Category Dreams of Other Worlds

The Earth and its atmosphere emit infrared radiation in all direc – tions,9 swamping any signals from space. It’s analogous to trying to see stars in the noontime sky. For a faint, cool source to stand
out against a warmer backdrop, an infrared telescope must lower thermal backgrounds. Which is to say, the longer the wavelength of light being observed, the colder a telescope and its detector need to be to see an infrared source from space. By the 1960s, infrared astronomy took off with the development of a new type of solid-state detector that had greater sensitivity than old-style radiometers.10 Another obstacle to detecting infrared radiation from cosmic sources is the fact that those waves are absorbed by water vapor in the air we breathe. Astronomers initially chose high mountaintops for their observations, to be above as much of the infrared-absorbing atmosphere as possible. For example, some of the best infrared observations are made 4,200 meters high atop the dormant volcano Mauna Kea in Hawaii, and new telescopes are being constructed in the arid Atacama Desert in Chile, includ­ing a major new millimeter array on the Chajnantor plateau at an altitude of 5,000 meters. An extreme version of this strategy was the Kuiper Airborne Observatory, which operated from 1974 to 1995. This 36-inch telescope in a converted Air Force C141 jet made observations for a few hours at a time at an altitude of 45,000 feet. The successor to this facility is a 2.5-meter telescope in a converted Boeing 747-SP, the Stratospheric Observatory for Infrared Astronomy.11 High-altitude balloons and rockets carrying small telescopes have been used to reach even higher altitudes of over 100,000 feet.

It proved essential to cool infrared telescope detectors to liquid nitrogen or even liquid helium temperatures so that radiation from the detector wouldn’t wash out signals from the sky. On ground – based telescopes observing the near infrared, two to six times the wavelength of visible light, liquid nitrogen is used to cool the de­tector to 77 Kelvin or -177°C. Though seemingly exotic, liquid nitrogen is basically liquefied air and costs no more than milk. For observing at the mid-infrared, 20-200 times the wavelength of visible light, liquid helium is used to cool the detector to 4.2 Kelvin or -269°C, a hair’s breadth from absolute cold. But the Earth environment emits so much radiation at these wavelengths that astronomical signals are swamped by background radiation and deep mid-infrared observations can only be made from space.

Space infrared astronomy was kicked into gear with the very successful NASA Infrared Astronomical Satellite (IRAS) mission,

which in 1983 surveyed the entire sky in the infrared for the first time. In less than a year, IRAS detected about 350,000 infrared sources. Around the same time, ESA launched the Infrared Space Observatory, which had higher sensitivity and much broader wave­length coverage.12 In 1997, an infrared camera was added to the instrument suite of the Hubble Space Telescope. This camera has been very successful but it has a small field of view and shares time with four other instruments, so infrared astronomers had always planned for their own facility.

Spitzer, from its earliest inception, was especially designed for infrared astronomy and is sensitive enough to detect infrared sig­natures of stars and galaxies billions of light-years away. The space telescope has been instrumental in unveiling small, dim objects like dwarf stars and exoplanets and can even determine the tem­perature of their slender atmospheres. Originally proposed in the late 1970s as NASA’s Space Infrared Telescope Facility, the Spitzer Space Telescope suffered from uncertainty, a delay after the loss of the space shuttle Challenger, near-cancellation, congressional limbo, budget cuts, and “descoping.” Nevertheless, in 2003 the telescope was finally launched, after being renamed subsequent to a public opinion poll conducted by NASA. The last of NASA’s four Great Observatories, the $800 million telescope was named after Lyman Spitzer, an early advocate of the importance of or­bital telescopes.13 After launch, the spacecraft took about 40 days to cool to its operating temperature of 5 Kelvin. Once cooled, it took just an ounce of liquid helium per day to maintain its detec­tors at their operating temperature. A solar panel facing the Sun serves to gather power and protect the telescope from radiation. On the anti-solar side, a series of concentric shells and a black shield radiate heat into space. Spitzer’s most precious resource was its liquid helium, used to cool the telescope and its instruments to the phenomenal temperature of just 1.2 Kelvin, within an iota of the temperature where atoms and molecules have no motion. This extra cold relative to the liquid helium operating temperature comes from venting the helium into the vacuum of space, much as your skin would cool if a liquid evaporated from it. Background radiation is reduced by a factor of a million relative to a similar­sized telescope on the Earth’s surface. The telescope’s cryostat held 350 liters at launch, as much as the gas tank of a large minivan. Helium evaporates and is boiled away by tiny heat inputs from the instruments and from the telescope structure, so the act of observ­ing steadily depletes the cryogen.

