Category Dreams of Other Worlds

In August 1929, the New York Times science section ran an article titled “The Star Stuff that is Man.” Astronomer Harlow Shapley had been popularizing the point that humans are the mere by­products of stars. In a radio talk series a few years earlier produced by the Harvard College Observatory, Shapley pointed out that “we are made out of the same materials that constitute the stars.”26 In the Times article, Shapley similarly observed: “We are made of the same stuff as the stars, so when we study astronomy we are in a way only investigating our remote ancestry and our place in the universe of star stuff. Our very bodies consist of the same chemical

elements found in the most distant nebulae.”27 At the time, some were disconcerted to think that humans might be little more than the product of fission and fusion occurring in stars.28

Cosmologists have good evidence that all the hydrogen and most of the helium in the universe have existed since close to the time of the big bang. Hydrogen, the simplest element, is truly primor­dial, while most of the helium was formed soon after in a process known as primordial nucleosynthesis or big bang nucleosynthe­sis. “The primordial nuclei of the matter constituting the universe were formed in the first three minutes,” explains CERN theoretical physicist Luis Alvarez Gaume. “The cosmic oven produced a num­ber of nuclei, made up of about 75% hydrogen and 24% helium. Small amounts of deuterium, tritium, lithium and beryllium were also produced, but hardly any of the other atoms that make up our bodies and the matter around us: carbon, nitrogen, oxygen, sili­con, phosphorus, calcium, magnesium, iron, etc. All of these were formed in the cosmic ovens of subsequent generations of stars. As Carl Sagan put it, we are just stardust, remains of dead stars.”29

The Oxford English Dictionary lists a definition for the term stardust as “fine particles supposed to fall upon the Earth from space; ‘cosmic dust.’” The OED likewise cites one published com­mentary from 1879 that claimed “the very star-dust which falls from outer space forms an appreciable part” of the mud accumu­lated on the ocean floor.30 In fact, the estimate for interplanetary dust particles swept up each year by the surface of the Earth as it churns through space is a substantial 10,000 tons, but that’s a tiny fraction of the material added to the seafloor. The most pristine reservoirs of stardust or interplanetary dust are comets. Having formed early in the life of our Solar System approximately 10 mil­lion years after the Sun’s protoplanetary disc stabilized, comets spend most of their time in the cold extreme outskirts of the Solar System. As a result, comet nuclei are largely preserved from heat­ing, melting, and collisions with other planetary bodies. The ice composition and dust grains of comets reveal elements present in the Solar System’s primordial past.

Nuclear fusion reactions in stars create heavier elements by pro­cessing or fusing together lighter atomic nuclei. The base of the fusion “pyramid” is the fusion of hydrogen into helium, which oc­curs in the Sun and low-mass stars. Stars approximately the mass of our Sun can also synthesize helium, carbon, and oxygen, but thereafter, they never reach a dense enough or hot enough state to fuse yet heavier elements. Massive stars have the ability to fuse their hydrogen to produce helium, carbon, and oxygen, but then progress in successive stages of evolution to neon, magnesium, sili­con, sulfur, nickel, and iron. Iron is the most stable element, so typ­ically the fusion chain stops there. Still heavier elements are forged two ways: by the slow capture of neutrons in the atmospheres of massive stars and more rapidly by stellar explosions known as novae and supernovae.31 Since rarer, larger mass stars are required to generate the heaviest elements, these elements are cosmically scarce compared to the light elements hydrogen and helium.

The first generation of stars probably formed approximately 200-300 million years after the big bang.32 According to cur­rent theoretical ideas, none of those stars still exist and certainly none have yet been found. They have since exploded in novae or massive supernovae, collapsed into black holes, or have oth­erwise expended their fuel and burned out. The oldest stars cur­rently are generations removed from the first stars that shone in the universe.33 Astronomers estimate that approximately twenty – five novae occur each year in an average galaxy generating inter­stellar dust enriched with heavier metals. From the clouds of gas, dust, carbon, silicon, and metals produced by repeated generations of novae and supernovae, new stars and their attendant planets are born.

A nova is a sudden brightening of a star believed to occur when the outer layers of a star are pulled by gravity onto the surface of a companion white dwarf. Pressure due to the mass that accumu­lates on the white dwarf causes nuclear fusion of hydrogen into he­lium at its surface that in turn blows material off the surface of the white dwarf. One type of supernova is an extreme form of a nova, where mass acquired from a young companion star causes a white dwarf to begin carbon fusion and explosively detonate. Another type, a core collapse supernova, occurs when a single massive star exhausts its nuclear fuel and suffers a core collapse followed by explosive detonation. In comparison to a nova, a supernova will produce a million times the energy and can for weeks shine as brightly as all the stars of an entire galaxy. In the case of the Milky Way, that’s equivalent to about 400 billion Suns. A supernova is a prodigious alchemical event; one that detonated in a nearby gal­axy in 1987 generated enough stardust to build 200,000 Earths.34

The Milky Way formed approximately 10 billion years ago, but our Sun is only 4.5 billion years old. The Sun and its neighbor­ing stars were likely born in a nearby region of dense, coalescing interstellar gas and dust. Isotopic studies of meteorite samples in­dicate that our Sun formed from the detritus of a massive super­nova explosion in nearby space roughly 1 million years prior to the formation of the protoplanetary disc that became our Solar System.35 We know this simply by the presence of metals that our Sun could not have produced, such as the iron that makes up the Earth’s core. Other than hydrogen, which is primordial and dates back to cosmic genesis, almost all the other atoms in the universe have been recycled, silent witnesses to amazing trips through fiery cauldrons and into frigid space, some experiencing multiple adven­tures. Unfortunately, no atom bears the imprint of its particular passages through the core of a star; astronomers can only describe the origins statistically.

