The Interstellar Age: Inside the Forty-Year Voyager Mission. By Jim Bell. New York: Dutton Books, Published by the Penguin Group, 2015. xi + 321. Notes and further reading, acknowledgments, index. ISBN: 978-0-525-95432-3. Hardcover with dustjacket. $27.95 USD.
Part memoir, part anecdotal history, and part sermon on the delights of science, Arizona State University planetary scientist Jim Bell presents here a captivating story of the missions of Voyagers 1 and 2 to the outer edge of the Solar System and eventually beyond. Bell is a veteran of many space science missions, including several of the recent Mars probes. He brings an in-depth, nuanced understanding of the nature of big planetary science efforts and a deft writing style to this popular account of the Voyager mission.
Voyager had assumed legendary proportions before Bell’s book, although The Interstellar Age certainly adds to it. Conceived in the 1960s, launched in the 1970s, and encountering all of the larger outer planets of the Solar System between the latter 1970s and the 1990s, the Voyager spacecraft continue on an interstellar mission at the Heliopause where the Sun’s solar wind meets the interstellar medium. The twin Voyager probes might best be characterized, and this may be an understatement, “the little spacecraft that could.”
Jim Bell writes about how in the early 1960s several scientists realized that once every 176 years both the Earth and all the giant planets of the Solar System gather on one side of the Sun, making possible close-up observation of them all in what has been dubbed the “Grand Tour.” Moreover, the flyby of each planet could through “gravity assist,” something like a slingshot effect, increase velocity and reduce flight times between planets by several years. Such a configuration occurred in the 1970s, and the Voyagers took advantage of it.
Bell describes how politics entered into planning; even though the four-planet scenario was possible NASA deemed it too expensive to build a spacecraft that could go the distance, carry the instruments needed, and last long enough to accomplish such an extended mission. With insufficient money for the Grand Tour, Voyager had to be down-scoped to a Jupiter/Saturn flyby. Nonetheless, engineers designed as much longevity into the two Voyagers as the $865 million budget would allow. NASA launched them from the Kennedy Space Center; Voyager 2 lifted off on August 20, 1977, and Voyager 1 entered space on a faster, shorter trajectory on September 5, 1977.
Hanging out with the science team during encounters while still a student, Bell describes how they achieved their objectives and then some at Jupiter and Saturn and then added flybys of the two outermost giant planets, Uranus and Neptune. Bell reports how as the two spacecraft flew, ground controllers reconfigured the Voyagers for extended operations. It was no easy task, the technology was old even then, but mission engineers and scientists made it work.
Eventually the Voyagers explored all the giant outer planets, 48 of their moons, and the unique systems of rings and magnetic fields those planets possess. They sent back to Earth well over 100,000 images of the outer planets, rings, and satellites, and took magnetic, chemical spectra, and radiation measurements. The two spacecraft returned information that revolutionized Solar System science, helping resolve key questions while raising intriguing new ones about the origin and evolution of the planets. Bell is at his best in telling the human stories of discovery, excitement, and public engagement.
By 2015 Voyager 1 had reached more than 130 Astronomical Units (AU) from the Sun, and Voyager 2 was at more than 107 AU. After picking up velocity from gravity assist, Voyager 1 has the greatest velocity and is leaving the solar system at about 3.6 AU per year. Voyager 2 has a slightly lower velocity, at 3.3 AU annually. They continue to take readings of the Heliopause.
It is very unlikely that either of these spacecraft will ever be seen by any alien civilization, but they are prepared if it ever happens. Thanks to Carl Sagan, one of Jim Bell’s heroes, they both contain messages from Earth. Affixed to the spacecraft’s exterior are gold-covered phonograph records and covers with instructions for use. Encoded on the records are 115 images showing scenes from Earth, audio greetings in several languages, and musical selections ranging from Bach to Chuck Berry. This “message in a bottle” is one of the most popular attributes of this mission, and Bell explains well its publicity value as well as its general “feel good” sentiment about the possibility of life beyond Earth someday encountering it.
If Apollo was the greatest achievement in NASA history, and I believe it was, the success of the Voyagers is also exceptionally high on the space agency’s list of accomplishments. Bell appropriately quotes historian Stephen Pyne on this significance: “The Voyagers were special when they launched. They have become more so thanks to their longevity, the breadth of their discoveries, the cultural payload they carried, and the sheer audacity of their quest” (pp. 291-92).