baby­lonians held that the moving planets in the night sky were home to their gods. Hindu and Buddhist traditions assume a multiplicity of worlds with inhabiting intelligences. For example, in one myth the god Indra says, “I have spoken only of those worlds within this universe. But consider the myriad of universes that exist side by side, each with its own Indra and Brahma, and each with its evolv­ing and dissolving worlds.”9 In cultures around the world, dream­ers’ imaginations soared. The Roman poet Cicero and the historian Plutarch wrote about creatures that might live on the Moon, and in the second century CE, Lucian of Samosata wrote an extraordi­nary fantasy about an interplanetary romance. A True Story was intended to satirize the epic tales of Homer and other travelers, and it began with the advisory that his readers should not believe a single word of it. Lucian and his fellow travelers are deposited by a water spout onto the Moon, where they encounter a bizarre race of humans who ride on the backs of three-headed birds. The Sun, Moon, stars, and planets are locales with specific geographies, human inhabitants, and fantastical creatures. This singular work is considered a precursor of modern science fiction.10

For more than a millennium it was dangerous in Europe to es­pouse the idea of fully fledged worlds in space with life on them. Throughout medieval times, the Catholic Church considered it heresy. There was an obvious problem with this position: if God was really omnipotent, why would he create only one world? Thomas Aquinas resolved the issue by saying that although the Creator had the power to create infinite worlds, he had chosen not to do so, and this became official Catholic doctrine in a pro­nouncement of the Bishop of Paris in 1177. Nicolas of Cusa sorely tested the bounds of this doctrine. In 1440, he produced a book called Of Learned Ignorance where he proposed that men, ani­mals, and plants lived on the Sun, Moon, and stars.11 He further claimed that intelligent and enlightened creatures lived on the Sun while lunatics lived on the Moon. It’s said that friendship with the Pope shielded him from repercussions, and he went on to become a cardinal.

Giordano Bruno was less fortunate. The lapsed Dominican monk had deviated from Catholic orthodoxy in a number of ways, but his espousal of the Copernican system, which displaced the Earth from the center of the universe, brought him extra scrutiny. He believed that the stars were infinite in number, and that each hosted planets and living creatures.12 Bruno was incarcerated for seven years before his trial and was eventually convicted of her – esy.13 A statue in the Campo de’ Fiori in Rome marks the place where he was burned at the stake in 1600 as an “impenitent and pertinacious heretic.” Religion had cast an ominous shadow over the idea of the plurality of worlds.

The same year Bruno was put to death, a twenty-nine-year – old mathematician named Johannes Kepler, an assistant to Tycho Brahe, was working with data that would cement the Copernican model of the Solar System. As he published his work on planetary motion in 1609, he dusted off a student dissertation he had writ­ten sixteen years earlier, where he defended the Copernican idea by imagining how the Earth might look when viewed from the Moon. Kepler elaborated on his youthful paper and added a dream nar­rative to turn it into a sophisticated scientific fantasy: Somnium.14 Kepler was inspired by Lucian and Plutarch’s earlier work, but un­like them, and unlike the mystic Bruno, he was a rational scientist who wanted to realistically envisage space travel and aliens. His narrative is rich with comments on the problems created by ac­celeration and varying gravity. The geography and geology of the Moon are realistically rendered. He even speculates on the effect of the physical environment on lunar creatures, foreshadowing Dar­win and Lyell.15 Kepler had every reason to take refuge in a dream. He was frail and bow-l egged, covered in boils, and was cursed with myopia severe enough that he would never see the celestial phenomena he enunciated so elegantly. Somnium was known to Jules Verne and H. G. Wells, and it’s a crucial step in the progres­sion toward rational speculation about other worlds.

The Copernican revolution was not a single event; it was a se­ries of realizations over a period of a century that the cozy idea of Earth as a singular place at the center of the universe was wrong. Displacing the Earth into motion around the Sun was the first wrenching step, but another was recognizing that the Earth was one of many worlds in space. The Copernican principle is more than just a cosmological model; it’s a statement that the Earth is not in any central or favored position in the universe. A heuristic that extends from the work of Copernicus is the principle of medi­ocrity, which goes much further by supposing that there’s nothing special or unusual about the situation of the Earth, or by exten­sion, the fact that humans exist on this planet. That is of course a central tenet of modern astrobiology, but four hundred years ago it was a radical idea.

The Scientific Revolution recast the debate over the plurality of worlds. Within months of Kepler’s dream piece, Galileo pointed his telescope at the Moon and affirmed it as a geological world, with topography similar in scale to the Earth. He also showed that Jupiter had orbiting moons and that the Milky Way resolved into points of light that seemed to be more distant versions of the bright stars.16 The word world was no longer confused with kos- mos; it meant a potentially life-bearing planet orbiting the Sun or, hypothetically, a distant star.17 Speculation about life on the Moon became routine, almost mundane. However, theology and phi­losophy still colored the debate in several ways. One theological concept was the principle of plenitude—everything within God’s power must have been realized, so inhabited worlds should be abundant. Another was the strong influence of teleology—purpose and direction in nature that implies a Creator, who would surely not have gone to the trouble of creating uninhabited worlds.18

For a long time, scientific arguments could do no more than sup­port the general plausibility of the plurality of worlds. Telescopes could easily track the motion of stars and planets, but gaining a physical understanding was much more challenging. The blurring effect of Earth’s atmosphere prevented astronomers from resolv­ing anything smaller than continent-sized surface features on any Solar System body other than the Moon. Even the nearest stars are a hundred thousand times farther from us than the size of the Solar System. In addition, planets do not emit their own light, so astronomers must gather the hundred million times dimmer light that they reflect from their parent stars. Three centuries of im­provements in telescope design after Galileo yielded only two new planets, a dozen or so moons, and no success in detecting worlds beyond the Solar System.