To judge the scientific contributions of Hipparcos, we start by rec­ognizing that measuring the positions of stars is both fundamental and unglamorous. It’s fundamental because it’s the key to measur­ing the physical properties of celestial objects. Positions are the keys to the trigonometric determination of distance, and distance is needed to calculate the size, mass, and intrinsic brightness of any planet, star, or galaxy.28 Without distances we’re stuck with the appearance of stars in the sky, and a star that’s far away and lumi­nous can appear to be the same brightness as a star that’s nearby and dim. That ambiguity is fatal to any reliable understanding of the denizens of the night sky. It’s unglamorous because measuring a position is the simplest and most obvious way to characterize a star: there’s no image, just two angles to identify a unique spot on the sky, with no units. Needless to say, the people who do such prosaic work don’t always get their due.

It was not always that way. On the spinning Earth, the measure­ment of star positions is critical for keeping time and navigating. Early cultures noticed and tracked star positions as if their lives depended on it, which they did! In the third century BC, Timo – charis and Aristillus produced the first star catalog in the Western world while working for the Great Library at Alexandria. About a century later, Hipparchus extended their work, generating a cata­log with 850 star positions. He also divided the stars into intervals of logarithmic brightness that form the basis for a system that as­tronomers still use. This was a natural way to classify brightness since the eye has a nonlinear or logarithmic response to light. Ptol­emy increased the catalog to 1,022 stars.29 These star catalogs are among the most impressive intellectual achievements of antiquity; later generations of admiring astronomers called Ptolemy’s stellar compendium Almagest, which means “greatest” in Arabic.30

As in many other aspects of astronomy, the torch for mapping stars was then taken up by Arabs for a millennium. Around AD 964, the Persian Abd al-Rahman al-Sufi wrote his Book of Fixed Stars, which depicted the constellations in glorious, natural color. Al-Sufi was the first to catalog the Large Magellanic Cloud and the Andromeda Nebula, two distant star systems whose true na­ture would not be fully understood until the 1930s. The pinnacle of pre-telescopic observations was reached by Tycho Brahe in the sixteenth century. Through relentless attention to detail and the control of systematic errors, he improved on the positional er­rors of earlier catalogs by a factor of fifty. His reputation didn’t suffer from doing these mundane measurements; Brahe was cel­ebrated in his lifetime and is considered the greatest observer before Galileo.

The big prize in astrometry was its use to measure the distance to a star. Stars are so far away that the apparent seasonal shift of a nearby star with respect to more distant stars—the effect called parallax—was not observable for the first two centuries of use of the telescope. Friedrich Bessel won the race to detect parallax by showing that 61 Cygni, one of the closest stars, was nearly 10 light – years away, or a staggering 60 trillion miles.31 Bessel didn’t have a university education, but his meticulous calculations elevated him to fame as one of the most noted scientists and mathematicians of the nineteenth century. The parallax shift is extremely subtle, and far more difficult to detect than the large-scale migration of constellations through the night sky as the Earth spins on its axis and orbits the Sun (figure 8.4). Almost all stars have parallax shifts over the course of a year of about one second of arc or less, and most stars visible to the naked eye have parallax shifts smaller than 0.1 second of arc. For comparison, each of the letters on an eye chart that defines 20/20 vision spans an angle of five minutes of arc, a 3,000 times larger angle.

Thereafter astrometry lapsed into the status of a worthy but dull aspect of astronomy. In part, this was because it was so chal­lenging to measure parallax; in the half century after Bessel’s mea­surement, new star distances were only added at the rate of about one per year. Through the twentieth century, photographic plates made it easier to capture and measure star positions, and Her – schel’s project to map the Milky Way galaxy was carried out by researchers in Europe and the United States. But the air blurs out the light of all stars to about 1 arc second in diameter (1/3600 of a degree), larger than the size of the angle that had to be measured to detect parallax. Refraction and telescope flexure also complicate a parallax measurement. Astronomers were bumping up against the limitations of the atmosphere and the only solution was to go into the pristine environment of space.

Gil Levin has never wavered. The principal investigator of Viking’s labeled release experiment is now approaching ninety, but he’s very active and stays current with research on Mars. Levin had an unusual career path, starting as a sanitation engineer before join­ing NASA. In addition to authoring 120 scientific papers, he owns fifty patents for items ranging from artificial sweeteners to thera­peutic drugs.46 Levin insists that at two locations on Mars, and in seven out of nine tests, his experiment detected biological activity. No purely chemical reactions have been identified that would fully reproduce the labeled release results, which explains the cautious and equivocal wording of the science mission summary.