The goals of cooling the telescope and preserving the liquid he­lium as long as possible dictated the telescope’s orbit and design. Careful design minimized these heat inputs, which are typically measured in milliwatts. As a reference, your fingertip radiates 25 milliwatts. The Spitzer facility is 4.5 meters tall and 2.1 meters in diameter and weighs about as much as a minivan. Previous space observatories had used either a low Earth orbit to be serviceable by the Space Shuttle (such as the Hubble Space Telescope) or a high Earth orbit with a period of one to two days (like the Chan­dra X-Ray Observatory). Spitzer is in an unusual Earth-trailing solar orbit. The big benefit is in escaping from the Earth’s heat and being situated in a good thermal environment where the tele­scope can cool in the vacuum of space. The orbit also avoids the Earth’s radiation belts, and there’s excellent efficiency of observing because the Earth and Moon rarely intervene. But one disadvan­tage is that Spitzer’s radio signals are getting weaker as it moves away from Earth. Spitzer slips away from Earth at about 10 mil­lion miles per year and is now farther away than the Sun. Only the huge 70-meter dishes of the Deep Space Network are sensitive enough to gather its precious data. Mission planners estimate that they will lose touch with Spitzer in early 2014.

If you were a field geologist, your most valuable asset would be your eyes. Geologists are trained to recognize rocks and minerals and crystals, using archetypal images in textbooks and samples in the lab. But hand-picked samples, viewed in isolation, can never prepare a geologist for the complexity and apparent chaos of a real landscape (figure 3.1). Rocks are sometimes layered and sometimes jumbled, their color and texture can change according to lighting conditions and perspective, and each rock’s story only makes sense in the light of the surrounding terrain. Experienced geologists must take all this in through their eyes. With little atmosphere on Mars to carry sound or smell, sight is the critical sense.

The rovers’ eyes sit at the top of a mast, roughly five feet off the ground. Twin CCD cameras are used to form stereo images. The cameras can rotate 360 degrees to make a complete panorama, and pivot 18 degrees to scan the landscape or the sky. With 16 megapixels, they are similar to high-end digital cameras you could buy, and each one weighs nine ounces and could fit in the palm of your hand. But unlike a commercial device, the rover cameras have a large set of color filters designed to help diagnose the composi­tion of rocks and minerals, and a set of solar filters for looking at the Sun. Like some new cars, the rovers also have “hazard avoid­ance” cameras, one pair on the front and one pair on the back,

each looking ten feet out to help avoid any unexpected collisions or obstacles.

Spirit and Opportunity may be one-armed geologists, but those arms are highly capable, with an elbow, a wrist, and four modes of motion. In the mechanical “fist” of each arm is a pint-sized im­ager that acts like the hand-held magnifying lens carried by many geologists. Just as no geologists would do any fieldwork without their trusty hammer, so each rover arm is equipped with a Rock Abrasion Tool (or RAT). The RAT is a muscular, diamond-studded grinder; in two hours it can carve out a hole 2 inches across and 1/5 inch deep into even the hardest volcanic rock.20 Rock interiors can be quite different from the exteriors, which have been subject to weathering and radiation and may be coated in dust. After the RAT has finished its work, the microscopic imager can peer into the hole and two other science instruments can be pivoted into po­sition to study the rock. One, called a Mossbauer spectrometer, is specifically designed to study iron-bearing minerals with high pre­cision. It takes about twelve hours to do a single measurement. The other is called an Alpha Particle X-ray Spectrometer. It has an on­board source of alpha particles—high-energy helium nuclei—and it can bounce either those or X-rays off the rock sample. The re­sults give the elemental composition, which is important in decid­ing how different chemicals came together to form minerals within the rock. These measurements take about ten hours per sample.

The rovers’ suite of sophisticated instruments is completed by the Miniature Thermal Emission Spectrometer. All objects emit heat, and the spectrum of their heat can be used to deduce their composition. This instrument is particularly designed to look for minerals that can only be made in the presence of or by the action of water, such as carbonates and clays. It weighs five pounds and it sits in the rover’s body, but it has a periscope so it can look out alongside the rover’s eyes, and it can also gaze upward to gather measurements on the temperature, water vapor content, and pres­ence of dust in the atmosphere.

Finally, like many good science fair projects, the Mars Explora­tion Rovers make use of magnets.

