Category Dreams of Other Worlds

In fact, NASA had lost a big battle, but they weren’t yet ready to concede the war. With Hubble working at part strength, the agency immediately began planning to diagnose and correct the problem with the optics. Although a backup mirror was available, the cost of bringing the telescope down to Earth and re-launching it would have been exorbitant. It was just as well that the telescope had been built to be visited by the Space Shuttle and serviced by astronauts. The instruments sitting behind the telescope fit snugly into bays like dresser drawers and they could be pulled out and replaced with others of the same size.

It also helped that the mirror flaw was profound but relatively simple, and the challenge was reduced to designing components with exactly the same mistake but in the opposite sense, essentially giving the telescope prescription eyeglasses. The system designed to correct the optics was the brainchild of electrical engineer James Crocker, and it was called COSTAR, or Corrective Optical Space Telescope Axial Replacement.10 COSTAR was a delicate and com­plicated apparatus with more than five thousand parts that had to be positioned in the optical path to within a hair’s breadth. Install­ing COSTAR raised the difficulty level of an already challenging first servicing mission that was planned for late 1993. Seven astro­nauts spent thousands of hours training for the mission, learning to use nearly a hundred tools that had never been used in space before. They did a record five back-to-back space walks, each one grueling and dangerous, during which they replaced two instru­ments, installed new solar arrays, and replaced four gyros. This last fix was needed because of the disconcerting tendency of Hub­ble’s gyros to fail at a high rate, in a way never seen in lab testing. Without working gyros, the telescope could not point at or lock on a target. Before leaving, the crew boosted Hubble’s altitude, since three years of drag in Earth’s tenuous upper atmosphere had started to degrade the orbit.

The first servicing mission was a stunning success. It restored the imaging capability to the design spec and added new capabili­ties with a more modern camera. It also played significantly into the vigorous debate in the astronomy and space science commu­nity over the role of humans in space. NASA had always placed a strong bet that the public would be engaged by the idea of space as a place for us to work and eventually live. But after the success of Apollo, public interest and enthusiasm waned. (It even waned during the program; a final three planned Moon landings were scrapped.) Also, most scientists thought that it was cheaper and less dangerous to create automated or robotic missions than to ser­vice them with astronauts. NASA’s amazingly successful planetary missions from the 1970s to the 1990s were seen as evidence of the primacy of robotic spacecraft. Hubble was of course designed to be serviced by astronauts, but the often-unanticipated problems they were able to solve in orbit, coupled with the positive public response (and high TV ratings during the space walks, at least the early ones), persuaded many that the human presence was essen­tial and inspirational.

More servicing missions followed. Each one rejuvenated the facility and kept it near the cutting edge of astronomy research. The second mission in 1997 installed a sensitive new spectrograph and Hubble’s first instrument designed to work in the infrared. Astronauts also upgraded the archaic onboard computers. The third mission in 1999 was moved forward to deal with the vex­ing problem of failing gyros. Before the mission flew, the telescope lost a fourth gyro, essentially leaving it dead in the water.11 All six gyros were replaced and the computer was upgraded to one with a blistering speed of 25 MHz and capacious two megabytes of RAM (that’s fifty times slower and thousands of times less storage than the average smart phone). The fourth mission in 2002 replaced the last of the original instruments and also removed COSTAR since it was no longer needed—each subsequent instrument has had its optics designed to compensate for the aberration of the primary mirror. The infrared instrument was revived, having run out of coolant two years early. New and better solar arrays were installed, along with a new power system, which caused some anx­iety since to install it the telescope had to be completely powered down for the first time since launch. Except for its mirror, Hubble is reminiscent of the ship of Theseus, a story from antiquity where every plank and piece of wood of a ship is replaced as it plies the seas.

The fifth and last servicing mission almost didn’t happen. Once again, the mission was affected by a Shuttle disaster, in this case the catastrophic loss of Columbia and crew in February 2003. NASA Administrator Sean O’Keefe decided that human repairs of the telescope were too risky and future Shuttle flights could only go to the safe harbor of the International Space Station. He also studied engineering reports and concluded that a robotic servicing mission was so difficult that it would most likely fail. At that point, Hubble was destined to die a natural death as gyros and data links failed.

When in January 2004 it was announced that the Hubble Space Telescope would be scrapped, the public’s response was overwhelm­ingly in support of refurbishing the instrument in orbit. David De – vorkin and Robert Smith report that a “‘Save the Hubble’ move­ment sprang into life.”12 Robert Zimmerman similarly comments: “The public response. . . was, to put it mildly, loud. Very soon, NASA was getting four hundred emails a day in protest, and over the next few weeks editorials and op-eds in dozens of newspapers across the United States came out against the decision.”13 Zim­merman details the history, development, and deployment of the telescope as well as its more significant findings. He suggests that Hubble, more than any other telescope or major scientific instru­ment, has allowed humankind to explore what lies at the furthest depths of space and that this is one reason for the widespread pub­lic sense of ownership of what some newspapers have called the “people’s telescope.”14 Not since Edwin Hubble did his pioneering work on cosmology with the 100-inch reflector at Mount Wilson Observatory has public interest in a particular instrument been so intense or pervasive.15

O’Keefe and other NASA officials were taken aback by the pub­lic response. They’d expected astronomers to lobby hard for an­other servicing mission, and they weren’t surprised that astronauts weighed in, saying they’d signed up knowing the risks and they wanted to service the telescope. But it turned out that a large num­ber of people felt attached to Hubble through its spectacular pic­tures and newsworthy discoveries. They felt clear affection for the facility, and since the taxpayer had indeed paid for it, the agency took notice and O’Keefe first reevaluated and then reversed his decision.16 The fifth servicing mission went off without a hitch in May 2009, leaving the telescope in better shape overall and with certain capabilities a hundred times more effective than its original configuration.17 Two new instruments were installed, two others were fixed, and many other repairs were carried out since it was agreed by everyone that this was the last time the telescope would be serviced (figure 11.1).

