The Interstellar Age: Inside the Forty-Year Voyager Mission

  • Jim Bell
Dutton: 2015. 9780525954323 | ISBN: 978-0-5259-5432-3

Part memoir, part anecdotal history and part sermon on the delights of science, The Interstellar Age is a captivating read. In it, planetary scientist Jim Bell presents the eventful story of NASA's Voyager 1 and Voyager 2 missions to the edge of the Solar System and beyond. Bell, a veteran of many space-science missions, including several Mars probes, brings deft writing and an in-depth, nuanced understanding of big planetary-science efforts to this popular account.

Jupiter's Great Red Spot, a massive stable storm photographed by Voyager 2 in 1979. Credit: NASA/SPL

Voyager's legendary status has been long assured, although The Interstellar Age will add to its cachet. Conceived in the 1960s and launched in the 1970s, the twin probes encountered the larger outer planets of the Solar System — Jupiter, Saturn, Uranus and Neptune — between the late 1970s and the late 1980s. Having gone far beyond their original remit, they now continue an interstellar mission at the edge of the Solar System. The probes are, in essence, 'the little spacecraft that could'.

Bell writes about how, in the early 1960s, aerospace engineer Gary Flandro and other scientists realized that once every 176 years, Earth and the Solar System's four giant planets gather on one side of the Sun. This would enable close-up observation of the giants, in a planetary 'grand tour'. During the fly-by of each planet, a gravity assist — a slingshot effect, harnessing the planet's movement and gravity — could increase the spacecraft's velocity and reduce flight times.

Bell describes the politics of the planning stage. The four-planet scenario was possible, but NASA deemed it too expensive to build a 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 Institute of Technology in Pasadena, describes how the craft achieved their objectives — and then some. Voyager 1 was, for example, programmed for a close encounter with Saturn's moon Titan, during which it revealed a complex 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 continue on to Uranus and Neptune, the craft's planetary mission ended.

Voyager 2 continued on to the two outermost giant planets. Bell reports how, as the probes flew, controllers constantly reprogrammed the on-board computers, which had only about 5,000 words' worth of memory each, to take advantage of scientific opportunities. Successfully capturing data was hugely taxing, but mission engineers and scientists made it work.

The identical Voyager probes launched in 1977 and are still travelling. Credit: NASA/JPL/SPL

The probes explored the giant planets' systems of rings and magnetic fields, finding previously unknown geological activity on Io, a moon of Jupiter with numerous volcanic features. The Voyagers also explored a total of 48 moons around the gas giants. They sent back more than 100,000 images of the planets, rings and satellites, and took magnetic measurements, chemical spectra and radiation readings. The information revolutionized Solar System science, helping to resolve key questions about how it was formed and raising intriguing new ideas such as the possibilities of life beyond Earth.

Bell is at his best in telling the human stories of discovery, excitement and public engagement. He describes the extension of the Voyager mission to the heliopause, where the Sun's energy is overpowered by interstellar forces. Ed Stone, who has been chief scientist for Voyager since its inception, evinced the excitement of a self-confessed non-party animal. “I can still remember taking the data home every night, and putting the plots on the refrigerator,” he tells Bell. “I couldn't stop thinking about them, wondering what would happen next.”

Voyager 1 is now more than 130 astronomical units (AU) from the Sun (1 AU is the distance between the Sun and Earth, around 150 million kilometres), and Voyager 2 is at more than 107 AU. They continue to take readings of the heliopause.

Thanks to astronomer Carl Sagan, one of Bell's heroes, both probes contain messages from Earth: gold-plated copper phonograph records encoded with 115 images of scenes from Earth, audio greetings in 55 languages, and 90 minutes of music from Bach to Chuck Berry, along with playing instructions. This message in a bottle is one of the mission's best-known attributes, and Bell explains well its publicity value and how it represents a feel-good sentiment about the possibility of encountering interstellar life.

The Voyagers demonstrate the remarkable advances in robotic space exploration over the past almost 40 years, and suggest that subsequent missions may yield even more exciting results. Such follow-ups as the Galileo mission to Jupiter, Huygens–Cassini to Saturn and New Horizons to the Kuiper belt including Pluto may herald even more ambitious missions — to Titan, for instance, where they might sail on a hydrocarbon sea, or to Jupiter's moon Europa, to explore an ice-covered liquid-water ocean that has the potential to harbour life.

I believe that NASA's greatest achievement is the Apollo Moon programme. The odyssey of the Voyagers certainly vies for second place. Bell appropriately quotes historian Stephen Pyne: “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.”