BOOKS & ARTS COMMENT SPACE SCIENCE Beyond the heliopause Roger D. Launius savours a masterful account, by a veteran of interplanetary space science, of the Voyager probes’ mission to the giant planets. spacecraft for an extended mission. Voyager was downgraded to a Jupiter–Saturn fly-by, but engineers designed as much longevity into the probes as the US$875-million budget would allow. NASA launched Voyager 2 on 20 August 1977, and Voyager 1 followed on a faster, shorter trajectory on 5 September. Bell, who hung out with the science team as an undergraduate at the California Insti- tute of Technology in Pasadena, describes how the craft achieved their objectives — and then some. Voyager 1 was, for example, pro- grammed for a close encounter with Saturn’s moon Titan, during which it revealed a com- plex world with an atmosphere, thick clouds and water ice. It showed that Titan was ripe for scientific investigation, paving the way for the sustained investigations of the Huygens– Cassini mission at the dawn of this century. But this fly-by deflected Voyager 1 out of the Solar System’s elliptical plane; unable to con- tinue on to Uranus and Neptune, the craft’s planetary mission ended. Voyager 2 continued on to the two outer­ most giant planets. Bell reports how, as the probes flew, controllers constantly repro- grammed the on-board computers, which had only about 5,000 words’ worth of mem- ory each, to take advantage of scientific opportunities. Successfully capturing data was hugely taxing, but mission engineers and scientists made it work. The probes explored the giant planets’ systems of rings and magnetic fields, find- NASA/SPL Jupiter’s Great Red Spot, a massive stable storm photographed by Voyager 2 in 1979. ing previously unknown geological activity on Io, a moon of Jupiter with numerous vol- art memoir, part anecdotal history late 1980s. Having gone far beyond their canic features. The Voyagers also explored and part sermon on the delights of original remit, they now continue an inter- a total of 48 moons around the gas giants. science, The Interstellar Age is a capti- stellar mission at the edge of the Solar System. They sent back more than 100,000 images Pvating read. In it, planetary scientist Jim Bell The probes are, in essence, ‘the little space- of the planets, rings and satellites, and took presents the eventful story of NASA’s Voy- craft that could’. magnetic measure- ager 1 and Voyager 2 missions to the edge of Bell writes about how, in the early 1960s, ments, chemical the Solar System and beyond. Bell, a veteran aerospace engineer Gary Flandro and other spectra and radiation of many space-science missions, including scientists realized that once every 176 years, readings. The infor- several Mars probes, brings deft writing and Earth and the Solar System’s four giant mation revolutionized an in-depth, nuanced understanding of big planets gather on one side of the Sun. This Solar System science, planetary-science efforts to this popular would enable close-up observation of the helping to resolve key account. giants, in a planetary ‘grand tour’. During questions about how it Voyager’s legendary status has been long the fly-by of each planet, a gravity assist — was formed and rais- assured, although The Interstellar Age will a slingshot effect, harnessing the planet’s ing intriguing new add to its cachet. Conceived in the 1960s movement and gravity — could increase the The Interstellar ideas such as the pos- Age: Inside the and launched in the 1970s, the twin probes spacecraft’s velocity and reduce flight times. Forty-Year Voyager sibilities of life beyond encountered the larger outer planets of the Bell describes the politics of the planning Mission Earth. Solar System — Jupiter, Saturn, Uranus and stage. The four-planet scenario was possible, JIM BELL Bell is at his best Neptune — between the late 1970s and the but NASA deemed it too expensive to build a Dutton: 2015. in telling the human 30 | NATURE | VOL 519 | 5 MARCH 2015 © 2015 Macmillan Publishers Limited. All rights reserved BOOKS & ARTS COMMENT The identical Voyager probes launched MATHEMATICS in 1977 and are still travelling. stories of discovery, excitement and public engagement. He describes the extension Groping in the dark for of the Voyager mission to the heliopause, NASA/JPL/SPL where the Sun’s energy is overpowered by interstellar forces. Ed Stone, who has glimpses of beauty been chief scientist for Voyager since its inception, evinced the excitement of a Amir Alexander relishes two accomplished accounts self-confessed non-party animal. “I can of the life mathematical. still remember taking the data home every night, and putting the plots on the refrig- erator,” he tells Bell. “I couldn’t stop think- ne evening in 2009, French math- Birth of a Theorem: A Mathematical ing about them, wondering what would ematician Cédric Villani stepped Adventure happen next.” into his children’s room. He