Hubble’s continual rejuvenation is a major part of its scientific impact. The instruments built for the telescope are state-of-the-art, and competition for time on the telescope has consistently been so intense that only one in eight proposals gets approved. All this comes with a hefty price tag. Estimating the cost of Hubble is dif­ficult because of how much to assign to the Shuttle launches and astronaut activities, but a twenty-two-year price tag of $6 billion is probably not far from the mark. For reference, the budget was $400 million when construction started and the cost at launch was $2.5 billion. Compared to slightly larger 4-meter telescopes on the ground, Hubble generates fifteen times as many scientific citations (one crude measure of impact on a field) but costs one hundred times as much to operate and maintain.18 Regardless of its cost, the facility sets a very high bar on any subsequent space telescope. As Malcolm Longair, Emeritus Professor of Physics at the University of Cambridge and former chairman of the Space Telescope Sci­ence Institute Council, has observed: “The Hubble Space Telescope has undoubtedly had a greater public impact than any other space astronomy mission ever.” He also notes that this small telescope’s “images are not only beautiful, but are full of spectacular new sci­ence,” much of which was unimagined by the astronomers who conceived and launched the instrument.19

On July 20, 1976, a small spacecraft emerged from a cloudless, apricot-colored Martian sky and fell toward the western Chryse Planitia, the “Golden Plain.” Its heat shield glowed as it buffeted through the tenuous atmosphere.27 About four miles up, the para­chutes deployed, the heat shield was jettisoned, and three landing legs unfolded like a claw. At one mile up, the retrorockets fired, and less than a minute later the Viking 1 lander decelerated to six miles per hour, reaching the surface with a slight jolt.28 It was a landmark of technological prowess, the first time humans had ever soft-landed an emissary on another planet.

The twin Viking missions were the most complex planetary probes ever designed. Their total price tag was around $1 billion, equivalent to $4 billion today after adjusting for inflation. That can be compared to the $80 million cost of Mariner 4. Mission plan­ners were well aware of the challenges; the Soviets had previously failed four times to soft land on Mars.29 Each Viking consisted of an orbiter designed to image the planet and a lander equipped to carry out detailed experiments on the surface.30 For the most part, the hardware worked flawlessly, but there were tense moments for the engineers and scientists on the team. After ten months and 100 million miles of traveling, the Vikings reached Mars two weeks apart. The first landing had been planned for July 4, 1976, the na­tion’s bicentennial, and the landing sites were selected after years of deliberation. But as the twin orbiters started mapping the planet with ten times sharper images than had ever been taken before, mission planners were shocked to see that the planned Viking 1 landing site was not the benign plain they’d expected, but the rock – strewn bottom of what appeared to be a riverbed.

The landing site was abandoned. Gentry Lee, the director of Sci­ence Analysis and Mission Planning for Viking, vividly recalled the turmoil the new images caused: “For almost three weeks the Viking Flight Team operated at an unbelievable pace and intensity. Many of the key members of the team, including not just the engineers, but also [Jim] Martin and [Tom] Young and many of the world’s foremost planetary scientists, worked fourteen or more hours a day for the entire period. Landing Site Staff meetings, to synthesize the results and look at all the logical options, were held every day. Carl Sagan, Mike Carr, Hal Masursky, and other famous Viking scientists argued eloquently about the safety of each of the candi­date landing sites. Finally, the exhausted operations team managed to reach a consensus.”31 A new site at Chryse Planitia was selected. Once the lander separated from the orbiter, it would not be pos­sible to redirect the lander with any additional commands. The die was cast. Few team members slept much that night.

Media coverage and public interest were intense. Viking 1 and 2 marked the first close-up glimpse of the red rocky soil of Mars. “Despite the early hour, the von Karman Auditorium [at NASA’s

Jet Propulsion Laboratory] was packed. In addition to 400 jour­nalists from around the world, there were 1,800 invited guests watching a closed-circuit television view showing the control room, with Albert Hibbs, one of the mission planners, provid­ing the commentary.”32 For nineteen agonizing minutes, everyone waited—that was how long it took for telemetry to reach the Earth saying the lander was safe. Its first picture was of its own foot, to see how far it had sunk into the Martian soil. When, on its sec­ond day on Mars, Viking 1 sent back the first color panoramic views of the Martian terrain, scientists and public audiences alike recognized a kind of reddish, iron-rich soil familiar to them from the deserts of the American southwest (plate 2).33 In fact, among the first panoramic photos released to the press was a view of the Martian landscape under blue skies, though JPL scientists quickly realized the sky should be salmon colored. As Paolo Ulivi and David Harland note, “Initially, the image-processing laboratory combined the red, green and blue frames to produce the dark blue – black sky that the thin atmosphere had been expected to yield, but after [the images] had been recalibrated the sky was found to be pinkish-orange.”34

The missions galvanized global fascination with the stark Mar­tian landscape, and they continue to provide a compelling story of discovery and of the sheer difficulty of trying to do science so far from a conventional laboratory. The feelings were best described by NASA’s Gentry Lee: “The Viking team didn’t know the Martian at­mosphere very well, we had almost no idea about the terrain or the rocks, and yet we had the temerity to try to soft land on the surface. We were both terrified and exhilarated. All of us exploded with joy and pride when we saw that we had indeed landed safely.”35

To many people, participation in the space program seems com­pletely unreachable. It requires too much training, too much spe­cialized knowledge, and the hardware is too expensive for nonex­perts to understand the issues, let alone be players. This mind-set ignores the great facility of the Net Generation with computers, games, and simulations, and it ignores the soaring aspirations of young people who have not yet tested their limits. To baby boom­ers who lived through the fallow years that followed the Apollo Moon shots, space travel is hard. To Millennials who are witness­ing the opening of space to the commercial sector for the first time, space travel is natural and inevitable.

The Student Astronaut program and its precursors were spurred by an unusual collaboration between a nonprofit advocacy organi­zation, the Planetary Society, an agency of the federal government, NASA, and a well-known corporation, the Lego Company. Their project was called “Red Rover Goes to Mars.” It was preceded by a project called “Red Rover, Red Rover” which began in 1995. The executive director of the Planetary Society, Louis Friedman, saw a teacher at an educational workshop using a Lego product called Control Lab, which let students build motorized devices that could be controlled by a computer. He immediately saw a parallel to the robotic exploration of Mars. If kids could sit at a computer and control a rover in another room, or another country, they could experience the challenges of controlling a rover on another planet, a world that can only be explored through the limited senses of a robot. Starting in 2003, a network of “Mars Stations” was set up around the United States, and in Britain and Israel, each equipped with a different Mars-l ike terrain and a Lego rover with a web camera. Anyone who wants to can drive these rovers over the In – ternet.34 Lego Education has products such that any kid can build and drive their own rover, assuming their parents don’t object to the creation of an artificial Mars in the family home.

