Category Dreams of Other Worlds

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.

Robert Burnham points out that it was not until the Space Age that astronomers realized just how pervasive cometary and asteroid impacts are throughout the Solar System. Beyond the Moon’s cra­tered and craggy surface, NASA’s planetary science missions have charted numerous bodies in our star system riddled with impact craters. Those missions reveal, explains Burnham, “that by an over­whelming margin the most common feature in the Solar System is the impact crater. Entire planets, moons, and asteroids turned out to be covered with them, at all scales ranging from global to micro­scopic. Craters or their traces can be found on every solid surface from Mercury to Pluto and surely beyond.”39 It’s likely the object that devastated the Siberian taiga near the Stony Tunguska River at approximately 7:15 a. m. on June 30, 1908, was a meteorite. However, the unknown projectile was not made of iron like the meteorite that excavated Meteor Crater in Arizona approximately 50,000 years ago. It is estimated the impactor exploded above ground with the force of 1,000 atomic bombs, more than adequate to destroy a modern city. The explosion reportedly flattened 830 square miles of forest, leveling 30 million trees, and was heard 500 miles away. Multiple remarkable witness accounts of this event included a man, 40 miles from the impact site, who reported being knocked off his feet by a searing blast wave.40 Atmospheric pressure waves registered on barographs after racing around the world at hundreds of miles per hour. The explosion apparently caused a local geomagnetic storm that lasted for days. People in London wondered about the strange, glowing atmospheric effects and pink-fringed clouds appearing in evening skies. One British meteor specialist published his observations in Nature in 1908, saying that the night sky over Bristol, England, was so bright few stars could be seen.41

In July 1994, Comet Shoemaker-Levy 9 broke into fragments that subsequently slammed into Jupiter with an estimated explo­sive force of six hundred times humanity’s entire nuclear arsenal, and spewed plumes of material thousands of miles above Jupiter’s clouds. Millions globally, including astronomers, world leaders and politicians, and a fascinated public, found themselves capti­vated by television news reports and images streaming from JPL’s designated website. That single comet impact raised global aware­ness of the very real danger such events pose for Earth’s popu­lations (figure 6.4). The comet began to impact Jupiter on July 16. Within three days, the U. S. House Committee on Science and Technology voted to establish a NASA program to track comets and asteroids that could threaten the Earth. As co-discoverer of the comet David Levy recalls, “This vote reflected the nation’s fascina­tion, and its growing awareness that such a trail of destruction could have been headed at Earth. The initiative was designed to protect future generations of people.” Should the threat of a comet or an asteroid impact be successfully deflected, Levy contends the global attention focused on Comet Shoemaker-Levy will have in effect “rescued our planet.”42

Though the devastation of comet impacts is obvious, these pri­mordial snowballs may very likely be the reason for life on Earth. Given their ubiquity, it’s believed that cometary impacts are a com­mon vector by which life may have emerged on Earth. As noted previously, researchers working with the Stardust data reported finding the amino acid glycine in Comet Wild 2 samples. Amino acids are comprised of what is commonly referred to as CHON, or carbon, hydrogen, oxygen, and nitrogen atoms, and the molecules that CHON, along with phosphorus and sulfur, can combine to

make are all chemical ingredients of DNA. Comet and meteorite impacts on Earth could have dispersed such organics or even syn­thesized them.43