And so the dreamers held sway. Many of them were grounded in science so they advanced the Copernican idea that our situation in the universe was not special.19 One striking work from the be­ginning of the Age of Enlightenment was Conversations about the Plurality of Worlds by Bernard de Fontenelle, published in 1686.20 He wrote about intelligent beings inhabiting worlds beyond the Earth, and incorporated the biological argument that their char­acteristics would be shaped by their environment. Fontenelle also followed Galileo’s lead by writing in his native language, French, rather than the scholarly language of Latin, and he was forward­looking in having a female protagonist and explicitly addressing female readers.21 A much later high-water mark was On the Plu­rality of Habitable Worlds by Camille Flammarion, which reached a wide audience in 1862.22 By the early twentieth century, scientific speculations and fictional accounts of worlds beyond the Earth proliferated, but technology and research weren’t able to address such conjectures.23 There’s an unbroken thread between earliest Greek thinkers and more recent explorations of science fiction writers. Anaxagoras was a visionary, but it would probably have taken his breath away to know that one day we would actually visit other worlds.

IIIII

Isaac Newton’s Mathematical Principles of Natural Philosophy, a three-volume masterwork published in 1687, is a landmark in the history of science. Principia, as it is known, laid down the foun­dations of classical mechanics and gravitation.24 Tucked away in one of the volumes is the drawing of a cannonball being launched horizontally from a tall mountaintop. This “thought experiment” sustained the dreams of space travel for nearly three centuries. Oc­tober 4, 1957 was a pivotal moment in the history of the human race; on that day a metal sphere, no bigger than a beach ball and no heavier than an adult, was launched into orbit. The world was transfixed, and amateur radio operators monitored Sputnik’s steady “beep” for three weeks until its battery expired.25 Within two years the Soviets had crashed a probe into the Moon—the first manmade object to reach another world—and the Space Age was in full flight. Humans have never been any farther than the Moon but we’ve sent our robotic sentinels through most of the Solar Sys­tem and slightly beyond.

For the universe beyond our backyard in the Solar System, we have no direct evidence and we cannot gather and analyze physical samples. The data are limited to electromagnetic radiation. New­ton improved on Galileo’s simple spyglass with a design for a re­flecting telescope. All research telescopes are now reflectors. In un­derstanding distant worlds, the complement to direct exploration with spacecraft is remote sensing with telescopes. A succession of larger and larger telescopes over the past century have now ex­panded our horizons, and extended the Copernican revolution.26 We know that we orbit a middle-aged, middle-weight star, one of 400 billion in the Milky Way, which is one of 100 billion galaxies in the observable universe. The pivotal moment in the remote sens­ing of distant worlds happened on October 6, 1995, when Michel Mayor and Didier Queloz announced that they had discovered the first planet beyond the Solar System.27 We’re now “harvesting” Earths from deep space, and our dreams have moved on to the nature of life that might be found there.

This book explores how our concepts of distant worlds have been shaped and informed by space science and astronomy in the past forty years. Scientific understanding of the universe has been intertwined with culture since the time of Anaxagoras, and the popular imagination continues to be fueled by insights from space probes and large telescopes. What follows is not a survey of the many facilities that have furthered our understanding of the cos­mos. Rather, it’s an exploration of twelve iconic space missions that have opened new windows onto distant worlds. Most are in NASA’s portfolio, but all have non-U. S. investigators, as space sci­ence and astronomy have become increasingly international.28 In general, the arc of the book is chronological and moves from the proximate toward the remote. From comets to cosmology, from the Mars rovers to the multiverse, these missions have given us a sense of our cosmic environment and have redefined what it means to be the temporary tenants of a small planet.

The journey starts with Mars. Six years to the day after humans left their first footprints on the Moon—still the only world hu­mans have ever visited in half a century of the Space Age—the first Viking lander touched down on Mars, with its twin reaching the opposite side of Mars six weeks later. The Vikings dashed hopes that Mars might be habitable, but they opened up the modern age of exploration of the red planet. Nearly three decades later, an­other pair of intrepid machines bounced to a safe landing on their cushioning airbags. The Mars Exploration Rovers were embraced by the public as they trundled across the rocky red soil, inspecting interesting rocks, sending back pictures in 3D, and gathering evi­dence for a warmer and wetter Mars in the distant past. Mars may have hosted life in the past, and life might still be there in under­ground aquifers, and it is this oscillation in the popular imagina­tion between hostile and hospitable that makes it an uneasy dop- pelganger of the Earth.

Next up are two spacecraft that made a grand tour of the outer Solar System during the 1970s. Where before we had had nothing more than rough sketches, the Voyagers painted detailed portraits of the gas giant planets and their moons. These epic missions each ventured billions of miles from home, and they taxed the ingenuity of the scientists and engineers involved, many of whom aged and retired in the years between the first concept and its completion. The successor to the Voyagers was Cassini, which will soon enter its second decade of exploring the Saturn system. Cassini bristles with complex instruments and it dwarfs its predecessors. Together, these three missions have recast our understanding of the frigid realm beyond the asteroid belt. Giant planets may be cold and miniature versions of the Sun, but their moons are anything but dull and lifeless. Some have active geology and liquid water under their crusts. Others spew out sulfur or tiny ice particles. Many of them have distinct “personalities,” like the more familiar worlds in the inner Solar System.

The planets and moons of the Solar System are intriguing enough to have earned the names of gods and mythological figures. Yet they are just side shows in a process that concentrates most of the mass into a central sphere of glowing gas. The star is the central character in this drama, and the plot line is alchemy: the creation of the heavy elements that make up the planets and moons. Star­dust was the mission that caught not just one but two comets by the tail and in doing so told us how the Solar System was likely to have formed. The story of Stardust is our story, since most of our atoms were forged in the central cauldrons of long-dead stars. The Solar and Heliospheric Observer, by contrast, focused on the Sun itself and taught us what it means to live with a star. Belying its steady light, the Sun leads an active life that manifests in invisible forms of radiation. Distant worlds will also have to deal with the vagaries of their nurturing stars. After that, we drop back to take in a view of the solar neighborhood, from the unsung but impres­sive Hipparcos satellite. Hipparcos has refined the work of Wil­liam Herschel over two hundred years ago by placing us accurately within the city of stars we call the Milky Way. If the Copernican principle holds, the “grit” from stellar fusion that gathered to form the Earth is not unique to our region of space, and similar worlds have formed across the galaxy.