Another idea designed to appeal to kids was the inclusion of a Lego mini-figure on each of the Mars Exploration Rovers. Biff Starling and Sandy Moondust were named in another competition and they each “authored” freewheeling online diaries as the rov­ers explored Mars. Each Lego figure was attached to a mini-DVD, and before the rovers rolled off into the Martian dust, they took a few pictures looking back—it’s a little incongruous to see Lego on Mars. Four million people have their names on the mini-DVDs; many are members of the Planetary Society and the rest signed up on the Planetary Society’s special website.35 This was only the sec­ond time privately developed hardware has flown on a planetary mission. Through these partnerships, the lure of space travel has been extended to a very wide audience. The Martian dreams of today’s kids are very different from the dreams of kids four genera­tions ago, which were fueled only by pulp fiction and fantasy.

To get a sense of the gulf of space that lies between us and the near­est star systems, let’s return to the scale model from the start of the chapter. Visualize the Earth as a golf ball, a little less than half a mile from a glowing, 20-foot diameter globe that represents the Sun. Light speed shrinks as distance does in the model, slowing to just under three miles per hour, a steady walking pace. The Moon is a few feet away, a light travel time of just over a second. Mars is 1,100 feet away, a fifteen-minute stroll. The outer gas giant, Nep­tune, is 12 miles away, which is four hours of light travel, or walk­ing in our scale model. We have shrunk space by a factor of 230 million; think of it as a map with a scale of 1:230,000,000. At the moment Voyager 1 is 48 miles away and Voyager 2 is 37 miles from Earth (figure 4.4). Proxima Centauri, the nearest star system, is 2,000 times farther away, just over 100,000 miles in this scale model. In the real universe, if it’s taken the Voyagers thirty-five years to get where they are today, it will take them around 70,000 years to get as far as the nearest star. This daunting isolation makes other worlds seem out of reach. The trajectories of the Voyagers were never intended to aim at a particular star. They will each drift past other stars as the eons pass. In approximately 40,000 years, Voyager 1 will come within 1.5 light-years, or 9 trillion miles, of an anonymous star in the constellation Camelopardalis. Mean­while, Voyager 2 is heading toward Sirius, the brightest star in the sky, and will pass within 25 trillion miles of it in 300,000 years.63 Long before then, in less than 25 years, both spacecraft will lose power for all of their instruments and become silent sentinels glid­ing through the Milky Way.

The stars seem as far away as ever. With the Space Shuttle pro­gram over and the International Space Station expensive and un­popular among most scientists, who don’t see it as a cost-effective or compelling platform for research, NASA is struggling to recap­ture the vision that fueled the Apollo program and the “Golden Age” of planetary exploration epitomized by the Voyager probes. Reaching the stars will take much greater speeds than are cur­rently possible. Traveling at the highway speed limit of 55 mph, Proxima Centauri is a 50-million-year trip. At the speed of the

Apollo spacecraft, it’s a million-year trip. And Voyager traveling at 37,000 mph would take 80,000 years to get there. Sending a probe to a nearby star in a human lifetime runs into the obdurate principles of physics. Reaching a speed a thousand times faster than Voyager requires a million times the energy, since kinetic en­ergy is proportional to velocity squared. The space program so far has been powered exclusively by chemical rockets, which are inef­ficient because they get their energy from chemical bonds. Fusion releases energy from atomic nuclei and is 10 million times more ef­ficient.64 Sending a Shuttle-sized craft to Proxima Centauri in fifty years would take about 1020 Joules, which is the amount of energy consumed in the United States in a year. Unfortunately, that would require 500,000 kg of hydrogen fuel, and fusion technology is no­where near being able to put that capability into a Shuttle-sized package.65 Ideally, there would be no need to carry and accelerate all that fuel, but solar sails don’t work efficiently when far from a star, and there’s not enough interstellar hydrogen to scoop up and use along the way. It sounds like a bridge too far.

At the speed of light, traversing the span of our galaxy would take 100,000 years. Even so, we dream of plying the eternal wastes beyond that. It is a remarkable aspiration for such a fragile spe­cies. Some engineers consider interstellar travel an impossible goal. That may be, but in launching spacecraft in the direction of Earth’s rotation, we use in a very simple way a natural energy resource of our planet. Similarly, Voyager harnessed the rotational energy of the large outer planets to sling itself from one planet to the next. In essence, through the gravity assist deployed in Voyager and other missions, we tap into a resource of the Solar System itself and, however minutely, borrow energy from, and leave our mark on, the rotation of nearby planets. Sagan, who characterized walk­ing and driving on the Moon or using gravity assist as natural steps in human evolution, wrote of Voyager: “They are the ships that first explored what may be homelands of our remote descen­dants. . . . [U]nless we destroy ourselves first we will be invent­ing new technologies as strange to us as Voyager might be to our hunter-gatherer ancestors.”66

Research on propulsion concepts for interstellar travel was un­dertaken at NASA’s Glenn Research Center from 1985 to 1992, and then the seed funding of $1.5 million ran out. One tangible result was a 740-page volume that quickly became the “bible” of propulsion science.67 Project leader Marc Millis offers a caution­ary note to NASA’s Breakthrough Propulsion Physics website:

On a topic this visionary and whose implications are profound, there is a risk of encountering, premature conclusions in the literature, driven by overzealous enthusiasts as well as pedantic pessimists. The most productive path is to seek out and build upon publications that focus on the critical make-break issues and lingering unknowns, both from the innovators’ perspective and their skeptical challengers. Avoid works with broad-sweeping and unsubstantiated claims, either supportive or dismissive.68

Yet the visionaries are alive and well, and planning for a future when we will slip the bonds of the Solar System.69 In a move that mirrors the U. S. government’s encouragement of the private sector to develop new launch capabilities, in 2011 the Defense Advanced Research Projects Agency (DARPA) initiated an annual strategic planning workshop and symposium to bring together a wide range of technologists, engineers, members of space agencies, entrepre­neurs, space advocates, science fiction writers, those working in medicine, education, and the arts, as well as the general public. Organized to foster collaboration among these groups as well as academicians and financiers, the “100 Year Starship” project isn’t immediately planning to design a starship that could travel to the stars, but is hoping to kick-start private-public partnerships that will address the technical problems of interstellar travel in the next one hundred years. Colloquia for the project focus on propulsion at light speed, medicine and space travel, possible destinations, the economic, legal, and philosophical implications, as well as the need to communicate a vision for interstellar travel through narra­tive, or storytelling.70