Science writer Connie Barlow claims: “Our ancestors include ancient stars. Stars are part of our genealogy.”44 She means that if we want to discover our origins, we can’t simply peer into a micro­scope at possible progenitor organisms. Instead, we need to look to the stars, where heavy elements necessary for life, like carbon, are forged. But we also can attribute our genetic makeup to com­ets and asteroids that may have deposited on Earth the chemistry for life. All known organisms are the products of their DNA, and some components of DNA have been detected in meteorites. In 2011, NASA scientists reported finding in meteorites “adenine and guanine, two of the four so-called nucleobases that, along with cytosine and thymine, form the rungs of DNA’s ladder-like struc – ture.”45 DNA is the coded information in our cells that determines biologically who and what we are. “At the center of the ladder-like DNA molecule lie ring-like structures called nucleobases. It’s these tiny rings that scientists at NASA and the Carnegie Institution for Science in Washington found in 11 of 12 meteorites they scruti­nized.”46 Meteorites, if not jettisoned from a neighboring planet, are remnant chunks of rocky debris left over from the Sun’s for­mation. Given that such meteorites deliver organic molecules to Earth, molecules from our star’s earliest days are inevitably in our DNA. Scientists reported in the journal Nature having found fossils of multicellular life dating back 2.1 billion years, roughly 1.5 billion years prior to the Cambrian explosion. These fossils in black shales in West Africa rewrite the history of when multicel­lular organisms first emerged.47 Microbial life on Earth predates even this. Biologists, astronomers, and astrobiologists now faced with recalibrating the timescales on which life began on Earth are reworking our appreciation for comets and their critical role in dispersing stardust throughout the Solar System.

Stardust’s mission to capture the interplanetary dust grains that pervade our star system is an attempt to understand not only how our Sun originated and evolved, but also the characteristics of the early Solar System that led to the origin of life. What we learned from the Stardust mission is that comets and asteroids harbor the building blocks of life. Twenty different amino acids, occurring naturally in a wide number of arrangements, produce the proteins that create all living organisms on Earth. John and Mary Gribbin report, “Formic acid (the stuff some ants squirt out as a defensive weapon, and the stinging ingredient in stinging nettles) and metha – nimine are two of the polyatomic organic molecules that have been identified in dense interstellar clouds. Together, they combine to form an amino acid, glycine.”48 As James Lovelock, originator of Gaia Theory, has observed, “It seems almost as if our Galaxy were a giant warehouse containing the spare parts needed for life.”49 Perhaps the universe is built for life. The Gribbins suggest that the chemistry for life is endemic to the entire universe. They assert that “one of the most profound discoveries made by science in the twentieth century” is that the universe comprises “the raw materi­als for life, and that these raw materials are the inevitable product of the processes of star birth and star death.”50

Though the normal matter that makes up stars, planets, and stel­lar dust clouds constitutes only a small fraction of the total matter in the universe, most of which is dark and still mysterious, the stuff of stars nevertheless provides all the components necessary for life. “The raw material from which the first living molecules were assembled on Earth was brought down to the surface of the Earth in tiny grains of interplanetary material, preserved in the fro­zen hearts of comets,” write Gribbin and Gribbin, who eloquently observe, “Those grains themselves literally, not metaphorically, formed from material ejected by stars. The ‘manna from heaven’ that carried the precursors of life down to the surface of the Earth was literally, not metaphorically, stardust. And so are we.”51 That we are made of stardust is not to be disparaged. It’s likely that life has emerged on planets orbiting other stars in galaxies far, far away. If so, it’s certain that the blood in their veins and the calcium in their bones, should they have them, were forged in the fires of their suns.

For a direct sense of what Hipparcos learned, ESA offers a sky map on its website that can easily fit in the palm of your hand.34 The “Hipparcos Star Globe” represents the brightest stars and the major constellations measured by the satellite as charts that can be printed out on two sheets of paper and then assembled into a sky sphere. In fact, for ease of construction, the sky is projected onto an icosahedron, a polyhedron with 20 triangle-shaped faces. Instructions for constructing this astronomical origami are also provided on the ESA website. This simple sky chart conveys no more than the “bony skeleton” of the night sky’s stars; Hippar – cos mapped the anatomy in exquisite detail. Its main instrument charted the positions of 118,218 stars with the highest precision.35

In addition, a beam-splitter was used with a secondary detector to map out the sky with slightly lower precision—the resulting Tycho Catalog lists 1,058,332 stars.36 Years after the satellite ceased op­eration, astronomers produced the definitive Tycho-2 Catalog, containing a prodigious 2,539,913 stars.37 That number is 99 per­cent of the stars down to 11th magnitude, which is a level 100,000 times fainter than the brightest star, Sirius. With this exquisite level of detail, Hipparcos has mapped our location in the “city of stars” called the Milky Way galaxy (plate 14).