The two missions that follow illustrate the revolution in as­tronomy when astronomers’ blinders were removed after centuries of learning about the universe through visible light. Spitzer and Chandra are two of NASA’s Great Observatories, straddling the electromagnetic spectrum from waves hundreds of times longer to hundreds of times shorter than the eye can see. Each telescope looks at regions of space that are hidden from view. Spitzer pen­etrates the murk of gas and dust that permeates interstellar space and reveals new worlds being formed. Young stars and planets are shrouded in placental dust that is opaque to light but nearly transparent to long infrared waves. This is a huge advantage when looking for exoplanets because their contrast relative to the host star is hundreds of times better at infrared than at optical wave­lengths. Chandra, by contrast, has revealed the violence of dark objects like neutron stars and black holes, where such tiny worlds distort space-time and accelerate particles beyond any capability of our best accelerators. We would be ignorant of all these phe­nomena without space-based telescopes.

Closing the book are two missions that venture to the edges of space and time. The Hubble Space Telescope is the only space facil­ity that has embedded itself deeply into the consciousness of the general public, to the level where the prospect of not servicing the telescope generated a backlash and an eventual reversal of NASA’s original decision. Hubble has contributed to every area of astro­physics, but in particular it has quantified the limits of our vision, a region spanning 46 billion light-years in any direction, which con­tains roughly 100 billion galaxies. The inferred hundred thousand billion billion stars, with their attendant (and similar number of) habitable worlds, form the prodigious real estate of the observable universe, a census inconceivable to Anaxagoras and his colleagues. The Wilkinson Microwave Anisotropy Probe was a specialist mis­sion to map the microwave sky and pin down conditions in the infant universe. By gathering exquisitely precise data, this satellite has confirmed the big bang model in great detail. It has also shown that there are likely to be innumerable distant worlds out there whose light hasn’t yet had time to reach us since the big bang.

The journey ends with the near future, and efforts to measure realms of the universe that are currently at the edge of our vision. Close to home the goal is to see whether Mars has hosted or could host life—finding Life 2.0 would reset our views of biology beyond the home planet. In the proximate universe, we have the hope of detecting Earth clones and seeing if these worlds have had their atmospheres altered by a metabolism. At the frontier of cosmol­ogy, the hope is to test the multiverse concept, where the planets around 1023 stars are just part of the story, and a suite of alternate universes may exist, with properties perhaps egregiously different or perhaps uncannily similar to our own. In this extreme version of plenitude, everything that can happen has happened, and the set of events that led to our existence are neither special nor unique.

IIIII

The authors are grateful to the two Steves—Dick and Garber— from NASA’s History Program Office for their careful attention to this project, and to NASA for financial support during the writing of the manuscript. We acknowledge Ingrid Gnerlich at Princeton University Press for her epic patience during the long and winding road that led to the completion of the project, and to the staff at the Press for their assistance during production.

CI is also grateful to the Aspen Center for Physics, which is supported by the National Science Foundation, for providing a congenial setting for substantial work on the manuscript in 2010 and 2011, and to his astronomy colleagues at the University of Arizona for answering questions too numerous to count when he strayed from his expertise. CI also acknowledges the hospitality of his colleagues in the Department of Astrophysical Sciences at Princeton University, where he finished work on the manuscript during an appointment as the Stanley Kelley Visiting Professor for Distinguished Teaching.

HH would like to especially thank NASA for supporting the project and the research. She also wishes to thank the administra­tors, faculty, and staff at the College of Arts and Letters, the Depart­ment of English, the Office of Academic Research and Sponsored Programs, and the Pfau Library at the California State University, San Bernardino, for their assistance throughout the project. HH is extremely grateful to the many colleagues, friends, and family members who discussed and recommended topics and sources. It has been a great pleasure to research and explore the breadth of ideas that inform the study and that affirm our deep connection to the universe around us.

Although the rovers subsequently got into some amazing scrapes, there was nothing “seat of the pants” about NASA’s planning for the mission. Each of the science instruments was exhaustively tested in the lab and in an environment designed to roughly mimic Mars.

Then, 150 potential landing sites were winnowed down to four and finally two. Site selection was an agonizing process, balancing science and safety, and taking into account any steep inclines and potentially hazardous rocks in the footprint of the landing area. To the scientists and engineers involved in the mission, the launch was a double-edged sword. On the one hand, after years of preparation and testing, nothing more could be done except watch the launch play out. On the other hand, $800 million of effort was on the line, and there were many opportunities for disaster. Everyone involved knew that over the previous thirty years, nearly half of the mis­sions to Mars had been lost or had failed.

On January 4, 2003, after a journey of 300 million miles and seven months, Spirit entered the Martian atmosphere traveling at 12,000 mph, or twenty-five times the speed of sound. What fol­lowed was what team members called “six minutes of terror.” The spacecraft heated up and was buffeted by winds in the upper at­mosphere. Five miles up, the parachute deployed on schedule and then rockets fired to further slow the descent. Cocooned by airbags on all sides, Spirit hit the surface and bounced four stories high in a lazy arc in the weak gravity. After bouncing a dozen or so more times and traveling a quarter of a mile, it came to rest (figure 3.2). The “bouncing bag” method had been used for Mars Pathfinder; it removes the need for a completely soft landing, but at the expense of unpredictable ricochets from the surface. In the control room at JPL in Pasadena, over two hundred scientists, engineers, and NASA managers exhaled. And then let out a raucous cheer.

Spirit landed in the Gusev crater, on a rolling surface of red soil and small rocks. Opportunity landed three weeks later on the far side of Mars in the middle of a flat plain called Meridiani Planum, bouncing into a crater just 70 feet in diameter. Scientists were elated and called it a “hole in one” since the crater rim seemed geologically interesting, but the result was pure luck since the spacecraft could not have been targeted that accurately. This was the first example of serendipity or circumstance playing a role in the mission. Spirit and Opportunity have identical hardware “DNA,” but their different environments began to play a role soon after landing. Both rovers went through a series of system checks before rolling gingerly off their platforms to begin exploration of the red planet.