The 100 Year Starship initiative exemplifies the potential of an idea, suggested in mere fiction, to become reality. Flip cell phones modeled on the Star Trek communicator, desktop computers, downloadable mp3 files, iPods, and iPads were all inspired by the fiction of Star Trek. Chief engineer and mission manager for NASA’s Dawn spacecraft, Marc D. Rayman, attributes his design of ion propulsion for interplanetary spacecraft to a Star Trek epi­sode titled “Spock’s Brain” in which the term was used (figure 4.5). The ion propulsion powering the Dawn mission allows continu­ous firing of its engine so the spacecraft can attain speeds surpass­ing chemical propulsion.71 Similarly, the Qualcomm Tricorder X Prize competition, which was launched in 2012 by the X Prize Foundation for the first team to develop an inexpensive device to readily diagnose illness, is yet another Trek-inspired potential innovation.72

Futurist Ray Kurzweil thinks we are facing a critical stage in human evolution due to exponential advances in information tech­nologies, genetics, nanotechnology, and robotics. Kurzweil and others anticipate that emerging technologies in three-dimensional

printing will radically change the world as we know it. Myriad items can be produced via 3D printers, including parts for flyable aircraft printed out of plastics or complete components for build­ings that could be printed from liquid concrete. Made in Space, a fledgling company inspired by Singularity University, based at the NASA-Ames Research Center in California and co-founded by Kurzweil and Peter Diamandis, has proposed using 3D printers on the International Space Station or for establishing outposts on the Moon or Mars. In 2011, NASA funded research focused on print­able spacecraft, as well as 3D printing technologies to construct planetary surface habitats. When astronauts return to the Moon or someday walk on the surface of Mars, they may carry 3D print­ers with them and computer files for printing the tools and habi­tat components needed to build livable spacehabs using nothing more than lunar regolith or the ruddy dirt of Mars. The process of 3D printing involves a sequential layering of powdered or liq­uid plastics or metals, even potentially human cells. The medical community is interested in using human stem cells to print three­dimensional vertebral discs for repairing spine injury, or a human heart or liver as organ transplants.73

In the next thirty years, Kurzweil predicts, disease will be eradi­cated through this and other medical advances in biochemistry, gene therapy, and bio and nanotechnologies. He may be on target, given the success of researchers at MIT who are in the develop­mental stage of a drug that can destroy human cells infected with viruses while leaving normal cells untouched. So far, trials with mice eradicated H1N1, the most common flu virus in humans, and the drug looks very promising in effectively treating stomach vi­ruses and the common cold.74 Kurzweil anticipates that in the next few decades we will begin to incorporate artificial intelligence the size of a blood cell into the human body to enhance our health as well as our intellectual and computational capacities so that it will be possible to extend our lives for hundreds or thousands of years, possibly even indefinitely.75

That is not an unprecedented idea. Consider Turritopsis Dohr – nii, the immortal jellyfish discovered in 1988 by marine biology student Christian Sommer and now being researched by marine biologist Shin Kubota. Upon reaching maturity, the jellyfish reverts in age to its nascent state and then begins life again. The organ­ism’s natural life cycle doesn’t ever end. Kubota believes that in unlocking the jellyfish’s genome humans too might become im­mortal. In such a scenario, human missions to nearby stars seem nearly feasible.

If we’re not attached to sending humans, with our present need for expensive life support, nanotech might propel us into interstel­lar space even sooner. Exoplanet hunters like Geoff Marcy and Debra Fischer are studying the two brightest stars in the nearby Alpha Centauri system, the nearest solar systems to Earth and comprised of three suns. In October 2012, a graduate student at Geneva Observatory, Xavier Dumusque, detected a planet compa­rable in mass to Earth orbiting the star Alpha Centauri B. Though this exoplanet orbits too close to its star to be habitable, the gen­eral consensus among astronomers is that where there is one planet there are likely to be more. Fischer, though not associated with the discovery, has called the finding “the story of the decade.”76 Prog­ress in the detection of exoplanets has been stunning. Starting with a first detection of a Jupiter-mass object in 1995, Earth-mass exo­planets are now routinely detected both with ground-based and orbiting telescopes.

Unfortunately, studying these planets in detail from afar will be extremely difficult; think of a golf ball seen at a distance of 100,000 miles. Listen to what Marcy hopes will happen next: “NASA will immediately convene a committee of its most thoughtful space propulsion experts, and they’ll attempt to ascertain whether they can get a probe there, something scarcely more than a digital cam­era, at let’s say a tenth the speed of light. They’ll plan the first-ever mission to the stars.”77 If there is an “Earth” next door, Alpha Cen – tauri will become the compelling destination that the Moon was fifty years ago. Assuming the tricky issue of miniaturized propul­sion can be resolved, nanobot probes can be small enough that the energy requirements are tractable. They’ll take pictures of the habitable worlds and we’ll see them back on Earth within a gen­eration. These pint-sized emissaries will be able to carry far more information than the quaint phonograph records of the Voyagers; their digital storage will contain the sum of all human knowledge. With the Voyagers, we took our first tentative baby steps beyond the Solar System. The future beckons.

Stardust had some surprises in store for scientists working on the mission.20 The first came during the flyby. Team members and comet experts expected Wild 2 to be bland-l ooking, like a large black potato. Instead, the seventy-two images sent back to Earth revealed a dramatic scene (figure 6.3). There were kilometer-sized holes bounded by vertical and sometimes overhanging cliffs, spiky pinnacles hundreds of meters high, and numerous jets of gas and dust surging into space. Some of the holes didn’t look like the kind of impact crater found on Mercury, the Moon, and every other airless surface exposed to space. The ravines and chasms indicated that the surface is very young and is constantly being created and altered by dynamic processes within the comet. Some of the jets were on the dark side of the comet, proving that they are not gen­erated by the action of the Sun. Wild 2 doesn’t look like any other comet or asteroid that’s been imaged by spacecraft (plate 10).

A related surprise came as the spacecraft flew through debris es­caping from the comet. It was expected that the rate of particle hits would increase with time, reach a smooth peak, and then decline as the comet rapidly disappeared in the rearview mirror. Instead, the impact rate surged and fell in bursts, presumably as the space­craft passed through jets of dust escaping from the surface and the breakup of cometary “clods” as the ice holding them together evaporated.21

The most exciting discovery came in 2009, when a team at Goddard Space Flight Center reported the discovery of the amino acid glycine in tiny particles they were analyzing.22 It’s not a total surprise that comets contain molecules like amino acids, but their survival intact in material traveling at thousands of miles per hour was unexpected. An isotopic study of the glycine showed it was not a contaminant from Earth. This discovery shows that at least

one of the essential ingredients for life can be delivered by comets, and since most stars are thought to have comet clouds, comets will be an important delivery system for delivering pre-biotic molecules to Earth-like planets, so “setting the table” for life. Another unex­pected result was the discovery of iron and copper sulfide minerals in Wild 2 that could only have formed in the presence of water.23 This means that the comet has spent some of its time at balmy tem­peratures in the range 50-200°C, where water is liquid or steam, so it’s not the “frozen snowball” that everyone expected.

