Growing up in the 1970s, I watched the progress of every NASA mission to the planets with great enthusiasm. Probably the one mission I waited for with the greatest anticipation was NASA’s proposed “Grand Tour” of the outer planets. I first learned of this mission in 1970 and followed its progress during the years that followed as best as I could in the pre-Internet Age through magazines and NASA publications (see “Growing Up in the Space Age: Summer Vacations of the 70s”). Eventually, a pair of Mariner Jupiter-Saturn 77 spacecraft, renamed Voyager, were launched in the summer of 1977 destined to flyby Jupiter and Saturn with a longshot option of Voyager 2 continuing on to Uranus and Neptune in the decade ahead (see “Voyager 2: The First Uranus Flyby”).
The launch of Voyager 1 from Cape Canaveral on September 5, 1977. (NASA)
While most space enthusiasts of the time seemed to give most of their attention to what Voyager would reveal about the planets Jupiter and Saturn themselves, my interest was focused instead on the large planet-sized moons orbiting these giants. This interest was only amplified by the tantalizing glimpses of the four large Galilean moons of Jupiter (which I had been regularly observing at the time with my first telescopes) from NASA’s Pioneer 10 and 11 which reached Jupiter at the end of 1973 and 1974, respectively. I knew that the far superior cameras being carried by the pair of Voyagers would yield spectacular views of these unexplored worlds during their Jupiter encounters in 1979.
Diagram of the major components and instruments of the Voyager spacecraft. Click on image to enlarge. (NASA)
Unlike the simple scanning imagers carried by Pioneer 10 and 11, each three-axis stabilized Voyager spacecraft carried a pair of vidicon-based slow scan television cameras attached on a pointable Science Scan Platform mounted on the end of one of the booms extending from the spacecraft’s bus. This would be the last NASA planetary mission to use vidicon-based imagers with improved solid state cameras employed on future missions. Despite the comparatively primitive nature of these cameras by today’s standards, JPL engineers used their 13 years of experience flying vidicon-based cameras on earlier Mariner and Viking missions to create highly sensitive and stable cameras suitable for use in the dim reaches of the outer solar system.
Diagram showing the layout of Voyager’s wide and narrow angle cameras. Click on image to enlarge. (NASA)
Each Voyager carried a wide and a narrow angle camera which employed 200 mm and 1,500 mm focal length lenses, respectively, to focus images on an 11 mm square photoconductor-coated vidicon plate. This resulted in 3.2° and 0.42° fields of view for these cameras – the equivalent of using 400 mm and 3,200 mm lenses on an old-style 35 mm format camera. A shutter assembly on each camera had a range of standard exposure settings available from 0.005 to as long as 15.26 seconds as well as an extended exposure mode which allowed the shutter to remain open for many minutes. The images were subsequently readout in as quickly as 48 seconds after being sliced into 800 lines consisting of 800 pixels each. Each pixel was digitized to 8 bits and could be transmitted live at a rate of as fast as 115,200 bits per second or saved on a magnetic tape recorder with a 100-image capacity (the equivalent of about 61 megabytes of data). Each camera had its own eight-position filter wheel which included orange, green and blue filters that could be combined to create near true color images. Because of the low sensitivity of vidicon cameras at redder wavelengths, a wide-band orange filter was employed instead of a red filter which would normally be used to create true color images. We got a foretaste of the capabilities of the Voyagers from a parting color image of the Earth and Moon together in space taken from a range of 11.7 million kilometers by Voyager 1 about two weeks after its launch on September 5, 1977 (see “First Pictures: Voyager 1 Portrait of the Earth & Moon – September 18, 1977”).
Here is a color image showing the Earth and Moon together for the first time as seen from a departing interplanetary spacecraft. Voyager 1 took this color image on September 18, 1977 from a range of 11.66 million kilometers during its outbound trip to Jupiter. (NASA/JPL)
By the beginning of 1979 when I was in the middle of my junior year of high school, Voyager 1 began the Observatory Phase of its encounter with Jupiter while still tens of millions of kilometers distant. In the weeks that followed, the trickle of new Voyager images turned into a veritable torrent as its March 5 close encounter date with Jupiter approached. The images of the Galilean moons returned by Voyager 1 did not disappoint. Instead of a quartet of dead, battered planet-sized moons that had been expected, each of the moons displayed a range of features indicating that they each had a unique geologic history very different from that of the Moon.
Here is a montage of full-disk images of the Galilean moons acquired by Voyager 1 during its approach to Jupiter in early-March 1979. Scaled to their relative sizes, they are Io (upper left), Europa (upper right), Ganymede (lower left) and Callisto (lower right). Click on image to enlarge. (NASA/JPL)
Most perplexing of all of the Galilean moons was Io. Only slightly larger than the Moon, Io had been expected to be heavily cratered like the Moon with various salts deposited on its desiccated surface, based on earlier spectroscopic studies. Instead, Io was a strange world covered in yellow, orange and white deposits along with enigmatic geologic features without any impact craters visible. Even a young budding planetary scientist like myself at the time knew that this meant Io’s surface had been reworked in the geologically recent past erasing billions of years of cratering events in the process. But what exactly caused this resurfacing was a mystery to everyone as Voyager 1 made its closest approach to Jupiter at 12:05 GMT on Monday, March 5 and began its long exit from the Jupiter system.