Linda Salzman Sagan coordinated the recording of greetings from the people of Earth. In an attempt to reflect the diversity of our species, the committee deliberately recruited volunteers from as many cultures, languages, and dialects as possible. The team de­cided to record greetings in the most commonly used languages as well as salutations in languages so ancient they are no longer spo­ken, such as Akkadian, Sumerian, and Hittite. To avoid complete anthropocentrism, the committee also included humpback whale song to give a voice, as Carl Sagan noted, to “another intelligent species from the planet Earth sending greetings to the stars.”41 Actually, humpback whale songs are a shared form of commu­nication and are now understood as cultural artifacts transmitted over hundreds or thousands of miles between humpback commu­nities. Whale researcher Ellen Garland reports that specific songs are passed between humpback whale groups from west to east across the Pacific Ocean. The humpbacks learn the songs from each other and teach them to other whales, indicating “cultural change on a vast scale,” asserts Garland, who along with her colleagues documented the same song, consisting of newly learned phrases, transmitted from one population to another.42 Though researchers in the 1970s were only beginning to recognize whale song as pur­poseful communication, when Druyan inquired about a recording of humpback whales for the interstellar record, zoologist Roger Payne responded: “Proper respect! . . . Oh, at last! . . . The most beautiful whale greeting was once heard off the coast of Bermuda in 1970. . . . Please send that one.” Druyan comments, “When we heard the tape, we were enchanted by its graceful exuberance, a series of expanding exultations so free and communicative. . . . We listened to it many times and always with a feeling of irony that our imagined extraterrestrials of a billion years hence might grasp a message from fellow earthlings that had been incompre­hensible to us.”43

Both fascinating and revealing are the fifty-five human saluta­tions on the Voyager record. As might be expected, several people sent wishes of peace. Many simply said “hello.” Carl Sagan’s son, Nick, who was then about six years old, offered, “Hello from the children of planet Earth.” Health and wellness marked a key re­frain among the messages. A significant number of volunteers ei­ther wished presumed inhabitants of exoplanets good health or expressed concern regarding their wellness. For instance, Stella Fessler, speaking in Cantonese, sent this greeting: “Hi. How are you? Wish you peace, health and happiness.” In Russian, Maria Rubinova said, “Be healthy—I greet you.” Saul Moobola in Nyanja, a language of Zambia, asked, “How are all you people of other planets?” Maung Myo Lwin in Burmese queried, “Are you well?” Liang Ku commented in Mandarin Chinese: “Hope every­one’s well. We are thinking about you all.” Frederick Ahl stated in Welsh, “Good health to you now and forever.” In Zulu, Fred Dube sent this message: “We greet you, great ones. We wish you longevity.” Andrew Cehelsky in Ukranian stated, “We are send­ing greetings from our world, wishing you happiness, good health and many years.” In Korean, Soon Hee Shin wondered, “How are you?” And Margaret Sook Ching and See Gebauer in the Amoy language of Eastern China asked: “Friends of space, how are you all? Have you eaten yet? Come visit us if you have time.”44 Slightly more than a quarter of the greetings explicitly mentioned wellness, good health, or longevity. Those salutations in particular seem to reflect a primal preoccupation with physical well-being. This is not surprising. For now, humans struggle with ubiquitous illnesses such as the common cold or influenza, severe and persistent back pain (a pervasive problem for upright walkers), or recovery fol­lowing traumatic injury. The many wishes of good health to pos­sible galactic neighbors on the Voyager Record suggest awareness of our own ephemerality.

Huygens was a pinnacle among many high peaks for the Cassini mission, marking the first time humans had landed a spacecraft on any body in the outer Solar System. It saw a terrain sculpted by wind and liquid, with vast hydrocarbon dunes and sinuous chan­nels carved into the shoreline. Volcanoes in the distance are likely to emit water instead of lava. Meanwhile, a tepid ocean sits under the icy crust of little Enceladus. These two moons are worlds at once alien and familiar, and if they host life, it will be unlike any form of life known on Earth. In 2011, the sum of all the evidence so far caused scientists at a major meeting to elevate Enceladus to the status of the “most habitable spot in the Solar System” beyond Earth, above Titan, and even above Mars.58

One of the conundrums of the search for life beyond Earth is the difficulty of moving away from anthropocentric thinking. All life on this planet is one thing, derived from a single common ancestor and a single implementation of information storage in genetic ma­terial. There’s no way to inductively formulate a “general theory” of biology or know how life might have evolved if conditions had been slightly different, or if it would have evolved at all. Since the hypotheticals cannot be answered, we don’t know whether the origin of life on Earth was a historical accident or the nearly inevitable outcome of physical and chemical conditions plus the passage of time. These two scenarios project very different roles for life elsewhere in the universe: sparseness or abundance. We do know that life has adapted to almost every conceivable ecological niche on the Earth, including environments below water’s freezing point and places where the source of energy is not sunlight or pho­tosynthesis. Life grips the planet like a fever, thriving in almost all places we can imagine and in some that are nearly unimaginable.

If Titan and Enceladus host life, we’ll not just have to expand the envelope of our traditional thinking about life processes, we’ll have to throw away the box completely. If the moon of a giant planet is hospitable to biology, then by focusing only on finding Earth-l ike terrestrial planets we’ll be missing a large part of the story. If a tiny moon far from the Sun can be habitable, the num­ber of potential living worlds throughout the Milky Way galaxy rises dramatically to several billion. Biochemistry based on ethane and methane, augmented by water and ammonia, would be unlike anything we’ve ever seen on Earth. Computer and lab simulations might help, but we still know too little about physical and chemi­cal conditions on these moons for them to be reliable. The next time window to get a gravity assist and revisit the Saturn system is 2015 to 2017, which adds a sense of urgency since the oppor­tunity following that one is not until 2030. Cassini and Huygens have given us just a taste of the potential for life beyond Earth; if we want to address this profound question we’ll have to go back.