Magnetic dust grains are freeze-dried remnants of the planet’s wetter past, where some types of molecules align with the Martian magnetic field when they are in solution, and then preserve that ori­entation when the mineral solidifies. Magnetic minerals give addi­tional clues to the geological history. One pair of magnets sits at the end of the robotic arms, where it can gather magnetized particles ground out by the RAT. Another pair of magnets is at the front of the rover, tilted such that non-magnetic particles will fall off. Two of the spectrometers then analyze any particles that stick there.

Attached to each Voyager spacecraft is a gold-plated, copper pho­nograph record conveying greetings from Earth to civilizations potentially inhabiting nearby exoplanets in our galaxy (plate 6). Though not the first spacecraft to carry salutations from Earth, Voyager is the first to send our voices, music, and photographs. Pioneer 10 and 11 carry a plaque that includes a map of nearby pulsars indicating the Sun’s location in the Milky Way, a schematic of our Solar System, their flight path from the third planet out, and an outline of the Pioneer spacecraft with the outlines of a man and woman standing in the foreground to demonstrate the humans’ shape and comparable size. The male figure has his right hand raised in a sign of greeting. By contrast, Voyager carries much more detailed salutations to potential species that might happen upon the spacecraft.

The idea of attaching greetings from Earth to the Voyager spacecraft apparently was suggested by John Casani, who served as project manager for Voyager from 1975 to 1977. In October 1974, Casani had scribbled on a routine JPL concern and action report: “No plan for sending a message to our extrasolar system neighbors,” as well as the phrase, “Send a message!”34 Casani asked Carl Sagan to coordinate putting together a message. Sagan was a natural choice to head the effort; he and Cornell astrono­mer Frank Drake had organized development of the Pioneer 10 and 11 plaques. Sagan understood the powerful public impact of a greeting to possible galactic civilizations, eagerly accepted Casani’s mandate, and recruited a group of scientists, profes­sors, and business leaders for the NASA Voyager Record Com – mittee.35 Sagan additionally consulted with science fiction writers Arthur C. Clarke, Robert Heinlein, and Isaac Asimov and gath­ered a core group of collaborators including Drake, who in 1961 developed an equation for calculating the number of civilizations that might populate the Milky Way, writer Ann Druyan, science writer and professor Timothy Ferris, astronomy-inspired art­ist Jon Lomberg, and artist Linda Salzman Sagan, who drew the images for the Pioneer plaque. This core group selected the mes­sages, sounds, music, photos, and diagrams now hurtling into the interstellar abyss.

Inscribed in the grooves of the Golden Record is an essay of sounds of Earth; spoken and written greetings; humpback whale song; 115 photographs and diagrams related to nature, science, and human activities; and ninety minutes of music. Murmurs of Earth, an account of the compilation of the interstellar record, reveals the intense deliberation that went into selecting its con­tents and determining the medium that would best preserve this special envoy from Earth. It was Drake who recommended using the equivalent of a phonograph record that could record sound as well as photographs that would render as television images. Since corrosion does not occur in the vacuum of space, it was calcu­lated that, excepting a direct collision with micrometeoroids or other space debris in our Solar System, the records might remain intact for a billion years. “We felt like we were on the commit­tee for Noah’s Ark, that we were deciding which pieces of music and which sounds of Earth would be given eternal life, really a thousand million years. We took it as a kind of sacred and joyful task,” recalls Druyan, who gathered the audio clips for the record’s twelve-minute sound essay.36

The sound essay begins with a musical rendering of Kepler’s Harmonica Mundi, a mathematical treatise transposed into sound at Bell Telephone Laboratories. “Each frequency represents a planet; the highest pitch represents the motion of Mercury around the Sun as seen from Earth; the lowest frequency represents Ju­piter’s orbital motion. . . . The particular segment that appears on the record corresponds to very roughly a century of planetary motion,” explains Druyan. Following this are sounds of the pri­mordial Earth: volcanoes, earthquakes, thunder, and mud pots. To illustrate the evolution of life, ocean surf, rain, and wind are fol­lowed by crickets and frogs, then vocalizations by birds, hyena and elephants, and a chimpanzee. Human footsteps, heartbeats, speech, and laughter come next and fade to the sound of fire and of flint being struck by rock to indicate the evolution of tool use. These are followed by a tame dog’s bark, and the clamor produced by sheep herding, blacksmithing, sawing, and agriculture. Next is Morse code tapping out the phrase Ad astra per aspera, which translates as “to the stars through difficulties,” a suggestion by Carl Sagan. To trace the evolution of travel technologies in the twenti­eth century, the clatter of horse-cart and noise of an automobile segue to an F-111 flyby and the cacophony of a Saturn V liftoff. Concluding the sound-essay are juxtaposed sound files of electro­magnetic waves of human brain activity, actually those of Druyan, in case an advanced civilization could read human thoughts, and the radio output of a pulsar, a collapsed star in rapid rotation. As Druyan points out: “My recorded life signs sound a little like recorded radio static from the depths of space. The electrical sig­natures of a human being and a star seemed, in such recordings, not so different, and symbolized our relatedness and indebtedness to the cosmos.”37