Often in science, a simple model or theory formed based on limited data has to be modified or even discarded when better data are gathered. Nature is messier than our wishful thinking. The Stardust mission has played an important role in retooling the conventional ideas of comet structure and formation mechanisms. It was long thought that short-period comets formed from materi­als that condensed beyond the orbit of Neptune in the cold Kui – per Belt. Long-period comets, by contrast, were presumed to have formed at higher temperatures closer to the Sun, among the giant planets, and then subsequently ejected (primarily by the gravity of Jupiter) to orbits that extend halfway to the nearest star. The discovery of trans-Neptunian objects that feed both the short – and the long-period comet population complicates this simple picture. Data from Stardust have complicated it further.

Our direct information on comets is still modest. The first probe to reach a comet was launched in 1978 and approached Comet Giacobini-Zinner, providing support for the “dirty snowball” the­ory. Comet Halley’s first visit to the inner Solar System in the age of spacecraft was in 1986, and received visits from an armada of spacecraft—Giotto, Suisei, Sakigake, Vega 1, and Vega 2. Fifteen years later, Deep Space 1 flew by comet Borrelly. Stardust came next, and in 2005, Deep Impact excavated 10,000 tons of ma­terial from comet Tempel 1 by smashing into it at 23,000 mph. When Stardust returned its payload in 2006, it was the first time we had gathered physical samples from any other place than the Moon. As the only human relic to go on a multi-billion-mile jour­ney and return home, Stardust earned a very auspicious resting place. In 2008, on NASA’s fiftieth anniversary, the sample return capsule went on display in the Smithsonian Museum’s “Milestones of Flight” gallery, alongside iconic artifacts such as the Apollo 11 command module, Charles Lindbergh’s “Spirit of St. Louis,” and the Wright brothers’ 1903 Flyer.

From time immemorial we have projected our stories, myths, and legends onto the night sky. Seeing patterns among the stars was a serious preoccupation from primordial time. Our shared narra­tives about the constellations, across cultures and millennia, served as a survival mechanism not to be underestimated or discounted as simple tales of mythological figures. Biocultural theorist Brian Boyd claims that human attention to pattern emerged as an evo­lutionary adaptation. Boyd and other scholars contend that arts relying on patterns such as storytelling, song, and cave painting emerged via natural selection and allowed individuals and clans to better collaborate and share information that enhanced survival.4

Recent analyses of prehistoric paintings and markings in caves in France have revealed a series of twenty-six symbols that may reflect humankind’s earliest attempts at pictographic writing, traditionally thought to begin about 3,000 BC. Anthropologists Genevieve von Petzinger and April Nowell collated a database of cave signs from 146 sites in France dating from 35,000 to 10,000 years ago. “What emerged was startling: 26 signs, all drawn in the same style, appeared again and again at numerous sites.”5 The symbols range from straight lines, to circles, spirals, ovals, dots, Xs, wavy lines, and various hand symbols among others. Similar symbols have been found at Paleolithic sites around the world.

Certain symbols studied in France frequently appear in deliberate groupings, such as the occurrence of dots with a particular hand symbol, which may indicate the beginnings of a system of writing. As Nowell speculates, “We are perhaps seeing the first glimpses of a rudimentary language system.”6

The celebrated Lascaux cave paintings not only incorporate these symbols, but may also include representations of the Pleia­des and Hyades star clusters and an ice age panorama of the night sky. Clive Ruggles and Michel Cotte, who in 2010 headed the In­ternational Astronomical Union’s Working Group on Astronomy and World Heritage, reported to UNESCO that some archaeoas – tronomers contend a series of dots above the aurochs in the Hall of the Bulls represents the Pleiades star cluster and that one of the auroch’s eyes and adjacent dots may depict the star Aldebaran and the Hyades cluster.7 The French government’s website on Las – caux’s cave art notes that in all of the renderings, the horses were painted first, then the aurochs, and then the stags. These animals apparently correspond to the seasons of spring, summer, and au­tumn, respectively, providing “a metaphoric evocation that, in this setting, links biological and cosmic time.”8 Even more fascinating is Chantal Jeques-Wolkiewiez’s assertion that two panels of these cave images may depict the night sky as perceived by Magdalen – ian people from the top of Lascaux hill during a summer solstice roughly 17,000 years ago. Besides comparing the cave paintings to computer models of the night sky in the last ice age, she also found that light during sunset at the summer solstice stills enters the cave to illuminate some of the paintings.9

While it may be very difficult to determine whether the dots in the Lascaux paintings are indeed asterisms, the markings ice age peoples at Lascaux left next to their remarkable paintings entic­ingly suggest they were meaningful forms of communication. Ex­trapolating from such biocultural and archaeological research, it seems likely that our attention to patterns seen in constellations in the night sky extends back to our earliest days.

The names of stars and designation of constellations as we know them in Western culture are so ancient that their origins remain elusive.10 Long before the earliest written records, humankind told narratives about the stars and clustered them into constellations, which served as mnemonics for travel and navigation and as a re­pository of knowledge about the seasons, but also of legends and myth. Only in the last 3,600 years do we find unequivocal evidence of tracking the stars. Chinese rulers employed court astronomers to record information regarding stellar motions, transients, and as­tronomical events such as supernovae. But even older than ancient Chinese records is the Nebra sky disc. Discovered near the town of Nebra in Germany, the Nebra sky disc is believed by archaeo – astronomers to be a Bronze Age durable sky map dating back to 1600 BC.