People have become familiar with the fact that climate change not only involves global warming but also greater extremes in weather. Rare but damaging events like tornados and hurricanes and floods appear to be increasing.27 Another type of weather that can impact human affairs is space weather. In addition to emitting a continu­ous stream of superhot hydrogen and helium called the solar wind, the Sun periodically ejects billions of tons of plasma in a cataclysm called a coronal mass ejection. Researchers think this happens when magnetic fields are stretched and then snap into a different arrangement, like rubber bands pulled to the breaking point.28 The ejections are huge clouds of material, as much as 10 billion tons of hot gas, and when they head toward us, they can cause fierce magnetic storms in the upper atmosphere. This extreme “weather” in space can impact the functioning and the reliability of technol­ogy on the ground and in low Earth orbit. Strong electromagnetic fields resulting from space weather induce surging currents in elec­trical wires, disrupt power transmission lines, cause widespread blackouts, and affect the infrastructure that forms the backbone of the Internet. Power grids are particularly vulnerable to electrical overloads caused by space storms. Extremes of space weather also dislocate the radiation belts, and damage the satellites used for essential functions like telecommunications, weather forecasting, and GPS or global positioning systems.

Throughout history, people at high latitudes have experienced the Sun-Earth interaction through auroras, but the effect is not always benign. We’ve already seen that in the dog days of summer in 1859, telegraph wires in the United States and Europe spon­taneously shorted out and caused numerous fires. Soon after, the northern lights (or Aurora Borealis) were observed as far south as Rome and Hawaii. This was a “perfect storm” in space, where several changes conspired to produce a mammoth solar flare. The 1859 solar storm was the strongest ever recorded and it occurred when electrical technology was in its infancy. A weaker storm in 1921 induced ground current strong enough to shut down the New York City subway system. Much weaker storms in 1989 and 1994 knocked out communication satellites and parts of the North American power grid.

Arthur C. Clarke’s visionary proposal in 1945 in a letter to the editor of Wireless World to place communication and television satellites in geostationary orbit helped launch the Information Age. As of October 2009, the U. S. Satellite Database listed over 13,000 operating satellites in Earth orbit.29 In 2001, it was estimated that the satellite fleet in Earth orbit was worth $100 billion.30 During a single solar storm, billions of dollars of this orbiting hardware can be destabilized or destroyed. After the 1989 storm, Sten Odenwald reports that “satellites in polar orbits actually tumbled out of con­trol for hours” as others attempted to “flip upside down.”31 John Freeman explains why solar storms can decimate satellites despite their sophisticated electronics: “Storms in space produce miniature lightning storms on the surface of the satellite” and “immerse the satellites in a ‘cloud’ of hot electrons” that mimic commands from ground operators but are in fact phantom commands.32 If the cost of a fleet of satellites seems detached from our everyday existence, consider days or weeks without Internet access or GPS for vehicles, or communication and navigation systems for airline and ocean liner transport, cargo shipping, and road transport. Solar storms can even affect oil pipelines, which undergo accelerated corrosion from the bombardment of charged particles streaming from the Sun, eventually causing breaches in the pipeline.33

Space weather prediction remains an emerging science spear­headed by NASA, ESA, and the United Nations Office of Outer Space Affairs. Data from SOHO, and increasing concern over the impact of space weather, caused NASA to commission a new study in 2009. The resulting report provides clear economic data to quantify the risk to the near-Earth environment from episodes of intense solar activity. Extreme space weather is in a category with other natural hazards that are rare but have far-reaching con­sequences, like major earthquakes and tsunamis.34 It’s likely that more than once in the next twenty years there will be an “electro­jet disturbance” that disrupts the national power grid. In the 1989 event, the loss of some portions of the grid put stress on others and led to a cascade affect. The end result was power outages affecting more than 130 million people and covering half the country.

SOHO cannot prevent these natural disasters, but it can give two or three days’ notice of Earth-directed disturbances. And as we become more accurate in anticipating space storms, operators can place satellites in protective modes, shut down or limit power grids, redirect commercial flights, warn oceanic cruise and cargo ships, and place astronauts working on the International Space Station in the safest possible location on the station. Such steps will not only save lives but also protect the information systems that sustain our electronically fragile and networked global community.

Although SOHO’s bread and butter is looking for the high – energy radiation that’s a hallmark of solar activity, there was a recent concern that the Sun is becoming too inactive. Through 2008 and 2009 there were fewer and fewer sunspots. In part this was due to the natural solar cycle, but the calm was eerie. Entire months would go by without a single blemish on the Sun, and the solar minimum was as deep as it has been in a century. Bill Livings­ton and Matt Penn of the National Solar Observatory made the unsettling discovery that the magnetism of sunspots is declining by about 50 Gauss per year.35 Since sunspots have no substance— they’re just dim, cool markers of a concentration of magnetism— the worry is that the magnetic field would drop below 1500 Gauss, at which point no sunspots can form at all. Did decline like this presage a return to the Maunder Minimum of the late seventeenth century, when the European climate chilled markedly? Luckily, in 2010 normality asserted itself and the sunspot numbers started gradually rising.

The Sun is also fairly quiet in terms of the variation of its energy output. From peak to trough, the variation in total solar flux is only 0.1 percent and no long-term variation has been seen over thirty years. The extreme ultraviolet tail of the dog wags much more: 30 percent within a few weeks and a factor of 2 up to 100 over the whole solar cycle, depending on wavelength.36 This has important implications for our understanding of the Earth’s cli­mate, since human-i nduced change must be distinguished from natural variations in solar output.

Perhaps one reason the Sun is so equable is the fact that it steadily snacks on comets. One of the surprises of the SOHO mis­sion has been its prowess as a comet-hunter; the trawl is already well over two thousand. The Large Angle and Spectrometric Coro – nograph blocks the Sun’s face with an occulting disk, creating an artificial eclipse (figure 7.3). Most of the comets are “sun-grazers” that fly like Icarus very close to the Sun and then grow tails as their icy cores are heated. Toni Scarmato, a high school teacher from Calabria in Italy, discovered the one-thousandth comet in 2006, and two-thirds of them have been found by scouring coronograph data just after it’s taken and put on the web.37 SOHO found its two-thousandth comet in late 2010 and it has discovered half of

the comets for which orbits have been measured since 1761. The dance of fire and ice has become a way that the public has em­braced this protean space mission.

