13 March 2023

This Month in Astronomical History: March 2023

Loretta Cannon Rose City Astronomers

HAD LogoEach month as part of this series from the AAS Historical Astronomy Division (HAD), an important discovery or memorable event in the history of astronomy will be highlighted. This month's author, Loretta J. Cannon, writes about Voyager 1’s discoveries during its Jupiter flyby. Interested in writing a short (500-word) column? Instructions along with previous history columns are available on the HAD web page.

Voyager 1: The Jupiter Flyby

Forty-four years ago this month, on 5 March 1979, the Voyager 1 spacecraft made its closest approach to Jupiter,1 the first of four flybys on a tour of the outer planets, forever altering the course of human history and exponentially increasing our planetary science knowledge. Up until that moment, the project had involved twelve years of planning, engineering, construction, and after launch, two more years of waiting. But in March 1979, the science finally began.  According to Rich Terrile (then Voyager Imaging Science), “We called it drinking out of a firehose, you try to take a little sip and this torrent of data is coming out.”2 

The journey to Jupiter began in the summer of 1965, when a project called The Outer Planets Grand Tour was conceived. When approved in 1972 for only two planets, the mission became Mariner Jupiter/Saturn 1977.  By March 1977, just five months prior to launch, it was renamed Voyager. The initial idea was based on a rare – once every 175 years – alignment among the outer planets, a planetary alignment that would allow a tour of the gas giants in only twelve years rather than thirty, a planetary alignment plus gravity assist. Two spacecraft were built, Voyager 1 and Voyager 2, and launched on 5 September 1977 and 20 August 1977, respectively. Voyager 1’s trajectory allowed it to outpace Voyager 2 quickly, arriving first at Jupiter and Saturn before angling out of the ecliptic towards interstellar space. Voyager 2 was the only spacecraft to continue to Uranus and Neptune before it too angled out of the ecliptic. As of today, both Voyagers continue to communicate with NASA/JPL via the Deep Space Network.

Before the Voyager mission, planetary scientists gathered data on the outer planets from Earth-based telescopes and then Pioneers 10 and 11. Pioneer 103 flew by Jupiter on 3 December 1973, obtaining the first close-up photos while mapping both the planet’s intense radiation belts and strong magnetic field, information that proved crucial to Voyager’s designers. Pioneer 114 flew by Jupiter on 2 December 1974, making the first observations of its immense polar regions. And yet, no one was quite prepared for what Voyager discovered.

Many interesting features of the Jupiter system were seen, or heard, for the first time during Voyager 1’s weeklong flyby. Among these features was lightning in the clouds of Jupiter. Initially, the existence of lightning was based on photographic evidence (Figure 1).5 Subsequent analysis of wideband plasma data confirmed characteristics analogous to whistlers created by lightning.6, 7 One arc was reported to have flashed a distance equivalent to the width of the United States from coast to coast. One can listen to Jupiter whistlers in this recording archived by Prof Don Gurnett.8  

Jupiter’s system was examined using infrared spectroscopy and radiometry.9 In addition to learning that Jupiter’s northern atmosphere is warmer than the south, the authors learned that the Great Red Spot is not only a huge anticyclonic vortex with wind speeds approaching 650 kph, but is also significantly colder than surrounding cloud regions. But, according to the authors, that anticyclonic motion “implies a local pressure high, which, in the absence of a solid surface, requires a warm core.” No such warm region was detected by Voyager 1’s infrared instruments.

Voyager 1 examined the four Galilean moons photographically (Figure 2),5 which were previously known to be tidally locked with Jupiter with Io orbiting the closest, then Europa, Ganymede, and Callisto. For each moon, the quantity of data reveals something new. Multiple linear features on Europa seemed to indicate tectonics, while other indicators led the authors to conclude that “a large fraction (20 percent) of Europa is water,” with a possible water-ice shell (ocean) up to 100 km thick. Ganymede and Callisto uniquely exemplify “a class of large, low-density planetary objects that [had] never before been studied in detail,” with Ganymede estimated to be approximately 50 percent water by weight and having two distinct surface terrains – grooved and cratered. While dark Callisto, with the lowest density of the four moons, was also estimated to contain a lot of water but with large, bizarre multi-ring crater structures (Figure 3). 

And then there’s Io. Voyager Mission Navigator Linda Morabito’s job was to identify and confirm the position and orbit of the spacecraft.1 While processing an image of Io, she identified an enormously large object, large enough that it should have been seen previously with Earth-based telescopes – but it hadn’t. The umbrella-shaped plume she recorded rose over 250 kilometers above the surface of Io. Morabito identified the first evidence of active volcanism on a world other than Earth.5,10

Other studies involving magnetic fields,11 plasma waves,12,13 and radio astronomy14 provided insights into the Io Plasma Torus (IPT).  First identified in 1976,15,16 the IPT is now known to be a cloud of plasma generated by Io through sublimation of SO2 and extensive volcanic activity.17 But in 1979, not much was known beyond its being composed of sulfur compounds. The Voyager 1 studies described (a) intense electrical currents, up to 5 x 106 amperes, perturbing the magnetic fields that link Jupiter with Io (currents that may contribute to internal heating on Io); (b) a cold, corotating plasma on the Jupiter side of the IPT; (c) features within and near the IPT, including high-frequency electrostatic waves and both strong whistler mode turbulence and discrete whistlers with lightning; (d) the discovery of a kilometric wavelength radio source that might relate to the IPT nearest Io’s orbit.

Last, what was initially assumed to be a problem with a camera image became the first documented evidence of Jupiter’s ring,18 something never seen from Earth.5 The March 2022 article for This Month in Astronomical History, “Planets Have Rings”,19 discusses the discovery in detail.