Frank Drake and artist Jon Lomberg were tasked with putting together the Golden Record’s photo essay and selecting images that might make sense to a nonhuman species. Lomberg has sent more of his own art into space than perhaps anyone, having de­signed the sundial on the Spirit and Opportunity rovers and cre­ated the Visions of Mars DVD attached to NASA’s Phoenix Lander as a message to future generations who explore the red planet. For Voyager’s record, Drake and Lomberg compiled 115 photos and diagrams depicting insects, wildlife, ballet dancers, bushman hunt­ers, various landscapes, architecture, an X-ray image of a hand, a radio telescope, the Earth in space, an astronaut space walking on orbit, and even a sunset, chosen in part to illustrate Earth’s beauty and because “the reddening of the light contains informa­tion about our atmosphere.”38

The elegantly etched aluminum cover of Voyager’s record indi­cates how the record is to be played and is Lomberg’s work as well. On the right of the cover are illustrations of the appropriate verti­cal to horizontal ratio for television images of the photographs and what the first image, a simple circle, should look like. On the left are top-down and side view graphics showing proper place­ment of the enclosed stylus and the correct rpm speed in binary. The Golden Record is not a conventional 33 1/3 rpm long play­ing record, as is often claimed. Wanting to include as much data as possible, the committee realized they could embed more music and images if they slowed the playtime to 16 2/3 revolutions per minute. Near the bottom right of the record cover is a diagram to indicate that the transition period of a hydrogen atom between its two lowest states, 0.7 billionths of a second, should be taken as equivalent to 1 in binary code. This is intended to give an accurate speed for spinning the record and for interpreting the pulsar map at the bottom left that depicts our Sun relative to 14 pulsars whose precise periods are notated in binary code.

The committee was adamant about sending music as a form of art, particularly since music expresses a potentially universal, mathematical language. But with only a ninety-minute segment dedicated to music, agonizing decisions had to be made in repre­senting musical genres of the world. Hurtling into the depths of space are selections, among others, of Javanese gamelan, an ini­tiation song from Zaire, Japanese shakuhachi, a raga from India, Melanesian panpipes, panpipes and drum from Peru, a Bulgarian shepherdess song, a Navajo Night Chant, tribal music from New Guinea, Louis Armstrong performing jazz, Chuck Berry playing rock and roll, and classical selections by Mozart, Bach, Stravin­sky, and Beethoven. Also included is the Cavatina movement of Beethoven’s String Quartet No. 13 in B Flat, Opus 130, along with an image of sheet music from this selection. It was a piece that Beethoven so cherished he once told a colleague he could cry when thinking of it.39 In researching Beethoven’s Cavatina, Druyan found on the score of an adjacent opus that the composer had written: “What will they think of my music on Uranus? How will they know me?” Beethoven apparently, comments Druyan, “toyed with the thought that his music might leave [our] planet.”40 His intuitive query seems indicative of an artist who saw far beyond his time. For that, and for his deafness, he was often misunder­stood or considered eccentric. While reworking the finale rejected by his publisher for the String Quartet No. 13, Beethoven appar­ently strolled through fields waving his arms, shouting and likely singing, certainly in an attempt to feel since he could not hear, the music playing in his mind and that he found so compelling. So, it seems a perfectly harmonious outcome that Beethoven’s music indeed traveled to Uranus and at this moment, aboard Voyager, is barreling into the pristine interstellar void.