Exoplanets have quickly become the focus in the search for life beyond Earth and the quest to know whether we are alone in the unfathomable depths of space. Finding a large number of habit­able exoplanets would suggest that life abounds in the universe. For now, astronomers are scouring stars in the Milky Way most likely to host habitable worlds. Stars range in size from small, cool dwarfs no bigger than Jupiter to colossal stars topping out at about 200 solar masses and 10 million times brighter than our Sun. Blue giant stars are very hot, bright suns, such as those in the Pleiades Cluster or in Orion’s Belt, that emit mostly UV light. About half of all stars are binaries, in which one star orbits another. Binary

star systems can host planets with stable orbits around both stars, called circumbinary planets (figure 9.3). The first of such exotic exoplanets, reported in 2011, was dubbed the Tatooine planet for its dual sunsets as depicted in the film Star Wars.15 But in some binary systems gas interactions between the stars produce intense X-ray radiation deadly to life on their attendant planets. The most common stars in the Milky Way galaxy are M dwarfs. With less than half the mass of our Sun, M dwarfs emit most of their light at infrared wavelengths. Brown dwarfs, too small to generate nuclear fusion in their core, emit light solely in infrared. Among the many types of stars, it seems we couldn’t have wished for a better star to orbit, nor one more suited for life, than our Sun.

Any type of star can host planets, but the light stars pump out determines whether, and what kind of, life might survive on their

satellite worlds. Astronomers Ray Wolstencroft and John Raven contend that life works most efficiently with blue light, which they argue triggered photosynthesis on Earth.26 If that’s true, one might assume that planets orbiting hot, blue stars would be the most likely to host organisms. However, large blue stars emit dangerous ultraviolet radiation and expend their fuel in a mere 10-20 million years. By contrast, our Sun has existed for 4.5 billion years, and astronomers expect the Sun will not expend its fuel for another 4 billion. It’s our Sun’s long life span, in part, that allowed time for photosynthesizing organisms to emerge. As it happens, our Sun is the type of main sequence star that emits its greatest amount of radiation in the range of visible light. At some point 3 billion years ago, living organisms began to harness this light for processes in photosynthesis.27 Cyanobacteria or its progenitor began to use sunlight and carbon dioxide to generate food in the form of sugars, and in turn release a tiny gulp of oxygen. That seemingly simple development slowly, but radically, began to alter the chemistry of Earth’s atmosphere from one conducive to anaerobic life to an atmosphere rich in oxygen. But oxygen levels sufficient to sustain complex life as we know it didn’t evolve until approximately 550 million years ago. As Leslie Mullen reports, “If the evolution of photosynthesis is the same everywhere, then the lifetime of blue stars is just not long enough” for life to emerge on surrounding planets.28

Though no one knows for sure exactly how life emerged, a group of scientists at the Santa Fe Institute posit that the reductive Krebs cycle may have brought together chains of carbon molecules to produce the first organisms.29 The Krebs cycle, or citric acid cycle, describes how organisms produce energy by breaking down carbohydrates, fats, and proteins. In what has been called the “me­tabolism first” theory, the reductive Krebs cycle is proposed as the mechanism through which the first organisms emerged. “In some primitive single-celled organisms, [the Krebs cycle] operates in re­verse—it takes in energy and puts small molecules together into big ones. Scientists term the reactions in this mode as ‘the reductive Krebs cycle.’”30 Eric Smith, a professor at the Santa Fe Institute, has argued that geochemical processes on the early Earth might have combined smaller molecules into larger ones because it was energetically favorable to do so, similar to the chemical reactions in the reductive Krebs cycle. In other words, life might be built into, or be an expected outcome of, terrestrial geology.

Plants stretch their stems and leaves toward the Sun, and even the earliest organisms would have used a single photon of light for fuel and to obtain valuable information about the immediate envi­ronment. “The very pigments that led to photosynthesis probably also allowed those earliest creatures to react to light,” writes Ben Bova, who notes that even “single-celled organisms have light re­ceptors, sensitive spots of pigment that absorb light.” Michael So – bel’s book Light, Michael Gross’s Light and Life, and Bova’s The Story of Light explore the myriad ways life on Earth is enabled by, and is adapted to, the light of our Sun. Photoreceptor cells in bacteria, plants, and animals demonstrate the selective pressure of visible light, as such cells evolved due to the survival advantage they provided organisms on a planet with a star that emitted its greatest radiation in visible light. “The very earliest creatures cer­tainly did not have a sense of vision,” notes Bova. “All they could do was react to light the way a scrap of paper reacts to a puff of wind—move either toward the light or away from it.”31

By the Precambrian period, soft-bodied marine animals emerged that had at least a rudimentary sense of light and darkness. Under such conditions, finding prey and avoiding being eaten were dif­ficult prospects. But life on Earth would suddenly, on geological timescales, change. Neurobiologist Michael Land and zoologist Dan-Eric Nilsson note that “something remarkable seems to have happened in the interface between the Precambrian and the Cam­brian eras. Within less than five million years, a rich fauna of mac­roscopic animals evolved, and many of them had large eyes.”32 Land and Nilsson posit that a simple strip of photoreceptive cells in organisms could, in time, become curved to better sense the ori­entation of the incoming light. If that curved space became filled with a liquid, the light could be focused on a lens to create image­forming vision. Nilsson and Land conservatively estimate that the evolution of image-forming eyes could occur in a half million years, and perhaps as little as 300,000-400,000 years.