Space missions produce data of such complexity and abundance that it’s often years before all the results are known. Hipparcos is no exception. The Tycho-2 Catalog was published in 2000, seven years after the satellite returned its last data. As recently as 2007, Dutch astronomer Floor Van Leeuwen re-analyzed the Hipparcos data. He diagnosed many small effects that had been overlooked in the original analysis, such as tiny jogs in the spacecraft’s orienta­tion due to micrometeorite impacts and subtle changes in the image geometry each time the satellite went into Earth’s shadow and then emerged into sunlight. He also took advantage of great gains in the power of computers to improve the calculation of positions. With a million stars, the number of angles between any star and all of the others is a million squared, or a trillion. To pin down the errors in positions, those trillion angles must be calculated many times, a procedure that took six months at the end of the mission but only a week when Van Leeuwen did his work using much faster proces­sors. His analysis has shrunk the errors by a factor of three from the initial goal of 0.002 arc seconds, and a factor of ten for the brightest stars.38 This minuscule angle would be formed by draw­ing lines from the top and bottom of Lincoln’s eye on a penny in New York and having them come to an apex in Paris.

The trick of Hipparcos is to measure positions across the entire sky rather than picking off stars one by one. Thus it gains from the power of large numbers. Imagine you had to cover the floor of a large room with irregular but similar-sized tiles. You could lay them out tile by tile with a good chance of keeping the separa­tion of adjacent tiles uniform, but as you covered a larger region it would become very difficult to control the uniformity. Whether you worked from the center out or the edges in or from side to side, it’s likely you’d either have tiles left over or leave a gap. The optimum solution would be to be aware of the distance between any two tiles and regulate it over the whole area, thereby filling it uniformly. If the tiles are now stars on the sky, that’s what scien­tists did with the Hipparcos data. They made an optimal solution for all stars simultaneously. In practice it was a calculation that taxed the best computers at that time.

Behind the numbers are the people who work to make a mission successful, often devoting their entire scientific careers to the task. Michael Perryman was born in the dreary industrial town of Luton, just north of London, and he was interested in math and numbers from an early age. His math teacher at school advised him to study a subject with better employment potential. He ignored the advice and studied theoretical physics at Cambridge University. For his PhD he stayed at Cambridge but switched to radio astronomy, joining a group that was still buzzing with excitement from the discovery of pulsars and the award of the Nobel Prize in Physics to Martin Ryle and Tony Hewish in 1974. At this point he has spent thirty years of his career on the unglamorous but very important work of mapping star positions, exceeding the amount of time the illustrious Tycho Brahe spent on his observations.39 Appropriately, in 2011 he was awarded the prestigious Tycho Brahe Prize by the European Astronomical Society.

Perryman was just twenty-six when he was selected to be the project scientist for the Hipparcos mission, a great honor and re­sponsibility for someone so young.40 Soon he found himself re­sponsible for the coordination of two hundred scientists and for all the headaches that go along with a complex multinational project. The biggest challenge came soon after launch when a motor on the Ariane launch rocket malfunctioned and the satellite didn’t reach its desired geostationary orbit. The unplanned orbit exposed Hip- parcos to high levels of radiation twice a day and it was thought the satellite might not last more than a few months. The team adjusted and made the best of the situation, but for over two years Perry­man lived under a “sword of Damocles” as gyros were knocked out by the radiation. In the end, the mission exceeded its design goal of both lifetime and science. Away from the project, Perryman enjoyed hiking and caving, choosing to escape underground from his upward-looking day job.