In the introduction to the stunning coffee table volume Saturn: A New View, Kim Stanley Robinson comments on the amazing photographs Cassini has archived in its ongoing exploration of Saturn. Noting that “the gorgeous concentricities of Saturn’s rings look like gravitation itself made visible,” Robinson is wistful that astronauts have not yet journeyed to Enceladus or Titan. “Even­tually, we might even go to Saturn ourselves,” writes Robinson, “It would be a kind of pilgrimage: it would be a sublime experi­ence.”50 However, roboticist Rodney Brooks would likely argue that we have already journeyed to Saturn and landed on one of its moons. NASA’s planetary missions are extensions of ourselves. These little machines, with the ability to travel billions of miles across the chasm of interplanetary space, enhance our vision—like a pair of contacts or glasses—and extend our sense of touch, our ability to sample the atmosphere of another world. Analogous to cochlear implants, pacemakers, or titanium prosthetic legs that allow paraplegic athletes to run faster than Olympians, Cassini has taken us to the far shores of the outer Solar System and con­tinues to record, in fine detail, the state of affairs at Saturn and its moons. Cassini, like our other planetary science missions, serves as a highly technical extension of humankind. These robotic ex­plorers not only extend our fingertips into the frigid outer Solar System, but Brooks argues that our machines are “us,” and that biotechnology of the future will reconfigure what we think of as human. “Our machines will become much more like us, and we will become much more like our machines,” predicts Brooks. “The distinction between us and robots is going to disappear.”51

Futurist Ray Kurzweil couldn’t agree more. Kurzweil predicts that in the next thirty years we will use biochemistry, biotechnol­ogy, and nanotechnology to reconfigure the human body, in part, by readily incorporating technology into our bodies to enhance longevity and our intellectual capacity. Kurzweil points to the evo­lution of sight to illustrate how technology has exponentially en­hanced our biological capabilities:

There are many ramifications of the increasing order and complex­ity that have resulted from biological evolution and its continuation through technology. Consider the boundaries of observation. Early biological life could observe local events several millimeters away, using chemical gradients. When sighted animals evolved, they were able to observe events that were miles away. With the invention of the telescope, humans could see other galaxies millions of light-years away. Conversely, using microscopes, they could see cellular-sized structures. Today humans armed with contemporary technology can see to the edge of the observable universe, a distance of more than thirteen billion light-years, and down to quantum-scale subatomic particles.52

Kurzweil compares this exponential advance in visual observa­tion to the evolution of information technology. He notes that mi­croorganisms can respond to and communicate events in their im­mediate environment, but with the evolution of humans, language, and the technology of writing, we have recorded information that persists for thousands of years. The simple technology of writing, whether in cuneiform or in modern languages, has exponentially expanded our scientific knowledge and reach. Our robotic part­ners in space are no less an extension of ourselves than a telescope or the technology of writing and have powerfully shaped what we know about our planet and the Solar System, and the billions of worlds we have yet to explore.

Even now we are joint explorers with our smart machines. An­thropologist Stefan Helmreich comments, “What it means to do oceanography and ethnography is changing. In an age of remotely operated robots, Internet ocean observatories, multi-sited field­work, and online ethnography, presence in ‘the field’ is increas­ingly simultaneously partial, fractionated, and prosthetic; it is not just distributed across spaces—multi-sited—but cobbled together from different genres of experience, apprehension, and data col – lection.”53 This collaborative scientific exploration, already being undertaken between humans and machines, affords us a kind of distributed intelligence across the Solar System.

Helmreich, Brooks, and Kurzweil suggest that we think of our machines as symbionts, without whose help we could not explore Earth’s ocean depths, much less the depths of lakes and oceans on icy moons orbiting Jupiter or Saturn. Our collaboration with smart machines incites Helmreich to consider one other order of unsuspected collaboration—t hat between humans and microor­ganisms. He suggests that alien microorganisms, if such exist in the frozen ocean on Enceladus or in Titan’s hydrocarbon lakes, may be more akin to life on Earth than we imagine. As microbiologist Jo Handelsman points out, “We have ten times more bacterial cells in our bodies than human cells, so we’re 90 percent bacteria.”54 Of the microbes coexisting in our bodies, scientists explain that we have “evolved with them in a symbiotic relationship, which raises the question of who is occupying whom.”55

In fact, instead of thinking of microorganisms as alien to us, doctors have begun to recruit them in fighting cancer. Research­ers at the University of Pennsylvania are relying on our symbiotic relationship with viruses and other microorganisms to attack and kill cancer cells. They’re using viruses to insert DNA into patients’ T-cells that in turn causes the T-cells to selectively attack and kill cancer cells. As Stefan Helmreich makes clear:

Microbes are not simple echoes of a left-behind origin for humans, orphaned from all evolutionary association. Microbes are historical and contemporary partners, part of our bodies “microbiomes.” “The” human genome is full of their stories. . . . The bacteria that inhabit our bodies do not simply mirror the bacteria that inhabit the sea—as might brine in our blood. This is not human nature reflecting ocean nature. It is an entanglement of natures, an intimacy with the alien. Such dynamics shift the grounds upon which anthropos might be figured, perhaps transforming humanity into Homo alienus.”56

Evidence of this is the fact that people in different regions of the world have different genetic makeup partly due to local microor­ganisms. People of Japanese descent have “acquired a gene for a seaweed-digesting enzyme from a marine bacteria. The gene, not found in the guts of North Americans, may aid in the digestion of sushi wrappers.”57

As noted in the chapter on the Viking mission, Lynn Margu – lis’s contribution to Gaia Theory was to highlight the extent to which our existence is intimately bound up with the Earth’s micro­organisms. Having proposed the theory of symbiogenesis, which claims that the mechanism for evolution is the symbiotic sharing of genetic material, Margulis demonstrated that bacteria invad­ing single-celled organisms became their mitochondria and chlo – roplasts. We do not know whether extremophiles exist on Titan, Enceladus, or other worlds such as Europa, one of Jupiter’s moons. What drives our continued exploration of those distant shores is that our beginnings may be entangled with theirs.

The Greek mathematician Pythagoras imagined a universe de­scribed by numbers. When he talked about the “harmonies of the spheres,” he meant music that enlightened individuals might hear resulting from the translucent shells that carried the celestial objects. Kepler continued this line of thought and applied it to the elliptical orbits of the planets. These ideas sound archaic but they’re not misguided. The universe contains many periodic and oscillatory phenomena, and the formalism to understand them in­volves studying the resulting harmonic frequencies.23 Situations as different as planet, moon, and ring systems and their orbits and interactions, the spiral arms of the Milky Way, and the interac­tions between matter and radiation in the early universe are well described in terms of coupled frequencies and harmonics. We’ve seen beautiful examples of this in the complex gravitational dance of Saturn’s rings and moons, discussed in the Cassini chapter.

A hundred years ago, nobody imagined that the Sun could be studied in terms of harmonics. Apart from the sunspot “blem­ishes,” the surface seemed smooth and featureless. Solar properties vary smoothly through the region we see as the “surface.” The edge marks the distance out from the center at which the density of gas reduces to the point where light no longer interacts with particles and travels freely. Inside this region, which is called the photo­sphere, light is trapped and so the Sun’s interior is hidden from view. The first inkling that the interior was pulsating came when George Ellery Hale built his heliostat on Mount Wilson in the early twentieth century. High-magnification photographs showed a sur­face mottled with fine structure, and time sequences revealed that the Sun’s surface was a seething sea on which the pairs of sunspots floated like large lily pads. The field of helioseismology matured as the century progressed, and solar scientists identified thousands of different oscillatory modes—the Sun “rings” like a bell. The Sun is also like an echo chamber (plate 12). Sound travels through the plasma and sets up standing waves, like the vibrations of the head of a drum or the air inside an organ or woodwind instrument.24

Just as seismologists can infer the internal structure of the Earth from the way sound waves and earthquake tremors pass through the planet, helioseismologists can study the Sun by seeing how in­terior sound waves manifest at the surface. This work has led to measurements of the density, temperature, and chemical abundance of the interior, as well as inference of the age of the Solar System and the constancy of the gravitational constant.25 The workhorse instrument on SOHO is the Michelson Doppler Imager since it shows the oscillations of the entire Sun. This instrument discov­ered a layer about a third of the way to the Sun’s center where the orderly interior, within which energy flows radially, transitions to the turbulent outer region, where energy moves in convective loops. This is the place where the solar magnetic field is created. Just as large-scale flows like the Gulf Stream and the jet stream are important for the Earth’s climate, SOHO data have shown that such flows are important for solar weather.

SOHO’s data is of such high quality that 3D maps of the Sun were derived for the first time. The maps answered questions that puzzled Galileo: how deep do sunspots extend and how can they survive for weeks at a time? The answers: they are fairly shallow but they are rooted in places where the plasma converges and strongly flows downward. SOHO scientists have managed the amazing trick of holographically reconstructing features on the far side of the Sun.26 All these images are available daily on the web. In fact, a plethora of solar data is available online, since a small armada of satellites is monitoring our life-giving star all the time.

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.