Levin is in the minority, but he’s not alone in believing the Vi­king results bear revisiting. Rafael Navarro-Gonzalez and his team went to the highest and driest parts of the world, places like the Atacama Desert and Antarctic dry valleys, and duplicated the tests that Viking did more than thirty-five years ago.47 In northern Chile, they found arid soils with levels of organic material that would have been undetectable by the Viking instruments, yet there were bacteria in the soil. In other words, Viking was not sensitive enough to detect either organics or life in terrestrial locations that are the closest analogs to Mars.48 The team speculated that the organic material on Mars might have been too stable to turn into a gas, even at the blistering temperature of 500°C (932°F) reached inside the oven of the Viking experiment. They also noted that iron in the soil might oxidize the organic material and prevent its detec­tion by the mass spectrometer. This would account for the carbon dioxide that was detected in the experiments.

Mars is an alien chemical and physical environment, so it makes sense to think “outside the box” when considering how biology might operate there. If we only look for life as we know it, that’s all we’ll be able to detect and we may miss life with a different biochemical basis. Dirk Schulze-Makuch and Joup Houtkooper have speculated that Mars might be home to microbes that use water and hydrogen peroxide as the basis for their metabolism.49 Hydrogen peroxide is a toxic household disinfectant and seems implausible as a basis for life, but the researchers note that it’s more life-friendly under the extreme conditions typical of Mars. It attracts water and when mixed with water freezes at -57°C, yet it doesn’t form cell-destroying ice crystals at even lower tem­peratures. Hydrogen peroxide is tolerated by many terrestrial microbes and is the basis of the metabolism for Acetobacter per – oxidans. It’s used as a defensive spray by the bombardier beetle and even performs useful functions in the cells of some mammals. As Schulze-Makuch pointed out in 2007, “We can be absolutely wrong, and there might not be organisms like that at all. But it’s a consistent explanation that would explain the Viking results. . . . If the hypothesis is true, it would mean we killed the Martian mi­crobes during our first extraterrestrial contact, by drowning—due to ignorance.”50

Another speculation was spurred by the discovery of perchlo­rate in a Martian polar region by the Phoenix lander in 2008.51 Phoenix carried out a wet chemical analysis of Martian soil, find­ing it to be alkaline and low in the type of salts found on Earth, but it had enough perchlorate to act as antifreeze and allow the soil to hold liquids for short periods during the summer. Perchlorate is strongly oxidizing and so generally considered to be toxic for life, but some microbes metabolize it.52 In 2010, the Viking results were reinterpreted in the light of the Phoenix discovery, giving support to Gil Levin’s lonely position.53 When perchlorate was added to Atacama Desert soils and analyzed in the manner of the Viking samples, it released chlorine compounds. When these were seen in the Viking experiments back in the 1970s, they were presumed to be cleaning fluid contaminants from Earth. But if perchlorate is present in the mid-latitude Martian soil, it would explain the data. Since perchlorate becomes a strong oxidant when heated, it would have destroyed any organics and so explain why Viking didn’t detect any. It’s remarkable that the Viking legacy continues to provide so many surprises and unanswered questions.

A leitmotiv in the reinterpretation of the Viking biological ex­periments is the amazing range of life on Earth—ihere are mi­crobes that can tolerate or thrive in conditions that would be fatal to plants and animals. Collectively, forms of life found in physi­cally extreme conditions are called extremophiles. The envelope of habitability is much larger than we thought in the 1970s. There are microbes than can live below the freezing point of water and with toxic and alkaline conditions as seen on Mars. Even high doses of ultraviolet radiation are not an impediment to life, since mi­crobes can survive conditions in the upper stratosphere that match the radiation environment of Mars.54 In science, proof is the gold standard, but it’s a very high bar to clear, usually requiring copious amounts of evidence. While Viking didn’t find life on Mars, it was unable to prove the converse hypothesis, that biology is absent. It’s premature to declare Mars dead. Surprisingly, this harsh planet inspired a new way of thinking about life on Earth.

By the late 1960s and early 1970s, the Moon landings became the grand travel narratives of human history. Mars has subsequently emerged as the next great travel destination. There are multiple reasons why the planet so intrigues us. Through MER and other missions, Mars has become familiar and tangible. While our oceans obscure 70 percent of Earth’s topography, on Mars we can visually survey the entire surface, so that planetary scientists to some extent have a better understanding of its surface than of our home planet. Global Surveyor’s multi-colored elevation maps, for instance, re­veal that Mars’s northern hemisphere lies low in elevation com­pared to the southern highlands. Far fewer impact craters in the northern hemisphere may indicate that at one time this region was covered by a primordial sea. Prominent on the Tharsis bulge is the tallest volcano in the Solar System, Olympus Mons (figure 3.5), accompanied by three nearby shield volcanoes. Looking toward Mars’s north pole, Arsia Mons is the most southern, then Pavonis Mons, and finally Ascraeus Mons. These adjacent volcanoes occur in a chain-l ike sequence reminiscent of the tectonic plate move­ment that produced the Hawaiian Islands, which raises the ques­tion of whether geothermal processes are still at work on Mars.

