Wednesday’s Book Review: “The Interstellar Age”

The Interstellar AgeThe 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).

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5 Responses to Wednesday’s Book Review: “The Interstellar Age”

  1. spacegary says:

    Sounds like a great book thank you for the review. Off to buy it!


  2. Unfortunately, Bell parrots the JPL line that Gary Flandro invented gravity-assists and worked out that there was a once-in-176-years mystic alignment of planets. Neither is accurate. Michael Minovitch worked out the math behind gravity-assists while a UCLA student working at JPL, but he wasn’t very socially skilled, nor was he a JPL staffer. So, JPL cites Flandro’s 1965 paper – apparently hoping no one will look at it, because it cites two Minovitch JPL memos. In the Voyager Neptune Travel Guide, Charlie Kohlhase set the record straight. I’m given to understand that wasn’t a popular move.

    As for the 176 yrs thing, Grand Tour windows extended into the 1990s. They can in fact be made to occur any time. Jupiter can grav-assist a spacecraft every 13 months, and subsequent flybys are a matter of correct navigation. One could manage a three-planet Grand Tour now with a flight-time about the same as Voyager 2’s. The origins of the 176 yrs thing is somewhat mysterious – it might have been designed to give the Grand Tour a political push, though I’ve also heard that it was a JPL PR flack’s creation.

    If one is keen on grav-assist history, a more significant grav-assist – also calculated by Minovitch – was the Mariner Venus-Mercury example. It was in fact the first use of grav-assist to travel in a guided manner to a second planetary target and the first Minovitch grav-assist opportunity put to use.

    I wrote about this – – as part of a series about how our concept of how we would reach the planets evolved – rather rapidly despite resistance – during the late 1950s/early 1960s.

    I contacted Bell about this error and his response was along the lines of “I talked to Flandro and that’s good enough for me.” I’m glad you referred to this book as partly anecdotal. I’d call it a memoir, a genre that must be approached with caution. Memory is not always reliable. The sad thing is, researching the documents on this and giving credit where it’s due should not have been hard. So, the Flandro & 176 yrs mythology, which denies Minovitch due credit, gets another high-profile repetition. It’s sad.



    • launiusr says:

      Great comments. Thanks so much David.


    • Jay Gallentine says:

      Hi David,

      I spent three years carefully researching the whole Flandro/Minovitch/Grand Tour thing. Perhaps I can clarify.

      While Gary Flandro certainly did not come up with the gravity-assist concept, he should definitely be credited with discovering the Grand Tour opportunity. Flandro has repeatedly denied to me that he ever originated the idea of gravity assist, saying that he studied it in college.

      Mike Minovitch had nothing whatsoever to do with the Grand Tour, or Voyager.

      While Minovitch certainly calculated multi-planet spacecraft trajectories, he was absolutely not the first to do so. The technique was already well-understood long before he started work at JPL. Krafft Ehricke was lecturing about it at UCLA as early as 1959. Minovitch merely came up with his own method.

      The section about Minovitch in the Voyager Neptune Travel Guide was NOT written by Charley Kohlhase, and Kohlhase does NOT agree with what was stated in that Guide. Kohlhase never reviewed the text of the Guide while it was being prepared, and regrets what it says.

      The entire Minovitch saga, as well as gravity assist history and the contributions of Flandro, are covered in my award-winning book “Ambassadors from Earth.” Frankly, I don’t know of any other publication beyond mine which has told the story accurately.


  3. This is puzzling to me since the documents – and not just the VOYAGER NEPTUNE TRAVEL GUIDE – would seem to indicate something else. A general awareness of gravity-assist dates back at least to the 18th century – see my discussion of Lexell’s Comet – but it didn’t make much headway in the spaceflight world until Max Hunter promoted Minovitch’s work, per my blog post.

    JPL engineers rejected Minovitch’s work at first because they thought gravity-assist violated laws of physics – and, I suspect, because Minovitch was not a people person. Only after Hunter’s prestige got into the mix did it receive due consideration.

    Others then jumped on the bandwagon before Flandro – if you haven’t already, look into proposals for a “Galactic Jupiter Probe.” That’s the most obvious example, I think.

    I haven’t read your book, by the way. This past 15 years or so I’ve mostly worked with primary documents. There are so many award-winning space books out there, including at least one of mine. Where does one start? Mostly I buy reference books, not narratives, to supplement the primary documents I collect from archives or from their elderly authors.

    Minovitch’s early (pre-Flandro) papers describe “the Grand Tour” – though he didn’t call it that, since he understood that Jupiter can be used every 13 months to get to any other world in the Solar System and points beyond without using propellants. That’s a crucial point here.

    I don’t delve into it in my post, but Pioneer 11 used Jupiter to get to Saturn. One reason it needed as long as it did to reach Saturn is that the trajectory sent it inward toward the Sun after it left Jupiter. Work toward that mission design post-dates Minovitch/Hunter and pre-dates Flandro/Grand Tour.

    In his first “Grand Tour” paper, Flandro properly cites Minovitch’s work, which was mainly published in limited-circulation JPL internal documents and thus easy to ignore. Minovitch, whose connection was more with UCLA than JPL, was not good at self-promotion. JPL promoted Flandro and the “Grand Tour.”

    There was never a single “Grand Tour” opportunity. I do not know where the “once-in-176-years” thing got its start. I think I heard of it first from Carl Sagan on a 1970s NOVA program when I was a kid.

    Perhaps it depends on how one defines “Grand Tour.” Looking at the proposed Pluto Grand Tour mission, the definition would seem to admit any Jupiter mission that then travels to any other Outer Solar System world. Conversely, if one defines it narrowly, one can limit it to a once-in-176-years magic alignment.

    We will see new “Grand Tour” opportunities in the 2020s (they shade over into the 2030s). This is one reason why we see an upsurge in Outer Planets mission proposals for the 2020s decade. Few seek to fly past multiple planets – most seek to use a Jupiter gravity-assist to launch relatively fast, relatively heavy missions to individual Outer Solar System targets.

    It was a Minovitch trajectory that got Mariner 10 from Venus to Mercury. That was the first use of gravity-assist for interplanetary spaceflight. The earliest follow-on studies that detailed how we might use that opportunity to explore Mercury date from shortly after Hunter’s description of Minovitch’s work and give Minovitch credit.

    Bell admitted to me that he did not research the origins of gravity-assist. He merely spoke with Flandro and repeated Flandro’s anecdotes. That is not how an historian works. Ultimately, that’s the point of my comment on Roger’s post.



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