Together Lovelock and Margulis theorized that Earth’s habitability was not simply a function of its orbital position relative to the Sun, but was in large part due to biota metabolizing and cycling atmo­spheric gases and rock minerals. As Margulis explained, “The me­tabolism, growth, and multiple interactions of the biota modulate

the temperature, acidity, alkalinity, and, with respect to chemically reactive gases, atmospheric composition at the Earth’s surface.”63 While Lovelock considered the impact of all forms of life in using and maintaining Earth’s biosphere, Margulis’s contribution to Gaia in the early 1970s highlighted the shaping effect of microor­ganisms relative to atmospheric chemistry, rock weathering, and carbon deposition via phytoplankton onto the seafloor. Lovelock wrote: “Lynn brought her deep understanding of microbiology to what until then had been mainly a system science theory that saw a self-regulating Earth through the eyes of a physical chemist. By stressing the importance of the Earth’s bacterial ecosystem and its being the fundamental infrastructure of the planet, Lynn put flesh on the skeleton of Gaia.”64

However, in the early 1970s, when Lovelock and Margulis floated their ideas regarding Gaia, Earth sciences were largely studied in isolation. “Oxygen, for example, was thought to come solely from the breakdown of water vapour and the escape of hy­drogen into space, leaving an excess of oxygen behind,” explains

Lovelock. “Life merely borrowed gases from the atmosphere and returned them unchanged. Our contrasting view required an atmo­sphere which was a dynamic extension of the biosphere itself.”65 It’s not surprising that the Gaia hypothesis was initially contro­versial and slow to be accepted. By 1974, Lovelock and Margulis were publishing scientific papers on Gaia. However, they also pub­lished a paper for nonspecialists titled “The Atmosphere, Gaia’s Circulatory System” in Stewart Brand’s CoEvolution Quarterly.66 New Age connotations and the fact that the theory was champi­oned by many nonscientists didn’t help their cause. Another issue was the sheer complexity of the biosphere and its interlocking parts—scientists had trouble making robust and predictive models of its behavior.67 But contrary to the scientific community’s cool reception of the Gaia hypothesis, NASA was very interested in Lovelock’s ideas regarding planetary atmospheres.

Gaia Theory proposes that bacteria and other organisms not only consume and replenish atmospheric components like oxygen and methane but also maintain our biosphere’s temperature and habitability through rock weathering and the cycling of carbon, oxygen, sulfur, and other chemicals. The atmosphere is under­stood as a coupled system with the lithosphere, hydrosphere, and the planet’s biota, and these geospheres interact in cycling water, minerals, chemicals, and atmospheric gases. The theory, however, evolved over time and represented a spectrum of ideas. At one end of the spectrum was the uncontroversial premise that living organ­isms have altered Earth’s atmosphere and surface geology. Follow­ing from that is the idea that the biosphere and the Earth co-evolve in a way that is self-regulatory due to negative feedback loops. The most extreme form of the theory, containing the idea that the entire biosphere is somehow a living organism, with the teleological im­plication that the continuation of life is a purpose of co-evolution, has been discredited (and Lovelock was always clear about not subscribing to these views). Nor were Lovelock and Margulis the first to consider Earth’s biology and its geospheres as intimately coupled. Early proponents of such thinking were the eighteenth – century Scottish geologist James Hutton, the father of modern ge­ology, and the Russian mineralogist Vladimir Vernadsky. Hutton was among the first to theorize how rock cycling impacted our entire planet, referring to the Earth as a “superorganism,” while Vernadsky theorized in the early twentieth century that geologic processes contributed to sustaining the biosphere.68

Lovelock and Margulis’s integrated view supposed that micro­organisms not only maintained Earth’s habitable atmosphere and climate, but also could alter the environment on a planetary scale. For this and reasons noted above, Gaia hypothesis seemed pre­posterous to many people. However, as Lovelock explains in The Vanishing Face of Gaia (2009), “It is too often wrongly assumed that life has simply adapted to the material environment, what­ever it was at the time; in reality life is much more enterprising. When confronted with an unfavorable environment it can adapt, but if that is not sufficient to achieve stability it can also change the environment.”69 Stromatolites in Australia’s Shark Bay indi­cate, for example, that by 2.4 billion years ago the progenitors of cyanobacteria developed oxygenic photosynthesis and began to reconfigure the Earth’s early atmosphere from largely nitrogen and carbon dioxide to the nitrogen – and oxygen-rich composition we breathe today. But any correlation between the emergence of oxygen metabolizing life and global atmospheric composition is complex. Measurable levels of oxygen appeared 300 million to a billion years after the first evidence of oxygenic photosynthesis, which indicates other geologic or hydrologic processes were simul­taneously stripping oxygen from the atmosphere.70 The complex­ity of how microorganisms and geochemical processes combine to sustain or alter our atmosphere remains at the core of Gaia Theory and Earth System Science.