Moreover, extremophiles may thrive in possible underground water reserves, deep in the crust, or at the Martian polar caps. Re-

searchers have detected methane in the atmosphere, which could suggest active volcanism or a more exciting, possible microbial source.49 Despite Mars’s tenuous atmosphere, images by the Mars Reconnaissance Orbiter indicate groundwater sporadically breaks through the surface to briefly flow down steeply sloped terrain, and data from the 2008 Phoenix Lander indicate its soil is simi­lar in acidity to Terran soil. Added to this are recent findings that ancient fossilized life on Earth, dating to more than 3.4 billion years ago, metabolized and lived on sulfur instead of oxygen, as Earth’s atmosphere was not then oxygen-rich.50 All of this suggests that Mars could harbor life deep within its ruddy dirt. If the Mars

Science Laboratory’s rover Curiosity finds evidence of ancient or extant extremophiles, we might discover how life emerged on the early Earth.

The other tremendous appeal of Mars is that it is the only planet astronauts could travel to, and explore, in the near future. Allud­ing to the passion with which we dream of stepping foot on worlds beyond our Moon, Ray Bradbury has observed, “We know not why we thread an architecture of travel in a fiery path across a winter space and warm far worlds with our breath.”51 Perhaps we do so simply because we survive on our own relatively cold planet only as a result of technology. Anthropologist Ben Finney points out that we inhabit the globe, from pole to pole, solely with the aid of our technological ingenuity. Finney contends that the Moon or Mars represent a logical next step for human migration. He re­searched the ocean routes Polynesian peoples navigated via canoe over as much as 1,000 miles of open ocean, from South China and Southeast Asia to Hawaii and other islands, to demonstrate how humans might hopscotch to nearby planets. Finney has shown that Polynesian peoples sailed with all the supplies they needed to es­tablish themselves on islands that they could have only speculated existed but had never charted or seen. This was an enormous feat without even the aid of a simple compass or sea charts. And yet, they accomplished it using horizon markers and the rising and set­ting of the Sun and stars.

The Polynesians succeeded, asserts Finney, precisely because “[t]hey did not regard the ocean as an alien environment, but one which was utterly natural—and essential to the spread of human life.”52 Even as ancient seafarers ventured far beyond known shores to settle uninhabited islands in the wastes of the Pacific, Finney ar­gues that we could and should strike out across the Solar System. The extremely challenging act of humans venturing to Mars or an asteroid could mark the first step. Noted physicist Stephen Hawk­ing would agree. Hawking contends that the future of our species depends on our becoming a truly space-faring species as a means of surviving natural or human-produced global catastrophes.

Archaeologist Peter Capelotti observes that extreme explora­tion, “begun more than two million years ago with a few rough stone tools, in what would become Africa’s Olduvai Gorge, would culminate in Apollo 11.” Our desire for travel and exploration will take us far beyond Earth, suggests Capelotti:

By the time a human finally claimed to have stood at the North Pole, that icy coordinate had come to signify—and continues to represent—the human search for something larger in our culture and ourselves: the sense and the knowledge that our journey is a continu­ous one, and that the geographic “firsts” we credit to ourselves are merely waypoints on a continuum begun two million years ago. If our culture. . . (and, increasingly, our robots) allow it, this path may con­tinue for millions of years more. We have set human feet upon only one North Pole; in space, there are millions of North Poles waiting to be discovered.53

As of this writing, Opportunity continues sampling the Martian terrain, and with each turn of its wheels, we take in more of the landscape, not just in images downloaded to NASA’s archives or as attached links to Google Mars, but into the human imagina­tion and into our dreams. As Capelotti indicates, there are many mountaintop vistas in our Solar System yet to be glimpsed, through the stereoscopic eyes of our robotic co-explorers and, eventually, through our own eyes. The sublime heights of Martian volcanoes entice the explorer in us. Mountain climbers and space enthusiasts dream of summiting Olympus Mons, rising 69,480 feet above the Martian surface, and imagine the sweeping vistas they would take in from the rim of its calderas, or from the top of nearby Ascraeus Mons reaching 59,699 feet, or Arsia Mons’s summit of 57,085 feet.54 Our tallest peaks above sea level pale in comparison, with Mount Everest reaching a mere 29,035 feet. Rock climbers like­wise anticipate the grandeur of venturing into Valles Marineris, ex­tending more than 2,500 miles in length and whose canyon walls soar from three to six miles high. But we don’t have to wait for a brave explorer to rappel over the six-mile vertical scarp of Olym­pus Mons. Robotic eyes are even now taking us there.