The disc was uncovered in 1999 by treasure hunters who hit upon an ancient burial site in a circular earthwork enclosure at the top of Mettelberg Hill. The disc, 30 centimeters or 11.8 inches in diameter, was found with two bronze swords among other items. The sky disc is made of bronze with gold overlays of the Sun, the Moon in phase, and multiple stars. Gold bands on its sides indicate the east and west horizons and mark an angle of 82.5 degrees. At Nebra, sunset at the winter and summer solstices is visible on the horizon 82.5 degrees apart. As the angular separa­tion of those setting points varies at differing latitudes, some ar – chaeoastronomers are convinced that the disc was constructed in the Nebra region and is the oldest extant sky map in the world. A cluster of seven gold dots on the disc are thought to be the earli­est known representation of the Pleiades star cluster, used in the ancient past for identifying seasons of planting and harvest (figure 8.1). Investigation of its metal composition traces the disc to a Bronze Age mine in the Alps. The site where the disc was found is only 15 miles from Goseck, Germany, the location of a Neolithic ceremonial woodhenge dating to 7,000 years ago that researchers say clearly marks the position of sunrise on the horizon on the summer and winter solstices. Archaeologist Harald Meller, who posed as a buyer and worked with Swiss police in a sting opera­tion to capture underground traders attempting to sell the sky disc, points out that it predates “the beginning of Greek astronomy by a thousand years.”11

The ancient Greek writers Homer and Hesiod knew the names of recognizable stars and star clusters like the Pleiades and Hyades, and of constellations such as Ursa Major, the Bear. The Iliad and The Odyssey, attributed to the poet known as Homer, remain the

oldest extant Greek texts we have. Homer’s tales were rooted in an oral tradition that extends back centuries prior to their being recorded. In The Iliad, traditionally dated to approximately 700 BC, some of the constellations we know today appear on the shield that Hephaistos forged for Achilles:

He made the Earth upon it, and the sky, and the sea’s water, and the tireless Sun, and the Moon waxing into her fullness, and on it all the constellations that festoon the heavens,

The Pleiades and the Hyades and the strength of Orion and the Bear.12

James Evans also notes Homer’s description of Odysseus orient­ing his ships by keeping the constellation Ursa Major, which turns about the celestial north pole, left of his vessels as he sails East.13

One significant reason early agrarian societies marked the stars in the night sky was to develop an agricultural calendar, initially important for planting and harvesting. Evans points out that a few generations after Homer, Hesiod wrote Works and Days, the open­ing lines of which directly associate the rising and setting of star groups with agriculture:

When the Pleiades, daughters of Atlas are rising,

Begin the harvest, the plowing when they set.14

Evans explains that winter wheat was the only wheat planted in Greek antiquity, so that when the Pleiades were setting in the west in late fall it was time to plow the ground and plant the wheat.

Nick Kanas points out that ancient Greek astronomers supple­mented their knowledge of the night sky with what they could glean from the Egyptians and Babylonians: “From the Egyptians, they learned about the length of the year, its break-up into a 12-month calendar, the division of day and night into 12 hours each. . . . From the Mesopotamians, they learned a sophisticated system of constellations[,] especially involving the zodiac along the ecliptic.”15 Babylonian temple scribes conducted serious as­tronomical observation and carefully preserved their records on cuneiform tablets. Many of these astronomical records have been recovered, some of which date to the seventh century BC. A few of the most well known of these tablets are titled MUL. APIN, mean­ing “Plow Star,” the title of which apparently refers to the stars of the Triangulum constellation and the star Gamma Androme – dae, not Ursa Major, also known as the Plough, Big Dipper, or the Bear. Evans observes that the MUL. APIN tablets, copies of much older texts, begin with a list of dozens of stars and provide a star calendar indicating the rising and setting of stars at particu­lar times of the year. Also included in the tablets, which are more accurate than Hesiod’s agricultural calendar, are observations of various constellations as well as the planets Mercury, Venus, Mars, Saturn and Jupiter, the planet associated with the primary god of Babylon.

Ancient Greek astronomers thought of the stars as fixed or un­movable. However, the Greek astronomer Hipparchus, after whom ESA’s Hipparcos mission is named, was an accurate observer who “suspected that one of the stars may have moved, and. . . wished to bequeath to his successors data against which any future sus­pected movements might be tested.”16 Hipparchus was interested in what today is called astrometry, or the science of measuring the position and motions of stars and other astronomical objects. He produced a star catalog now lost or destroyed. According to Floor van Leeuwen: “The oldest catalog of stellar positions we know of is the compilation made around 129 BC by Hipparchus, a catalog that is still being investigated. Its only surviving copy appears to be a map of the sky on a late Roman statue, and is known as the Farnese Atlas.”17

For thousands of years, all we’ve known of Hipparchus’s star guide were descriptions by Ptolemy. But astronomer Bradley Schaefer asserts that, indeed, the Farnese Atlas (figure 8.2), a statue of the Greek figure Atlas kneeling while holding on his shoulders a globe of constellations, represents the stars and constellations known to the ancient Greeks. He contends that the statue “is the oldest surviving depiction of the set of the original Western con­stellations, and as such can be a valuable resource for studying their early development.”18 Schaefer realized after a detailed study of the globe that the constellations depicted match the night sky in the era and from the location where Hipparchus lived in 129 BC. As evidence in favor of this possibility, Schaefer writes: “First, the constellation symbols and relations are identical with those of Hipparchus and are greatly different from all other known ancient sources. Second, the date of the original observations is 125 ± 55 BC, a range that includes the date of Hipparchus’s star cata­logue (c. 129 BC) but excludes the dates of other known plausible sources.” Schaefer concludes that “the ultimate source of the posi­tion information [of the constellations on the globe] used by the original Greek sculptor was Hipparchus’s data.”19

Hipparchus was the first to identify the Earth’s precession, pro­duced by the gravity of the Sun and Moon on the Earth’s equato­rial bulge. The “precession of the equinoxes” refers to the gravi­tationally induced, gradual shift in the Earth’s axis of rotation, so

that the equinoxes occur earlier each sidereal year over the course of 25,765 years, when this cycle of precession begins again. Preces­sion is a changing view of the stars caused by a subtle variation in the Earth’s orbital orientation relative to the Sun; it’s not related to the kind of stellar movement that ESA’s Hipparcos mission has charted.20 As Michael Perryman explains, the Hipparcos mission is based on the concept of parallax: “The key to measuring stellar distances is actually based on the classical surveying technique of triangulation. It simply makes use of the fact, known since the time of Copernicus, that the Earth moves around the Sun, taking one year to complete its orbit. This yearly motion provides slightly dif­ferent views of space as we speed around the Sun.” We experience the same effect in observing an object by first closing one eye and then the other. Perryman points out that “this stereo vision gives us depth perception and allows us to estimate distances, at least to nearby objects. . . . Astronomers use the same stereo technique, but with views of the celestial sky separated by hundreds of millions of kilometers as the Earth moves around the Sun. In this way, Na­ture has generously and serendipitously granted us the possibility of measuring distances stretching across the vast expanse of our Galaxy.”21