As Voyager 1 left Jupiter, it gained almost 58,000 kph from gravity assist, speeding it on its way to a meeting with Saturn only eight months later on 12 November 1980.  Voyager 2 reached Jupiter on 9 July 1979, adding its observations to our wealth of knowledge about this enormous, enigmatic gas giant that dominates our little corner of the Milky Way galaxy. 

First image of lightning detected on Jupiter

Fig. 1: First image of lightning detected on Jupiter5
“Dark-side multiple-image view of Jupiter. This wide-angle image was taken 6 hours after closest Jupiter approach while the spacecraft was in eclipse. Jupiter's north pole is on the limb approximately midway along the auroral arc. Several areas of lightning are seen on the disk. Since the scan platform slewed twice during this 3-minute, 12-second exposure, the picture consists essentially of exposures of 35, 35, and 85 seconds, each displaced from the others. The arc of limb is about 30,000 km long.”

Color images of Galilean satellites, clockwise from top left: Io, Europa, Callisto, Ganymede

Fig. 2: Color images of Galilean satellites, clockwise from top left: Io, Europa, Callisto,  Ganymede5
“Global color images of the four Galilean satellies. The subspacecraft longitude and resolution for the images are: Io (~140o, 16 km/lp), Europa (~300o, 36 km/lp), Ganymede (~320o, 47 km/lp), Callisto (~350o, 22 km/lp). The satellites are shown to scale; Io is slightly larger than [Earth’s] moon, Ganymede larger than Mercury. Relative albedo and color are also qualitatively preserved, although colors may be somewhat exaggerated by the processing and the full range of albedo (over a factor of 3 between Europa and Callisto) cannot be easily displayed.”

Images of multi-ring crater on Callisto

Fig. 3: Images of multi-ring crater on Callisto5
Left: “Computer-generated four-frame color mosaic of Callisto (resolution 7 km/lp).” 
Right: “Sketch map of craters in the region of the large multi-ring structure on Callisto. The rings and large craters are outlined to show the lower density of craters superposed on this feature relative to the average on Callisto.”


  1. Stone, E. C. & Lane, A. L. (1979). “Voyager 1 encounter with the Jovian system,” Science, 204, 945-948
  2. The Farthest Voyager in Space. (2017). E. Reynolds, director; a Crossing the Line and HHMI Tangled Bank Studios production for PBS (https://www.pbs.org/the-farthest/)
  3. https://solarsystem.nasa.gov/missions/pioneer-10/in-depth/ (originally accessed 27 Feb 2019)
  4. https://solarsystem.nasa.gov/missions/pioneer-11/in-depth/ (originally accessed 27 Feb 2019)
  5. Smith, B. A., Soderblom, L. A., Johnson, T. V., et al. (1979). “The Jupiter system through the eyes of Voyager 1,” Science, 204, 951-972
  6. Gurnett, D. A., Shaw, R. R., Anderson, R. R., & Kurth, W. S. (1979). “Whistlers observed by Voyager 1: Detection of lightning on Jupiter,” Geophysical Res Letters, 6, 511-514 (https://doi.org/10.1029/GL006i006p00511)
  7. Cook, A. F., Duxbury, T. C., & Hunt, G. E. (1979). “First results on Jovian lightning,” Nature, 280, 794
  8. This is a sample from the collection of Prof Don Gurnett's favorite space audio recordings available from: http://space-audio.org/.
  9. Hanel, R., Conrath, B., Flasar, M., et al. (1979). “Infrared observations of the Jovian system from Voyager 1,” Science, 204, 972-976
  10. Morabito, L. A., Synnott, S. P., Kupferman, P. N., & Collins, S. A. (1979). “Discovery of currently active extraterrestrial volcanism,” Science, 204, 972
  11. Ness, N. F., Acuña, M. H., Lepping, R. P., et al. (1979). “Magnetic field studies at Jupiter by Voyager 1: Preliminary results,” Science, 204, 982-987
  12. Bridge, H. S., Belcher, J. W., Lazarus, A. J., et al. (1979). “Plasma observations near Jupiter: Initial results from Voyager 1,” Science, 204, 987-991
  13. Scarf, F. L., Gurnett, D. A., & Kurth, W. S. (1979). “Jupiter plasma wave observations: An initial Voyager 1 overview,” Science, 204, 991-995
  14. Warwick, J. W., Pearce, J. B., Riddle, A. C., et al. (1979). “Voyager 1 planetary radio astronomy observations near Jupiter,” Science, 204, 995-998
  15. Brown, R. A. (1976). “A model of Jupiter’s sulfur nebula,” Acta Pathologica Japonica, 206, L179 (https://doi.org/10.1086/182162)
  16. Kupo, I., Mekler, Y., & Eviatar, A. (1976). “Detection of ionized sulfur in the Jovian magnetosphere,” Acta Pathologica Japonica, 205, L51 (https://doi.org/10.1086/182088)
  17. Moriano, A., Gomez Casajus, L., Zannoni, M., Durante, D., & Tortora, P. (2021). “Morphology of the Io plasma torus from Juno radio occultations,” Jrl Geophysical Res Space Physics, 126 (https://doi.org.10.1029/2021JA029190)
  18. Candy Hansen relates a story about the first image of Jupiter’s ring in the documentary The Farthest Voyager in Space, see reference 2 above.
  19. Marotta, M. E. (2022). “Planets Have Rings,” This Month in Astronomical History, March 2022 (https://aas.org/posts/news/2022/02/month-astronomical-history-march-2022)

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