The journey to, and across, Mars’s cracked and cratered ter­rain is one of the great travel narratives of the twenty-first cen­tury. With our rovers, we have alighted upon and are motoring through its fascinating desertscapes. In July 1997, the Pathfinder Mission successfully landed the Sojourner Rover at Ares Val – lis. Project scientist Matthew Golombek commented that, at the time, “an entire generation had never witnessed a landing on an­other planet” and reported that within the first month of opera­tion, roughly 566 million people accessed the mission website.55 The twin MER rovers captivated even larger audiences. As sci­ence studies scholar Robert Markley points out, “Sojourner had traveled a few yards to take readings of the mineral composition of several rocks; the 2004 rovers provided a photographic record that seemingly mimicked the experience of humans walking across a Martian landscape reminiscent of terrestrial deserts.” Markley characterizes Spirit and Opportunity’s images as “the first vacation photographs from another world” that in turn have become “part of the semiotics of human visualization.”56 Through these rovers, we’ve sampled the planet’s sedimentary and volcanic rocks, as well as its ubiquitous dust. We’ve sniffed the air, measured barometric pressure, tasted the soil to determine its alkaline or saline qualities, and bored into rocks searching for fossilized evidence of water. On balmy afternoons, by Martian standards, we glimpsed the si­multaneous dance of multiple dust devils. We’ve witnessed silent sunrises, seen morning fogs cling to the valleys and dissipate, and watched from Husband Hill through Spirit’s cameras as the Sun sank below the horizon. In winter, the rovers weathered the cold, dim days and we too waited them out. Rather than our rovers exploring with us, it seems that we’ve tagged along with them as they traverse, photograph, and engage with this captivating and stark landscape.

Humans have a presence on Mars, even if for now it is limited to remote-controlled rovers ambling over its arid plains. Little could Steve Squyres know that his sturdy MER rovers would continue doing science for years beyond their ninety-day mission. Squyres wonders about the astronauts who might someday find the derelict rovers: “Above all, I simply hope that someone sees them again,” writes Squyres. “Spirit and Opportunity have become more than just machines to me. The rovers are our surrogates, our robotic precursors” until astronauts someday leave their “boot prints in our wheel tracks at Eagle Crater.”57 Those astronauts are just as likely to find remnant tracks of Mars Science Laboratory’s rover Curiosity. If W. H. Auden was right in his poem “Moon Landing” about the eventuality, from the first flaked flint, of humans walking on the Moon, then boot prints on Mars seem equally probable— even inevitable.58

The seashore marks a liminal space and appeals to us in part due to the sheer tenacity of life to survive there despite its harsh envi­ronment. Marine zoologist Rachel Carson characterized the sea­shore as a harsh but vibrant biome. Known primarily for her book

Silent Spring, Carson’s area of expertise and real passion was the ocean. She published three best-selling books on the topic, but The Edge of the Sea specifically focused on the rigor necessary for spe­cies to survive in the hostile terrain of the shoreline.17 Describing the seashore as “the place of our dim ancestral beginnings,” Car­son writes:

Only the most hardy and adaptable can survive in a region so mu­table, yet the area between the tide lines is crowded with plants and animals. In this difficult world of the shore, life displays its enormous toughness and vitality by occupying almost every conceivable niche. Visibly, it carpets the intertidal rocks; or half hidden, it descends into fissures and crevices, or hides under boulders, or lurks in the wet gloom of sea caves. Invisibly, where the casual observer would say there is no life, it lies deep in the sand, in burrows and tubes and pas­sageways. It tunnels into solid rock and bores into peat and clay. It encrusts weeds or drifting spars or the hard, chitinous shell of a lob­ster. It exists minutely, as the film of bacteria that spreads over a rock surface or a wharf piling; as spheres of protozoa small as pinpricks, sparkling at the surface of the sea; and as Lilliputian beings swimming through dark pools that lie between the grains of sand.18

Celebrated anthropologist and naturalist Loren Eiseley, in his essay “The Star Thrower,” similarly contemplated the formidable environment of the shoreline in Sanibel, Florida, which he de­scribes as “littered with the debris of life”: “Shells are cast up in windrows; a hermit crab, fumbling for a new home in the depths, is tossed naked ashore, where the waiting gulls cut him to pieces. Along the strip of wet sand that marks the ebbing and flowing of the tide, death walks hugely and in many forms. Even the torn fragments of a green sponge yield bits of scrambling life striving to return to the great mother that has nourished and protected them.”19

Eiseley recounts walking along the beach and encountering a man picking up starfish washed ashore by the rugged waves. Eise – ley pauses and together the men notice a starfish that “thrust its arms up stiffly and was holding its body away from the stifling mud.” The man picks up the star and tosses it beyond the breakers back into the sea. At some point, Eiseley joins in the effort to save at least a few of the beached starfish. He describes lifting up one star, “whose tube feet ventured timidly among my fingers while. . . it cried soundlessly for life.”20 In such prose Eiseley offers a snap­shot of life’s tenacity to survive in a threshold landscape. Evolved approximately 500 million years ago, starfish are ancestors to more complex organisms, including us. In that momentary em­brace between Eiseley’s fingers and the tube feet of the starfish, life primordial touches the present as two beings communicate a sim­ple and mutually understood message, each wishing only to cling to life. Such vivid depictions of species surviving by a bare foot­hold, or tubed foothold, inspire us to imagine the distant shores on Titan, with its carved river channels and methane lakes, and won­der what microbes or variants of terrestrial mollusks might subsist on its frozen shorelines. While Bonestell painted his spacescapes at the dawn of the Space Age, we have since sent our spacecraft bil­lions of miles from home, and in time we’ll seek out the answers to what must have been Bonestell’s questions regarding life-forms that might cling to the rocks and crevasses on Saturn’s icy moons.