Apollo 13 was crippled. Two hundred thousand miles from Earth, an oxygen tank had exploded. Damage to the Command Mod­ule was so extensive that the crew of three retreated to the Lunar Module, which they then had to use as a “lifeboat” to return home. Flight Director Gene Kranz aborted the Moon landing and consid­ered the options. The simplest would have been to use the Com­mand Module engines to reverse its direction and head for home, but it was unclear if the engines could be used safely, and Apollo 13 was already in the grip of the Moon’s gravity. So instead, Kranz authorized use of the Lunar Module engines to steer a course that would take the spacecraft behind the Moon and get a gravitational “assist” from the Moon that would send it on the correct trajec­tory for an Earth landing. It was a gutsy call. The world waited tensely as the spacecraft got a helping hand from gravity to bring the astronauts home safely.6

Traveling in the Solar System is a tussle with gravity, every as­pect of which affects the amount of fuel required for a mission and so also its cost. The initial problem is leaving Earth’s gravity. This is accomplished in two steps: first, by a large, expendable chemical rocket that can get the spacecraft into orbit, and then is jettisoned; second, by rockets onboard the spacecraft which give it the extra 40 percent of velocity needed to escape the Earth’s pull. The es­cape velocity of the Earth is 11.2 kilometers per second, or 25,000 mph. Once achieved, the spacecraft is in the Sun’s grip. Going to Venus or Mercury seems like the easy choice since the spacecraft can swoon into the Sun’s gravity, which will pull it inward. But the problem emerges when the destination is reached. How do you slow down enough to carefully study or land on your target? Going to Mars and the outer planets means swimming against the tide of the Sun’s gravity. The Sun is remote but extremely massive. So having left the Earth, it takes an extra 12.4 kilometers per sec­ond to reach Jupiter and an extra 17.3 kilometers per second to reach Saturn. Compared to what it took to leave the Earth, that’s a similar amount of extra energy to get to Jupiter and twice as much extra energy to reach Saturn. Leaving the Solar System entirely

requires a velocity of 42.1 kilometers per second, or a whopping 94,000 mph. Where is all this energy going to come from?

The answer to both questions lies in gravitational assist. Rus­sian theorists Yuri Kondratyuk and Friedrich Zander pioneered the idea in the early twentieth century, and American Michael Mi – novitch added important refinements later.7 Gravity assist was first used by the Russians to let Luna 3 photograph the far side of the Moon in 1959. NASA reprised the maneuver in 1970 to rescue Apollo 13. Here’s how it works. Imagine standing beside a railroad track as a train approaches. You throw a tennis ball at the front of the train and it comes back in your direction at a higher speed because the train transfers some energy to the ball. Similarly, if the train was traveling away from you and you threw the ball, it would come back more slowly because the train took away some of the ball’s energy. Notice that energy doesn’t appear or disappear mysteriously. When the train is coming toward you, hitting it with the ball actually slows it down, but by an incredibly tiny amount because it’s so massive. When the train is going away from you, hitting it with the ball speeds it up by a tiny amount. In the case of gravity, there’s no physical contact as the force operates through a vacuum. When a spacecraft approaches a slower moving planet from behind, the spacecraft slows down by transferring some en­ergy to the planet. Conversely, when a faster-moving planet ap­proaches a spacecraft from behind, the spacecraft speeds up by gaining some energy from the planet. The idea works with moons as well as planets.8