locked CÉDRIC VILLANI Bodley Head/Faber and Faber: 2015. Voyager 1 is now more than 130 astro- Othe door, turned off the light and began to nomical units (au) from the Sun (1 au pace, pondering the statistical properties of Mathematics Without Apologies: Portrait of is the distance between the Sun and plasma. A few metres away, his wife, Claire, a Problematic Vocation Earth, around 150 million kilometres), was in the kitchen, cooking dinner for the MICHAEL HARRIS Princeton Univ. Press: 2015. and Voyager 2 is at more than 107 au. family. The contrast, Villani concedes in They continue to take readings of the his engaging Birth of a Theorem, was “a heliopause. bit much”, yet immediately after dinner he Thanks to astronomer Carl Sagan, returned to the dark room to grapple for one of Bell’s heroes, both probes contain hours with his elusive proof. messages from Earth: gold-plated cop- Anyone reading that anecdote will feel per phonograph records encoded with sympathy for Claire as she tries to preserve JOEL SAGET/AFP/GETTY 115 images of scenes from Earth, audio family normality. Anyone who has seri- greetings in 55 languages, and 90 minutes ously engaged with mathematics will also of music from Bach to Chuck Berry, along understand her husband. Perhaps more with playing instructions. This message than any other field, mathematics pulls the in a bottle is one of the mission’s best- practitioner away from the ‘normal’ world known attributes, and Bell explains well of things and people into a strange alternate its publicity value and how it represents a universe, in which we catch glimpses of feel-good sentiment about the possibility beauty and coherence, but spend most of our of encountering interstellar life. time groping in the dark. In Birth of a Theo- The Voyagers demonstrate the remark- rem, Villani offers one way of straddling that able advances in robotic space explora- divide; in Mathematics Without Apologies, tion over the past almost 40 years, and fellow mathematician Michael Harris pre- suggest that subsequent missions may sents a very different one. Together, they yield even more exciting results. Such provide an unmatched perspective on life Cédric Villani studies the maths of plasmas. follow-ups as the Galileo mission to Jupi- in this “problematic vocation” by two of its ter, Huygens–Cassini to Saturn and New leading practitioners. portrayals of mathematicians and physicists Horizons to the Kuiper belt including Birth of a Theorem is the story of Villani’s past and present. The grumpy, grey-haired Pluto may herald even more ambitious quest to give a full mathematical account Étienne Ghys of Lyons and Chinese expat missions — to Titan, for instance, where of Landau damping. Whereas gas becomes Alice Chang of Princeton alternate with the they might sail on a hydrocarbon sea, or increasingly disordered over time as entropy autocratic Lev Landau in 1960s Moscow and to Jupiter’s moon Europa, to explore an increases, plasma spontaneously stabilizes, the ever-present shadow of Albert Einstein at ice-covered liquid-water ocean that has with no increase in entropy. Soviet physicist the Institute for Advanced Study in Prince- the potential to harbour life. Lev Landau was the first to mathematically ton. Much of Villani’s e-mail correspondence I believe that NASA’s greatest achieve- describe this improbable phenomenon, but with Mohout is reproduced, chronicling ment is the Apollo Moon programme. The he used a simplified model that left many moments of triumph and despair. odyssey of the Voyagers certainly vies for unconvinced. Working closely for several Charismatic and flamboyantly dressed, second place. Bell appropriately quotes years with mathematician Clément Mohout, Villani is the opposite of the ‘mathematical historian Stephen Pyne: “The Voyagers Villani succeeded, and he was awarded a hermit’ and annoyed by the stereo­type. He were special when they launched. They Fields Medal in 2010. attends a recital by one of his children, joins have become more so thanks to their lon- Villani’s quest takes him across the world, his family at the American Museum of Natu- gevity, the breadth of their discoveries, from Lyons, France, to Princeton, back ral History in New York and travels a long way the cultural payload they carried, and the to Paris and on to Hyderabad in India. At to attend a concert by rock band Têtes Raides. sheer audacity of their quest.” ■ every stop, he talks to local mathematicians, Yet he studies the mathematics of galaxy for- demonstrating that, mation during the recital, works out a step in Roger D. Launius is associate director for all its abstractness, NATURE.COM his proof on the bus from the museum, and for collections and curatorial affairs at the mathematics can be an For more on science explains his research to a stranger who drives Smithsonian Institution’s National Air intensely social activ- in culture, see: him back from the concert.