By number of atoms, humans and all other living creatures are roughly 63 percent hydrogen, 26 percent oxygen, 10 percent car­bon, and 1 percent nitrogen.36 Not only the iron in your blood, or the calcium in your teeth and bones, but every living organism is made from recycled stardust. John and Mary Gribbin emphasize that “the nitrogen in the air you breathe and in your DNA (along with most of the carbon in your body) had a previous existence as part of a planetary nebula, and was expelled from one or more red giant stars.” The lead in a pencil, potassium in bananas, zinc and selenium in vitamins were all forged in stars. Nickel in coins, oxy­gen in Earth’s atmosphere, mercury in a thermometer, as well as precious metals such as gold and silver are examples of the heavy elements forged as stars blow off their outer layers and die. “This is where neon in neon lighting, the sodium in common salt, and the magnesium used (appropriately) in fireworks comes from—carbon burning inside stars.”37 And, ultraviolet starlight irradiating the en­velope of gases surrounding nearby dying stars can produce large volumes of water, essential to the formation of Earth’s atmosphere and oceans and a key marker for life as we know it.38

The composition of a comet is the frozen record of the chemical environment of our region of the Milky Way as the Sun formed, the sum of generation upon generation of star birth and death. When a comet comes near the Sun, the energy reanimates chemi­cals that have been interred from 4.5 billion years. Fittingly, the comet lights to form a feast for our eyes, in a pale echo of the blinding stellar light in which those atoms formed, long before there were eyes to see them.

By the 1980s, astronomers had convinced their funding agencies that a space observatory to measure star positions in the vacuum of space would be a good investment. The High Precision Parallax Collection Satellite (Hipparcos) went through a series of design studies with the European Space Agency and was launched in 1989 on an Ariane 4 rocket from French Guiana.32 Hipparcos is the only facility in this book not supported or operated by NASA, but its importance transcends its country of origin, and U. S. astronomers have used it extensively for their research. National boundaries melt away in the night sky and international collaboration is the lingua franca of astronomy. Indeed, the stars belong to no one and yet to everyone.

The telescope that transformed the precision with which as­tronomers can map the sky was only 29 centimeters in diameter, not much larger than a dinner plate. Many amateur astronomers use bigger glass for the mirrors of their handmade telescopes. Its mission lasted for just three and a half years, from August 1989 to March 1993, yet the data are still generating scientific results and publications twenty years later.33 Hipparcos was one of the last space missions before the advent of CCD detectors. The satellite swept its gaze across two widely separated patches of sky and the starlight fell on a set of alternating transparent and opaque bands and then onto an old-fashioned photomultiplier tube. The primary goal of the mission was to measure the positions of 100,000 stars with an accuracy of 0.002 arc seconds. How small is this angle?

Five hundred times smaller than the typical angle by which a star image is blurred out by the Earth’s atmosphere, or equal to the angle made by lines to the two opposite sides of a penny in New York as seen from the apex of the triangle in Paris.

Imagine a great city ringed by a fence. It’s nighttime and you’re outside the tall fence looking in. As you walk around the fence, the lights of the city will appear to flicker on and off as they pass behind the slats of the fence and then reappear in the gaps. Now imagine a somewhat different situation: you’re inside the city and wearing a hood. The hole for each eye is covered with extremely thin vertical slats, like a miniature fence. As you turn, the lights from the streets and buildings brighten and fade as they pass in between and behind the slats. Hipparcos worked in this way, scan­ning a great circle on the sky every two hours, with its two imaging fields, or eyes, seeing a particular star 20 minutes apart. The preci­sion of the measurement came about because the angle between adjacent slats was only one arc second, and then combining a hun­dred or more observations of the same star gave a much smaller angular error. In addition to using the data to measure positions, astronomers used the repeated observations to search for variabil­ity in the light of hundreds of thousands of stars as Hipparcos pivoted to scan the entire sky.

A substantial legacy of Viking is that scientists have gained a better sense of the finely tuned ballet of biogeochemical cycles that sus­tain Earth’s vibrant biosphere. The Viking missions unexpectedly became integral to the late-twentieth-century view of the Earth’s biosphere as a self-sustaining system produced by biota interacting with the planet’s geochemistry. Brian Skinner and Barbara Murck assert that future historians will consider discovery of the com­plex interactions between Earth’s biota and geologic, hydrologic, and atmospheric cycles among the most significant scientific con­tributions of the twentieth century.55 This sea change in thinking about our planet began to emerge in the 1960s and was sparked by British scientist James Lovelock’s Gaia hypothesis. That idea later

evolved into Gaia Theory and the academic discipline referred to as Earth System Science.