Viking 1 was launched from Cape Canaveral on August 20, 1975. Its twin was launched on September 9, 1975, and Viking 2 reached Mars on August 7, 1976, a few weeks after the first triumphant landing. The second lander reached a site several thousand miles away at Utopia Planitia on September 3, 1976, after suffering its own small mishaps. The downward looking radar was probably confused by a rock or reflective surface, so the thrusters fired too long, cracking the soil and throwing up dust. It stopped with one leg resting on a rock, tilting the lander by eight degrees. Otherwise, it was unharmed. The hardware was designed to last for ninety days but proved to be very durable.36 Viking 1’s orbiter lasted nearly two years and Viking 2’s orbiter lasted just over four years. Meanwhile, on the surface, the Viking 2 lander ceased operating when its battery failed after three and a half years, while the in­domitable Viking 1 lander was going strong after more than six years, when simple human error during a software update made the antenna retract and communication with the Earth was lost.37

Public attention focused on the landers, with their life detection experiments and “you are there” images, but the orbiters were also very important in shaping a modern view of Mars. The orbiters carried the landers to Mars, scouted for landing sites, and relayed lander data back to Earth. Each equipped with optical and infrared imagers, they mapped 97 percent of the surface and sent back over 46,000 images. They could see features 150 to 300 meters across anywhere on the planet, and in selected areas they could resolve features the size of a small house.38 Whereas Mariner had only seen old, cratered terrain, the Vikings saw a rich and varied topogra­phy and geology. There were immense volcanoes, corrugated lava plains, deep canyons, and wind-carved features. The planet was divided into northern low plains and southern highlands that were pock-marked with craters. Mars had extensive elevated regions of volcanism, although no areas of fresh lava. There was weather: dust storms, pressure variations, and gas circulation between the poles. As NASA’s Thomas Mutch said of the Viking orbiter images, “They show Mars as an extremely diverse planet. . . . It is difficult to avoid the conclusion that, though Viking contributed immea­surably to breaking the code of the Mars enigma, we do not yet confidently understand its dramatic and turbulent past.”39

Most excitingly, Viking provided indirect but compelling evi­dence for water. Not currently—the air is so cold and thin that a cup of water placed on the surface would evaporate away in seconds. But the orbiters sent back images of rock formations all over Mars that could only have been produced by the action of

large amounts of liquid water in the past. Huge river valleys were seen, and places where it looked like rivers had once fanned out into a spider’s web of channels that ended in ancient shallow seas. The flanks of volcanoes had grooves that on Earth indicate water erosion. Many craters had shapes that were consistent with the impactor landing in mud. Other regions of chaotic terrain looked as if they had collapsed when underground volcanism melted ice, which then flowed away as water. The water flows implied by these features were equal to the greatest rivers on Earth (figure 2.2).40

In the sleepy community of Redlands, California, on the local li­brary shelves designated “Astronomy and Allied Sciences,” one book stands out for its well-worn, bent, and torn cover. It’s A Trav­eler’s Guide to Mars by planetary scientist William K. Hartmann. Advertised as an “extraordinary Baedeker,” the text is published in the format of the famous travel guide. Having sold so well the publisher reissued a second printing within the first two months of publication, the travel guide details features of the Martian surface. The adventures that await inside its pages include familiar and un­known craters, volcanoes, ancient river channels and flood plains, as well as the guide’s foldout maps, dramatic color photographs of key geographical locations, and sidebar articles on featured ter­rain. This isn’t a book for youth, but a serious guide for those in­terested in Mars. Readers are informed that they would be among the first to examine previously unpublished Mars Global Surveyor photos, as Hartmann served on the imaging team for that mission. His Martian travel guide offers serious analyses of geological for­mations in parallel with sidebars such as “What to Wear: A Look at Martian Weather.” Readers learn that typical daily temperatures span from -13°F during the afternoon to -125°F at night and that the extremely thin carbon dioxide atmosphere and low barometric pressure are inhospitable for human survival without protective gear. To familiarize travelers with the Martian night skies, Hart­mann explains: “The stars are brilliant at night after the glow of hazy sunsets fade, and the constellations are the same as the ones we see from Earth, with one exception: a blue-glowing ‘evening star’ with a faint companion ‘star’ is sometimes prominent for an hour or so after dusk.”36 One of Jim Bell’s panoramas, almost cer­tainly inspired by Carl Sagan, similarly captures from Gusev Cra­ter a view of our pale blue dot on the Martian horizon.

Travelers dulled by the “been there, done that” aspect of some Earth-bound excursions might consider the many fascinating des­tinations available via Google Mars, which offers virtual tours of Mars and commentary about major landmarks excerpted from Hartmann’s travel guide. Developed in collaboration with NASA by a Google team led by Noel Gorelick, and launched in 2009, Google’s virtual Mars was designed so that planetary scientists and general users might have ready access to a rich photo archive of past and current missions.37 Like Google Earth, click and zoom functions allow users to examine planetary features in 3D, as well as images from multiple NASA and ESA missions including Viking, Pathfinder, MER, Mars Global Surveyor, Mars Reconnaissance Orbiter, Mars Express, and Mars Odyssey Orbiter. Geographical and geological highlights are indicated with icons of two green mini-hikers, to reinforce Hartmann’s Baedeker motif. Users might follow the tracks of Spirit and Opportunity, locate the Viking land­ers, peer over the canyon rim into Valles Marineris, or alter the perspective to the canyon floor, from which Hartmann notes its walls can soar upward for 13,000 feet.

Even more stunning are the spectacular landscapes produced at the University of California, San Diego in what is called the StarCAVE, a 3D virtual, immersive environment the size of a large closet that allows researchers to explore stretches of Martian ter­rain. The MER rover pancams, developed through a collaboration of the NASA Ames Research Center, Carnegie Mellon University, and Google, produce high-resolution photos that can be config­ured as highly detailed 360-degree panoramas. The StarCAVE panoramas extend across the floor and to the ceiling so that plan­etary scientists can virtually explore the Martian landscape in situ to search for clues regarding soil deposition, wind and water ero­sion, and other geological processes. Using a hand-held device to navigate the immersive environment, researchers can zoom in to rock or sediment layers or zoom out to survey the broader lay of the land. Larry Smarr, who heads the California Institute for Tele­communications and Information Technology (Calit2) that oper­ates the StarCAVE, comments on the value of doing serious science in an immersive setting: “You can go into a room, and you’re on Mars.” He explains that the rendering is so fine that planetary sci­entists in effect can walk through the landscape, study rocks and geological features up close, as well as understand a site in relation to surrounding terrain.38