Here are views of a global color mosaic of Io created using images acquired by Voyager 1. The left image shows the moon’s eastern hemisphere while the other shows the western hemisphere. Click on image to enlarge. (NASA/JPL/USGS)
The mystery about Io’s youthful appearance as well as other odd observations about the moon and its environment uncovered by Voyager’s suite of instruments began to be solved almost by accident in the days after the encounter. At about 13:00 GMT on Thursday March 8, Voyager 1 acquired a long-exposure image of Io at a range of 4.5 million kilometers against a background of stars for the spacecraft’s navigation team. Later that day, an optical navigation engineer on the Voyager team named Linda Morabito was examining the image and noted what appeared to be a cloud hundreds of kilometers above the limb of Io’s overexposed crescent. Working with her colleagues the following day, Morabito eliminated all other possibilities including imaging artifacts and came to the conclusion that the cloud was real. And since Io had only a very tenuous atmosphere, the only conclusion left was that the cloud was the plume from a very powerful volcanic eruption on the surface of Io.
This is an enlargement centered on Io as it appeared in a long-exposure optical navigation image taken by Voyager 1 on March 8, 1979 at a range of 4.5 million kilometers. A volcanic plume is visible above the illuminated limb of Io in the lower right. A second plume was identified as causing the bright spot on the crescent’s terminator. (NASA/JPL)
Linda Morabito with a copy of the image in which she discovered the volcanic plumes of Io on March 8, 1979. (NASA/JPL)
Morabito showed the image to members of the Voyager Imaging Science Team later on Friday who agreed with her findings. Unfortunately, more senior members of the team, Brad Smith and Larry Soderblom, had already left for a well-deserved weekend off after a couple of weeks of nonstop work during the Voyager encounter. Late on Sunday night, March 11, Smith, Soderblom and other members of the Voyager Imaging Science Team finally got to see the image for themselves. Not only did it show a plume above Io’s limb, provisionally designated P1, it also had a bright plume on the terminator catching the rays of the Sun designated P2. Investigations showed that P1 was associated with a feature later called Pele which was also found coincide with a hotspot 200° C warmer than its surroundings observed by Voyager’s Infrared Spectrometer (IRIS). IRIS also detected sulfur and its compounds across the Ionian surface. P2 was found to be linked to a volcanic feature later called Loki which included a sulfur-rich lava lake. Morabito’s hypothesis had been confirmed: Io has active volcanoes and they must be responsible for the moon’s youthful appearance with the strange colors resulting from various allotropes of sulfur and light colored sulfur dioxide “snow” across the surface.
This color mosaic of closeup images from Voyager 1 shows the volcanic feature known as Pele surrounded by its hoof-shaped ring of volcanic deposits. The part of the mosaic above the limb of Io has been processed to reveal its volcanic plume reaching 300 kilometers above the surface. Click on image to enlarge. (NASA/JPL/USGS)
Here is a process closeup view of the volcanic feature called Loki taken by Voyager 1. The observed plume originated from a feature to the lower left of the dark linear fissure-like feature below the center of the image. The U-shaped feature is a sulfur-rich lava lake about 200 kilometers across. Click on image to enlarge. (NASA/JPL)
Members of the Voyager Imaging Science Team reprocessed other Voyager 1 images of Io looking for plumes missed in the originally processed versions. In the end, a total of eight active plumes were found in the images confirming that Io is the most volcanically active world known in the Solar System to this day. The media were informed about the momentous discovery of active volcanism on Io on Monday, March 12. With the July 9 encounter of Voyager 2 less than four months away, scientists on the Voyager team were able to plan a new series of Io observations to monitor the volcanoes from a much greater distance (since the closest approach of Voyager 2 to Io would be 1.13 million kilometers – 54 times greater than its predecessor’s minimum flyby distance). Voyager’s observations of active volcanoes on Io ranks among the top findings of this historic mission of discovery.
A specially processed color view of Io taken by Voyager 1 at a range of 490,000 kilometers showing Loki’s plume above the horizon. The hoof-shaped deposits surrounding the active volcano Pele are visible in the lower part of the image. Click on image to enlarge. (NASA/JPL/USGS)
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Related Reading
“First Pictures: Voyager 1 Portrait of the Earth & Moon – September 18, 1977”, Drew Ex Machina, September 18, 2020 [Post]
“Voyager 1: The First Close Encounter with Titan”, Drew Ex Machina, November 12, 2015 [Post]
“Voyager 2: The First Uranus Flyby”, Drew Ex Machina, January 24, 2016 [Post]
“Finishing the Grand Tour: Voyager 2 at Neptune”, Drew Ex Machina, August 29, 2019 [Post]
General References
Michael M. Mirabito, The Exploration of Outer Space with Cameras, McFarland, 1983
David Morrison and Jane Samz, Voyage to Jupiter, NASA SP-439, 1980
Bradford A. Smith et al., “The Jupiter System Through the Eyes of Voyager 1”, Science, Vol. 204, No. 4396, pp. 951-972, June 1, 1979
E.C. Stone and A.L. Lane, “Voyager 1 Encounter with the Jovian System”, Science, Vol. 204, No. 4396, pp. 945-948, June 1, 1979
Voyager to Jupiter and Saturn, NASA SP-420, 1977