In the early 1960s, Lovelock was working for NASA’s Jet Pro­pulsion Laboratory with other planetary scientists to develop the experimental means for determining whether microbial life might exist on Mars. NASA’s plan for a mission to robotically explore Mars in search of life was initially titled Voyager, not to be con­fused with the interplanetary mission launched in 1977. That plan was subsequently scrubbed and reconfigured into what became the Viking mission. In 1965, while helping to develop life detection experiments for the Mars landers, Lovelock came to the sudden realization that Earth’s atmosphere must be a natural extension, and a by-product, of Earth’s biota. This became the basis for the Gaia hypothesis and a paper Lovelock published in the prestigious journal Nature that year.56

In his first book, Gaia: A New Look at Life on Earth (1979), Lovelock, then sixty years old, explained that as he mulled over ways one could detect organisms in the Martian soil, he turned to our own planet and began to imaginatively “look at the Earth’s atmosphere from the top down, from space.” In the opening sen­tence, Lovelock observed: “As I write, two Viking spacecraft are circling our fellow planet Mars, awaiting landfall instructions from the Earth. Their mission is to search for life, or evidence of life, now or long ago. This book also is about a search for life.”57 The organisms on Lovelock’s mind, however, were those on Earth.

Lovelock thought it might be possible to answer the question of whether Mars harbored microbes by simply examining the com­position of the atmosphere. If a planet supported life, Lovelock posited, its atmosphere would be shaped in part by biota “bound to use the fluid media—oceans, atmosphere, or both—as conveyer belts for raw materials and waste products. . . . The atmosphere of a life-bearing planet would thus become recognizably different from a dead planet.”58 A mix of reactive atmospheric gases like ox­ygen and methane, Lovelock surmised, would be a biosignature of life as on Earth. Moreover, in 1965, researchers at the Pic du Midi Observatory in France reported that the atmospheres of Venus and Mars were largely made of carbon dioxide.59 Lovelock knew that Earth’s atmosphere, by comparison, contained reactive gases that dissipate if not continuously replenished by Earth’s biota. The Pic du Midi observations seemed a sure confirmation to Lovelock, who was then collaborating and publishing with NASA colleague Dian Hitchcock on analyses of infrared surveys of the Martian atmosphere.60 Methane in our atmosphere “has been fairly con­stant as ice-core analyses prove, for the past million years, as has oxygen,” notes Lovelock, who highlights the fact that “for such constancy to happen by chance is infinitely improbable” and there­fore must be sustained by life.61 While Lovelock set the wheels in motion for a systems approach to understanding the biosphere, it was his collaboration with microbiologist Lynn Margulis that formalized the Gaia hypothesis.

Margulis was just then developing her theory of symbiogen – esis, which posits that cell structures, and ultimately organisms, evolved from symbiotic relationships between progenitor cells or organisms. She is celebrated for her research in early cell evolution and was first to identify bacteria as the antecedent of chloroplasts and mitochondria in eukaryotic cells. Margulis also was interested in how bacteria and other microorganisms might impact their en­vironment. As it happened, Lovelock shared an office at JPL with planetary scientist Carl Sagan (figure 2.4). Lovelock biographers John and Mary Gribbin comment: “Margulis had independently become intrigued by the oddity of the Earth’s oxygen-rich atmo­sphere and asked her former husband, Carl Sagan, whom she ought to discuss the puzzle with. Sagan knew just the man, and put her in touch with Lovelock.”62 Upon Sagan’s recommendation, Lovelock and Margulis began exploring the question of how the highly reactive gas oxygen in our atmosphere has been sustained at a consistent level over billions of years.

There’s a NASA website where you can follow the two most distant human artifacts as they sail into the void of space. The real-time odometers for the Voyager 1 and Voyager 2 spacecraft flick silently upward. Single kilometers are a blur; even the tens of kilometers digit changes too fast to follow, while the hundreds of kilometers digit ratchets up by one every few seconds. These large and rapidly growing numbers are mesmerizing in the same way as counters of the national debt or the world’s population; numbers this large are difficult to fathom. By late-2012, Voyager 1 was 18.4 billion kilo­meters or 11 billion miles from Earth, and its near-twin Voyager 2 was 15 billion kilometers or 9 billion miles from Earth. Their feeble radio signals take more than a day to reach the Earth as the probes streak through space at approximately 58,000 kilometers per hour, or roughly 36,000 miles per hour.1

To see why these spacecraft represented such a leap in our voy­aging through space, consider a scale model of the Solar System where the Earth is the size of a golf ball. On this scale, the Moon is a grape where the two objects are held apart with outstretched arms. That gap is the farthest humans have ever traveled, and it took $150 billion at 2011 prices to get two dozen men there.2 Mars on this scale is the size of a large marble at the distance of 1,100 feet at its closest approach. As we’ve seen, it took an ar­duous effort spanning more than a decade before NASA success­fully landed a probe on our nearest neighbor. A very deep breath

is needed to explore the outer Solar System. In our scale model, Jupiter and Saturn are large beach balls 1.5 and 3.5 miles away from Earth, respectively, and Uranus and Neptune are soccer balls 7 and 12 miles from the Earth. This large step up in distance was a great challenge for spacecraft designers and engineers. On this scale, the Voyager 1 and 2 spacecraft are metallic “motes of dust” 48 and 37 miles from home, respectively.