Pioneer 10 was the first NASA spacecraft to benefit from gravity assist, using an encounter with Jupiter in 1973 to double its speed and send it someday out of the Solar System. Pioneer 11 followed suit a year later. Also in 1974, Mariner 10 passed close by Venus on its way to exploring Mercury. More recently, the MESSENGER probe needed one flyby of Earth, two flybys of Venus, and three flybys of Mercury to lose enough energy to be captured into an orbit of the innermost planet in 2011. For probes heading into the outer Solar System, it’s worth going out of your way to get a boost from gravity. Galileo and Cassini both took inward detours to Venus to get a “kick” that helped them explore the gas giants. Energy truly is conserved in this gravitational ballet. When MES-

SENGER used Mercury to slow down and go into an orbit, it gave the planet some energy and nudged it a tiny bit farther from the Sun. And when Pioneer “robbed” Jupiter of some of its energy, it pushed the giant planet very slightly closer to the Sun.

The two Voyagers launched from Cape Canaveral in Florida in the summer of 1977. Their Titan III/Centaur launch vehicle only provided enough energy to get to the distance of Jupiter. Without Jupiter’s help the spacecraft would have remained in elliptical or­bits that never got closer to the Sun than Earth and never got far­ther from the Sun than Jupiter. But NASA engineers had planned for Jupiter to be coasting by at the right time to give each of the spacecraft a boost. Although Voyager 1 left second, it took a faster and more direct route that got it to Jupiter first, and then Saturn, but at the cost of not visiting the outermost planets. It could have visited Pluto, but this possibility was sacrificed for a close look at Titan. In 1998, Voyager 1 overtook the slower moving Pioneer 10 to become the most distant human artifact. Voyager 2 took a more circuitous route through the Solar System, flying by each of the four gas giants and gaining a modest gravity boost from each encounter (figure 4.1).9 The Voyagers were originally con­ceived to take advantage of a once-i n-176-year opportunity: the near-perfect alignment of all four gas giants and Pluto. Aerospace engineer Gary Flandro pushed for NASA to take advantage of the alignment with a planetary “Grand Tour,” but the vision was com­promised by budget cuts so the Voyager executed a scaled-back version of the concept.

At its closest approach to the Earth, Saturn is about eight times the Earth-Sun distance, or 800 million miles away. Yet, by the time Cassini reached Saturn it had traveled 2.2 billion miles. NASA launched Cassini with the best rocket available, but it wasn’t pow­erful enough to get the spacecraft to Saturn directly since it would be fighting against the Sun’s gravity all the way. So mission design­ers used gravity assist, colloquially called the gravitational “sling­shot” mechanism, to get it to its target. As we saw in the last chap­ter with Voyager, this technique has been used since the 1970s to get nature’s help in hefting planetary probes away from the Sun’s gravity.24 The assist is provided by bringing the spacecraft along­side a planet from behind and letting it get a “kick” from the or­bital angular momentum of the planet. In principle the spacecraft can get a speed boost of up to twice the planet’s orbital velocity.

Cassini passed by Venus twice, then the Earth, and finally Jupi­ter before heading to Saturn. The terrestrial flyby was controver­sial because of the nature of Cassini’s power source. Solar panels aren’t feasible for a mission so far from the Sun’s rays. So Cas­sini has three generators where radioactive decay of plutonium – 238 generates electricity via a thermocouple. The power source had raised congressional eyebrows before launch, but as the flyby approached, NASA was told to do an environmental impact as­sessment on the possibility of Cassini impacting the Earth. For a worst case scenario of a shallow angle of entry to the atmosphere and slow dispersal of the radioactive materials, the odds were one in 10 million.25 NASA was allowed to proceed. There were dem­onstrators and a few lawsuits, but the launch and flyby went off without a hitch and now the plutonium is at a safe distance of a billion miles.