In the StarCAVE users must wear 3D glasses, but with the Per­sonal Varrier technology developed at the University of Illinois, Chicago, researchers can work in immersive virtual environments without any headgear, somewhat like the fictional holodeck pos­ited in the TV series Star Trek: The Next Generation. Even now re­searchers use this technology to engage with a variety of environ­ments, such as walking through the temples at Luxor, or exploring, from the inside, a molecule or a segment of the human genome. The public impact of these emerging virtual learning environments will be profound. Both NASA and the U. S. Congress are interested in using similar technologies to make planetary science more ac­cessible to everyone, so much so that the House of Representatives in its 2008 NASA Authorization Act invited the space agency to develop means by which general audiences can “experience mis­sions to the Moon, Mars, and other bodies within our solar sys­tem” through technologies such as “high-definition video, stereo imagery, [and] 3-dimensional scene cameras.”39 For now, Google Mars and the immersive environments of the StarCAVE or Per­sonal Varrier remind us that, whether in orbit or on the surface, our robotic partners precede us and increasingly unfold and make familiar nearby worlds.

“It is a drama as ancient as the sun, as unflinching as time. . . a never – ending whirl of celestial movements, scripted and precise, in a silent show of cosmic force, played out in light and shadow. It is a drama called equinox,” writes Cassini Imaging Team Leader Carolyn Porco in her “Captain’s Log,” an online diary of the Cas­sini spacecraft’s observations of the ringed world of Saturn and its moons. It takes approximately thirty years for Saturn to orbit the Sun, so the planet only experiences an equinox, when the Sun shines equally on its northern and southern hemispheres, every fif­teen years. In August 2009, equinox returned once again for Sat­urn as Cassini explored Saturn and its moons. “To its operators at significant remove, a billion miles away, it has been a long and gripping wait for this special season about to unfold. . . when the Sun passing overhead from south to north begins to set on the rings,” writes Porco. Observing Saturn in equinox from on site, she reminds us, is “a solemn celestial phenomenon no human has beheld before.”1

All of the large outer planets of our Solar System have rings, though none as magnificent as Saturn’s. The rings, no more than tens of meters thick yet spanning nearly 155,000 miles in diameter, come into sharp and rare relief during equinox when the angle of the Sun’s rays is lowered relative to the ring plane and casts long shadows across the rings (figure 5.1).2 In eloquent and poetic prose Porco comments, “Like the seas of Earth, this wide icy expanse

[ . . . ] froths and churns, not by wind but by the convulsive forces of Saturnian moons. This famous adornment, impressed deep in the human mind for four centuries as a pure, two-dimensional form, has now, as if by trickery, sprung into the third dimension.”3 Equinox on Saturn has since faded to northern summer and we along with Cassini have observed the clear, deep blue skies of Sat­urn’s northern hemisphere cloud over to reflect Saturn’s signature peach or faded orange hue. Robotic explorers like Cassini have given us entirely new perspectives of other worlds in the outer Solar System.

The comet cloud defines the outer edge of the Solar System, ex­tending a thousand times further than the planets from the Sun, and a significant fraction of the distance to the nearest stars. It had always been presumed that these small bodies were frozen rel­ics from the formation epoch, essentially unchanged in 4.5 billion years. The expectation was that comets would be made of dust from a previous generation of stars, or pre-solar grains. That’s why the mission was named Stardust.

Instead, something quite unexpected was found: fire and ice.24 Comets contain ice that formed in the frigid zone beyond Nep­tune, but the bulk of a comet’s mass is rock, and that rocky ma­terial seems to have formed under conditions hot enough to va­porize bricks. The comet particles embedded in Stardust’s aerogel included two ingredients that are found in meteorites, debris from asteroids that formed between the orbits of Mars and Jupiter. One is chondrules, rounded droplets of rock found in many primitive meteorites that melted and quickly cooled as they orbited the Sun. The other is a much rarer mineral called a Calcium Aluminum Inclusion, irregular white particles of very unusual chemical com­position that can only form at very high temperature. The Stardust team puzzled over this discovery, but the implication seemed clear. Matter that formed readily in the inner Solar System was some­how transported to the edge of the young Solar System where the comets formed. Wild 2 does contain grains of pre-solar stardust material, but they’re very rare. Comets are not made of material left over from other stars; they’re made mostly from material that formed close to the Sun. As such, they provide insights into how planets and moons were built 4.5 billion years ago.

Stardust’s primary contribution to planetary science is verifi­cation of the idea that there was extensive radial mixing of ma­terial in the solar nebula just as the Solar System was forming. Also, comets are more physically diverse and variegated than any­one imagined. Each of the comets where we’ve got up close and personal—Halley, Borrelly, Tempel 1, and Wild 2—have different shapes, surface textures and features, and levels of activity. Com­ets may be modest in size compared to planets and moons, but their role in planetary systems is anything but modest. They deliver water and organic building blocks of life to terrestrial planets, and as we know from studies of our geological record, when they hit a planet they can decisively alter the history of ecosystems and even the entire biosphere. It’s appropriate that all human cultures have been fascinated by comets, and viewed them as harbingers of life and death.

To learn more requires an increase in our modest amount of di­rect information. So it was very exciting when Stardust was given a new lease on life after its sample return. In 2007, NASA approved the New Exploration of Tempel 1 (NExT) mission, a return to the site of the Deep Impact mission in 2005.25 Stardust had con­served just enough hydrazine fuel to make a second journey to a comet, and the goal was to observe the impact made by the ear­lier spacecraft and see any changes on Tempel 1 caused by its last close approach to the Sun (Deep Impact’s cameras were blinded by dust released from the impact). In early 2010, a controlled burn ensured Stardust would approach at the optimal speed. The flyby was challenging because the camera had to catch the correct side of the tumbling comet nucleus while coming up on it at 7 miles per second. Appropriately, our first second date with a comet took place on Valentine’s Day, 2011. Stardust showed that the 500-foot wide crater created in 2005 was indistinct and had a mound in the center, indicating that much of the ejected material had fallen straight back down in the weak comet gravity. The results indi­cated that the comet nucleus was fragile and only weakly held together.

All good things must come to an end. For Stardust the end came on March 24, 2011, when mission controllers deliberately ordered a “burn to completion.” They told the spacecraft, which was al­ready running on fumes, to fire its main engines until there was no fuel left. NASA used the final burn to refine fuel consumption models for these kinds of engines, ensuring that Stardust gave use­ful data right up to its last gasp. The next day, Project Manager Tim Larson put the spacecraft into a state called safe mode, turned off the transmitter, and walked away from the console. Mission accomplished.