Land and Nilsson, as well as zoologist Andrew Parker, have re­searched how eyes may have emerged in response to the light pro­duced by our Sun. The first sophisticated eyes, dating to 530 mil­lion years ago, appeared at the beginning of the Cambrian period and they’re evident in the extraordinarily well-preserved fossils of the Canadian Burgess Shale. Parker refers to the Cambrian as the “big bang” in evolution: “It paved the way for the emergence of the vast diversity of life found today, whether in Australia’s Great Barrier Reef or Brazil’s tropical rainforests. It involved a burst of creativity, like nothing before or since, in which the blueprints for the external parts of today’s animals were mapped out. Animals with teeth and tentacles and claws and jaws suddenly appeared.”33 Parker argues that visible light acted as a strong selection pres­sure for the development of eyes in most species. In what he calls the “Light Switch” theory, Parker contends that sunlight has been the primary factor in determining animal evolution, behavior, and morphology. With the emergence of more complex eyes, Cambrian species could colonize niches within the water column of ancient oceans that in turn impacted their morphology. Trilobites, for instance, thrived in Cambrian seas (there were some 4,000 spe­cies) and apparently were the first animals to develop compound eyes. The trilobite could clearly see its environment, navigate, and seek prey. As a result, they and many other species developed exo­skeletons to protect themselves. Coincident with the evolution of eyes that could focus, Parker argues, the fossil record reveals the emergence of a radical diversity of all phyla of animals. Land and Nilsson agree: “If eyes had not evolved, life on earth would have come out very differently. More than any other organs, eyes have shaped the evolution of animals and ecosystems since the Cam­brian explosion.”34

Sometimes the dream is a nightmare. Mars has always had an ominous mien in myth and culture. Ancient civilizations regarded the planet as a malevolent agent of war and apocalypse. Similar myths emerged around the world.1 In late Babylonian texts, Mars is identified with Nergal, the fiery god of destruction and war. To the Greeks, Mars was Ares, one of Twelve Olympians and the son of Zeus and Hera. His attendants on the battlefield were Deimos and Phobos, terror and fear, and his sister and companion was Eris, the goddess of discord.2 Ares was an important but an unlikeable character. In Roman hands he morphed into a virile and noble god, one who facilitated agriculture as well as war. The third month of our year honors him and the time when winter abated enough that Roman legions could begin their military campaigns. In legend, Mars aban­doned his children Romulus and Remus and the twins went on to found the city of Rome.3 The mystique of Mars may have been enhanced by its retrograde motion: the fact that every few years it twice reverses its direction of motion among the stars.4 All exterior planets show this behavior, but the reversal is more dramatic for Mars than for Jupiter and Saturn. It’s curious that such a modest speck of reddish light could exert such power (plate 1).

Fast forward nearly two thousand years and Mars still exerts a grip on the imagination. It’s the night before Halloween, on the eve of World War II. Families across America are settling around the radio to hear “The Mercury Theatre on the Air,” a weekly pro­gram directed by the young Orson Welles and featuring him and

a talented ensemble cast. Listeners are enjoying salsa-inflected or­chestral music from a hotel in New York City when the announcer breaks in: “Ladies and Gentlemen, we interrupt our program of dance music to bring you a special bulletin from the Interconti­nental Radio News.”5 There’s a news report about unusual activity observed on the surface of Mars, then back to the music. A few minutes later the announcer breaks in with additional information about Mars. More music. The next interruption has the announcer talking in breathless tones about a meteor that just landed in New Jersey. A little later, on the scene, there’s horror in his voice as he describes creatures emerging from the meteor, which is in fact a spaceship. The Martians begin using a heat ray to incinerate by­standers, and as the announcer describes the engulfing flames, his voice is cut off in mid-sentence. Welles deliberately scripts several long seconds of silence, or “dead air,” to increase the tension and the verisimilitude.6 In New Jersey and elsewhere around the coun­try, people panic and many load their belongings into cars to es­cape the menace.7

To the modern ear, Welles’s broadcast has the tone of cheesy, B- grade science fiction. But this was a younger, more innocent world, worried about war and ignorant about the improbability of aliens actually visiting Earth. It was nearly twenty years before America would enter the Space Age. In fact, the story of invasion from Mars transcends particulars of time and culture. When H. G. Wells’s novel The War of the Worlds was published in 1898, it was an instant classic. His words retain their evocative power: “Yet across the gulf of space, intellects vast and cool and unsympathetic, re­garded our planet with envious eyes, and slowly and surely drew their plans against us.” More than a century later, when Stephen Spielberg adapted the book for a 2005 movie, the basic plot was unchanged.8 Fear of alien invasion taps into something deep in the human psyche, as primal as dreams themselves.

Water, water everywhere, but not a drop to drink. The most stun­ning finding of the Mars rovers, proclaimed the “Breakthrough of the Year” for 2004 by Science magazine,21 was the evidence for the prolonged presence of salty, acidic, and potentially life-supporting water on the surface. This water has long since disappeared, and climate change on Mars is the leitmotif of all the research done by Spirit and Opportunity. The Viking orbiter had provided sugges­tions of water, but the twin rovers provided indisputable evidence. Below the surface, there are large deposits of ice, with the possi­bility that some of it might be in aquifers kept liquid by pressure and a modest amount of natural radioactive heating from interior

rocks.22

All life on Earth—from the tiniest bacterium to the mightiest redwood tree—needs water. The jury is still out on whether or not Mars has ever hosted life, but within weeks of arriving on the

red planet, Opportunity showed that the Meridiani Planum had once been a water-soaked plain.23 Pay dirt was a stone’s throw from the landing site in the form of a rocky outcropping made of layers about as high as a street curb. The outcrop, nicknamed “El Capitan,” contained numerous clues to a watery past. Opportunity found small hard spheres, called “blueberries” by the science team, which were sometimes loosely scattered on the surface and some­times anchored into rock (figure 3.3). The blueberries were made of hematite, an iron-rich mineral that usually forms on Earth in the presence of water: oxygen atoms from the water bind to iron atoms in the mineral. The team speculated that groundwater car­rying dissolved iron had percolated through the sandstone to form the spheres.24 Later on, Opportunity discovered the mineral called jarosite, which only forms on Earth when acidic water is present.25 Water that’s acidic or rich in dissolved iron is quite capable of

hosting microbial life, as we know from the ecosystems found in places like the runoff from the Rio Tinto mine in Spain.

Opportunity also found inch-high rock layers that overlapped and cut into each other. Geologists call such formations cross-beds, and their sizes and shapes indicated that they had been formed by flowing water. Some layers showed weathering by wind so the water must have been present intermittently. The minerals in the cross-beds were rich in sulfur, chlorine, and bromine, which had apparently settled to the bottom of a salty lake or shallow sea. Similar briny deposits are found in desert regions of the Earth. Op­portunity also found small vugs, from the Cornish word for cave, and these inch-long or smaller cavities were probably left behind when concentrations of minerals were dispersed by groundwater. Mars minerals tell the tale of a watery past.