These flybys were just a warm-up for the amazing series of grav­itational dances that Cassini engaged in when it got to Saturn (fig­ure 5.3). Over its core mission, Cassini orbited Saturn 140 times. To see Saturn, its rings, its largest moons, and its magnetosphere from all conceivable angles, Cassini is using its rockets and sev­enty gravity-assist flybys of Titan to tweak its orbit size, period, velocity, and inclination from Saturn. As the largest moon, Titan is

40

Figure 5.3. The speed of the Cassini spacecraft relative to the Sun during its first decade of travel. The early peaks are gravitational slingshot maneuvers to enable it to reach Saturn with the appropriate velocity for insertion into orbit around Saturn. The later variations are cleverly arranged flybys of Saturn’s moons (Wikimedia Commons/YaoHua2000).

the most useful in “steering” Cassini around the Saturnian system. Each Titan flyby is engineered to return Cassini into the proper trajectory for its next Titan flyby. Encounters with other moons are performed opportunistically with what’s called a targeted flyby. About fifteen are planned by the end of the mission, half to the intriguing small moon Enceladus. From 2004 through 2011, Cas­sini did a dizzying hundred flybys, with another dozen completed in 2012. NASA hosts a clock counting down the time until the next swooping visit to a moon and coyly calls these “Tour Dates” to appeal to a younger generation.26 By clever planning, NASA engineers have doubled the length of the mission even though just a quarter tank of fuel remains.

Titan’s gravity has also been used to gradually tilt the inclina­tion of Cassini’s orbit, allowing it to see the rings from above and below, and to see atmospheric phenomena of Saturn’s poles for the first time. Flybys of Titan can’t get closer than 600 miles, or the large moon’s thick atmosphere would slow the spacecraft. Some flybys to small moons have been as close as 15 miles. At a distance of a billion miles, that’s like hitting a golf ball coast to coast and dropping it within an inch of the hole! There’s another problem with operating such a remote probe. Depending on where Earth and Saturn are in their orbits, the distance between them can vary from eight to ten astronomical units or Earth-Sun distances. It therefore takes 70 to 85 minutes for radio commands to travel
from mission control to the spacecraft, and the same for the re­verse journey. Controllers can’t give “real-time” commands. Even if they responded immediately to a problem, nearly three hours would pass before Cassini would get the response. This casts the flybys in an entirely new light, since the margin of error is less than a second.

Comets and asteroids provide evidence of the formation epoch of our Solar System. They represent the pristine stardust of which we are made. They’re implicated in life and death in the evolution of life on Earth. But they’re minor bodies of the Solar System. The three targets of Stardust’s study are a motley, misshapen collec­tion. There’s 5535 Annefrank, three miles across and shaped like a right-angled prism. There’s Wild 2, similar in size, with a pitted and coruscated surface. And there’s 9P/Tempel, slightly larger and shaped like a potato. Are they big enough to be called worlds? Are they substantial enough to have a place in our dreams?

Yes, absolutely. They may be too small to hold an atmosphere, but comets and asteroids have unique shapes and topographies that imbue them with personality. Our technology is good enough to throw a gravity “lasso” around a space rock that ventures near us and steer it into Earth orbit. Comets contain water ice and organic material needed to sustain life, and the hydrogen and oxygen in the water are all that’s needed to make rocket fuel. A comet retained in high Earth orbit would be a perfect jumping-off point for manned exploration of the outer Solar System and beyond. Asteroids also contain valuable metals and minerals; according to one estimate, the mineral wealth in the asteroid belt amounts to $100 billion for every person on Earth.52 Wild 2 probably contains around $20 trillion worth of precious and industrial metals, and over $100 bil­lion worth of platinum alone.53 In fact, the precious metals we do have were gifts from comets in the early days of the Solar System. Metals present when the Earth formed settled directly to the core while the planet was molten; those we can reach in the crust were “dusted” onto the planet by later waves of comets.54

Space mining aside, a small, barren rock might not be worth calling your own. The 1967 United Nations Outer Space Treaty prohibits any State owning or controlling the Moon, an asteroid, or a comet, but the treaty has a loophole since it doesn’t specifi­cally exclude the ownership rights of an individual.55 A small world might seem limiting, but think of the pleasure in owning a world the size of a small town and surveying the domain like a colossus. The gravity of Wild 2 is so weak you would literally be as light as a feather. A small push and you could escape your world and sail into deep space. And think of the glittering minerals—a hoard magnificent enough to power all the dreams ever dreamed.