The great thirteenth-century polymath and Dominican friar Albertus Magnus, like many before him, wondered about other worlds. He framed the issue in a way that would be familiar to a modern scientist, saying it was “ . . . one of the most noble and ex­alted questions in the study of Nature.”3 Before the Voyager space­craft did their “Grand Tour” of the outer Solar System, the gas giant planets were ciphers, barely resolved by the largest ground – based telescopes. Imagine trying to see details on beach balls and soccer balls that are miles away. Appetites had been whetted by the Pioneer 10 and 11 probes, which flew by Jupiter in 1973 and 1974, with Pioneer 11 going onward to Saturn in 1979, but the twin Voyagers promised to send back much sharper pictures. In the 1970s, theory suggested that the gas giants were spheres of hydrogen and helium similar in composition to the Sun. If they had solid cores at all, the surfaces would be at temperatures of tens of thousands of degrees and pressures many millions of times that at the Earth’s surface.4 Their moons were assumed to be inert and uninteresting rocks like Mercury or the Moon. The word “world” comes from the Old English woruld, referring to human existence and the affairs of life. Yet the outer Solar System seems inhuman and inhospitable for life.

Or is it? Just a year before the launch of Voyager, Cornell Uni­versity astronomers Carl Sagan and Ed Salpeter published a pro­vocative paper in which they argued that free-floating life-forms might populate the temperate upper reaches of the gas giants.5 The authors pushed the concept of life far beyond the bounds of ter­restrial biology; aerial “gas bags” sounded like a conceit of sci­ence fiction, but at the time no one could prove them wrong. Like explorers venturing into terra incognita, nobody knew what the Voyagers might find.

In October 1997, a six-ton spacecraft the size of a school bus set off on a billion-mile journey to Saturn. It was named after the seventeenth-century Italian astronomer Giovanni Domenico Cas­sini, who discovered four moons of Saturn—Tethys, Dione, Iape – tus, and Rhea—along with co-discovering Jupiter’s Great Red Spot and first spotting the gap in Saturn’s rings that bears his name. Cas­sini, along with its deployable probe Huygens, is one of the most complex and ambitious missions in NASA’s history.21 Scientists can spend their entire careers working on a planetary probe like Cas­sini. The first concept was floated in 1982 and it got a boost in the late 1980s when it was conceived of as a joint mission with the European Space Agency. That helped it survive budget-cutting by Congress in the early 1990s. More than thirty years after it was first conceived it’s still going strong. Cassini is an exemplar of international collaboration in space. More than five thousand scientists and engineers in seventeen countries have worked on the mission. It’s still the heaviest spacecraft NASA has ever launched to a destination beyond the Moon.

To date, Cassini-Huygens has cost about $3.5 billion, and the very high cost of complex planetary probes makes many people flinch. It was Cassini’s ballooning budget in part that made incom­ing NASA Administrator Dan Goldin embrace a faster, better, and cheaper approach in the early 1990s.22 During the Goldin years, NASA launched nearly 150 payloads at an average cost of $100 million, with a failure rate of less than 10 percent. But the move to crank out more frequent, stripped-down missions at lower cost was not universally popular; a NASA report in 2001 argued that the strategy had cut too many corners and produced an unaccept­ably high failure rate.23 The public took notice when the high pro­file Mars Climate Orbiter and the Polar Lander missions failed. The former was notoriously lost due to a failure to convert English to metric units, but the latter was successful in a later incarnation as the 2007 Phoenix mission to Mars.

The debate may be a false dichotomy. Special-purpose missions such as the recent LCROSS probe to the Moon and Mars Global Surveyor are only designed to do one thing. Billion-dollar missions typically have a dozen instruments and are very versatile; they’re like the Swiss army knives of the space program. Among them, the two Voyagers and Galileo lasted twelve years beyond their design lifetimes and have fulfilled all their scientific goals. Cassini has fin­ished its primary mission and in 2008 it was approved for a two – year extended phase called the Cassini Equinox Mission. In 2010, it was extended until at least 2017 and renamed the Cassini Sol­stice Mission. These names reflect the fact that the spacecraft will have witnessed an entire cycle of Saturn’s seasons by late 2017. So far, more than 1,500 research papers have been published based on Cassini and Huygens data, making this the most productive plan­etary probe ever. The mission will end with a dramatic flourish. Current plans call for Cassini to dive inside Saturn’s rings on Sep­tember 15, 2017, orbit Saturn twenty-two times, and then plunge to its death in the atmosphere. One hopes a musician will be in­spired to write a suitably operatic theme for this planetary finale.