Meanwhile, Spirit wasn’t spinning its wheels (yet). It had landed in a volcanic, rock-strewn plain with no obvious signs of sedimen­tation, but it soon roamed into its own discoveries. A volcanic rock called “Humphrey” had crevices filled with crystallized minerals that had most likely been dissolved in water. On another rocky target called “Clovis,” Spirit found traces of the mineral goethite, which only forms in a terrestrial environment in the presence of water, and telltale enhancements of sulfur, chlorine, and bromine. Spirit also measured a soil sample with a very high concentration of salt, another indirect indication of water.26 Science always deals with finite or incomplete information so it’s important to rule out plausible alternative hypotheses. The alternative explanation for cross-bedding and other sculpting of the surface is wind erosion, but on Earth the signatures of wind and water erosion are quite distinct. Mars was volcanically active in the past, and volcanism can produce spheres from molten drops and cavities, and occa­sionally minerals like sulphates and jarosite. However, the web of chemical evidence from the rovers, plus the features that can only be explained by the action of water, present a compelling case for an ancient Mars with water on its surface.27

The finding of past water on Mars raises more questions than it answers. Mars has an obliquity—or tilt of its axis as it orbits the Sun—that varies much more than the Earth’s. Orbital tilt causes seasons on any planet. Whereas our tilt is stabilized at close to 23.5 degrees by the Moon, the tilt of Mars is known to have varied from 10 to 60 degrees over the past 100,000 years, and dramatic climate variations were probably occurring in the distant past as well. The idea of a “warmer, wetter” Mars several billion years ago is appealing but is not well supported by climate models and evi­dence from Martian meteorites.28 The problem is that the planet’s modest gravity is incapable of retaining a thick atmosphere, and the early Sun was fainter than it is today. Some 65-70 degrees of greenhouse warming were needed to bring the surface up to the melting point of water, and while Mars did have early volcanism that generated carbon dioxide, which is a heat-trapping green­house gas, the early atmosphere was unlikely to have been thick enough to get the surface temperature above freezing. Mars cli­mate change continues to be enigmatic and difficult to pin down.

Thirty years after their launch and billions of miles from Earth, the Voyager spacecraft continue to transmit data on their way into interstellar space. NASA will track Voyager and listen for its transmissions until the spacecraft fall silent, so that we might learn something about what writer Stephen J. Pyne calls the “soft ge­ography” of the outer Solar System and what lies beyond. Their secondary mission to carry messages of greeting beyond our Sun may, to some, seem futile. Pyne and others have noted the very low likelihood that even if another species came across the spacecraft, they could figure out how to play the phonograph record or even recognize it as a message. Today’s teens, if presented with a copy of Voyager’s record, might find the task difficult, if not impossible. “By the time Voyager reached Jupiter and Saturn,” writes Pyne, “vinyl phonograph records were overtaken by magnetic tapes; by the time it reached Uranus and Neptune, tapes were fast fad­ing before CDs; by the time it reached the heliosheath, CDs were passe compared with digital drives and iPods. The phonograph was hopelessly archaic just as the golden record reached the edge of the solar system—in technology years, barely beyond cuneiform tablets.” As Pyne sees it, the Voyager spacecraft are in many ways ill-equipped for their journey into the unknown: “They were leav­ing the solar system with computer power inadequate to run a cell phone, and electrical power insufficient to animate a clock radio. Yet they had much yet to survey; the dynamics of the solar wind. . . reversals in the Sun’s magnetic field, interstellar particles, radio emissions from various sources within and beyond the he­liosphere, and of course the interstellar medium, if all went well.” However, in spite of Voyager’s technological limits, Pyne describes the spacecraft as the stuff of legend, whose tales will be told in future ages: “For now, it continued to send back reports from new settings. It was doing what no other spacecraft could. Its narrative simply defied closure from Earth.”45

Voyager’s ability to communicate with Earth is certainly lim­ited by its plutonium supply, but like the species that launched it, the spacecraft and its record demonstrate their resilience precisely when faced with seemingly insurmountable limitations. Timothy Ferris, who coordinated the music selections, speculates that even if Voyager’s record is someday retrieved but proves indecipherable, it nevertheless conveys a clear message: “However primitive we seem, however crude this spacecraft, we knew enough to envision ourselves citizens of the cosmos. . . . [W]e too once lived in this house of stars, and we thought of you.”46 Despite the possibility of becoming extinct, and precisely because we might, we sling these auspicious spacecraft into the unfathomable depths out of an innate optimism that runs deep in our species. The odds against another civilization retrieving and playing Voyager’s record are astronomical, and yet we sent them in the recognition that un­derstanding the universe and our place in it has mattered deeply from our earliest beginnings. Voyager’s interstellar mission inad­vertently and silently speaks of two intrinsic human traits: we are relatively physically fragile and, like other species, prone to extinc­tion, and yet we possess an inexplicable capacity to hope even in the most dire circumstances. This instinctual, undaunted ability to hope against all odds must have evolved early on in Homo sapiens as a survival mechanism. So far, it has worked; we are the only extant hominid.

And, such unyielding expectation despite seemingly insurmount­able circumstances has produced some of humankind’s greatest accomplishments. It was that kind of resilience that compelled Beethoven, though completely deaf and fatally ill, to neverthe­less compose some of his most acclaimed works. During Apollo 13, when it seemed the mission and possibly the crew were lost, NASA’s engineers and astronauts refused to give up and brought the crew home via gravity assist. It was with similar abandon and hope that we sent Voyager’s greetings to possible other galactic civilizations. Sagan eloquently illustrates this point: “Billions of years from now our Sun, then a distended red giant star, will have reduced Earth to a charred cinder. But the Voyager record will still be largely intact, in some other remote region of the Milky Way galaxy, preserving a murmur of an ancient civilization that once flourished—perhaps before moving on to greater deeds and other worlds—on the distant planet Earth.”47