A Jupiter Retrospective: From Pioneer to Juno
published during a new moon.


Credit: Kim Orr

NASA’s Juno spacecraft is arriving at Jupiter tonight and when it does, it will be the ninth spacecraft to visit the gas giant. Pioneers 10 and 11, Voyagers 1 and 2, Ulysses, Galileo, Cassini, and New Horizons, have all been there, done that. But all the previous missions, save Galileo, were fly-bys. Juno will be the first spacecraft in almost thirteen years to go into orbit around Jupiter.

With this exciting event on the horizon, I thought it would be fun to see just how much our views of Jupiter have changed since we first learned how to send our robotic ambassadors out into the Solar System.


The first of these was Pioneer 10, followed quickly by Pioneer 11. Launched in March of 1972, Pioneer 10 began its approach to Jupiter a year and a half later, passing closest to the planet in December 1973 at a distance just over 80,000 miles, about a third of the distance from Earth to the Moon. (For comparison, Juno will buzz just 2,900 miles above Jupiter’s cloud tops – roughly width of the continental US.) Pioneer 11 launched a year after its sister ship in April 1973, and reached Jupiter in November 1974.

Both spacecraft had primitive cameras on board that recorded only two color bands, red and blue, instead of the three required for true color images, so the pictures they returned have been enhanced to be a representative as possible. Pioneer 10 was the first to image Jupiter’s Great Red Spot in detail and Pioneer 11 was the first to image Jupiter’s polar region. Once of the most interesting things to note is that in 1973, the Great Red Spot was clearly in a band of white clouds, as opposed to the orange cloud band we observe it in today.

Juno Juno

Views of Jupiter taken by Pioneer 11 (top) and Pioneer 10 (bottom). Credit: NASA


Hot on the heels of the Pioneer spacecraft came Voyagers 1 and 2, both launched in 1977 just over two weeks apart and flew by Jupiter in 1979 just over four months apart. Voyager 1 got with one Earth-Moon distance while Voyager 2 flew past 150,000 miles farther out. Each encounter lasted less than two months, so the missions never overlapped.

Jupiter imaged by Voyager 1. Credit: NASA

Jupiter’s ring system was hinted at by radiation measurements from Pioneer 11, and in the spring of 1979, Voyager 1 confirmed its existence – in one single beautiful overexposed photo. Both Voyagers witnessed volcanic eruptions on Jupiter’s innermost moon, Io, the first time volcanic activity had been detected anywhere in the Solar System other than on Earth. All told, the probes observed a total of nine eruptions during their time at Jupiter and scientists hypothesize there were likely two more eruptions between their visits. Voyager 1’s images of another moon, Europa, showed evidence for linear surface features, but Voyager 2 revealed that they crisscrossed the entire surface yet appeared to lack any depth.


Jupiter’s rings captured by Voyager 2. Credit: NASA


This mosaic of Europa, the smallest Galilean satellite, was taken by Voyager 2. Credit: NASA



We managed to leave the Jovian giant alone for over a decade, until 1992, when the Ulysses solar probe flew past Jupiter in an effort to enter a polar orbit around the Sun. But as the mission was primarily to magnetic fields and radiation, there was sadly no camera on board so we can only guess at what Jupiter may have looked like in the early 1990’s. (Hint: I think it’s safe to say it stayed away from platforms, midriffs, and chokers.)


The first spacecraft to actually “hang out” at Jupiter was aptly named Galileo, after the man himself, Galileo Galilei, who was the first to make observations of Jupiter through a telescope more than 385 years earlier. Galileo (the spacecraft) launched in October 1989, aboard the Space Shuttle Atlantis as part of STS-34. Shortly after launch and obtaining low Earth orbit, the probe was deployed and sent on a long winding path of gravity assists that would eventually bring it to Jupiter. Five years later, it began its final approach and arrived just in time to see the fragments of Comet Shoemaker-Levy 9 crash into the cloud decks of the southern hemisphere.


These four images of Jupiter and the luminous night-side impact of fragment W of Comet Shoemaker-Levy 9 were taken by the Galileo spacecraft on July 22, 1994. Credit: NASA/JPL


Galileo officially entered orbit around Jupiter in December 1995 and remained there for seven years until taking a swan dive into the gas giant’s atmosphere in September 2003 (in order to prevent any accidental contamination of Europa – where more and more evidence points to the existence of liquid water under the surface, and the potential for life). In fact, magnetic field measurements made by Galileo provided strong evidence for a subsurface salt-water ocean on the icy moon. The mission also confirmed the existence of extensive volcanic activity on Io (akin to the early Earth) and was the first to flyby one of Jupiter’s tinier moons, Amalthea.


Jupiter’s moon Europa, as seen in this image taken June 27, 1996, by NASA’s Galileo spacecraft. Credit: NASA/JPL



Composite view of Amalthea and Io at the same scale. Credit: NASA/JPL/Cornell University

During its tenure at Jupiter, Galileo also managed to produce one of my all-time favorite photos of the Jovian system: a mosaic of the planet in shadow with its rim and rings backlit by the Sun.


This mosaic of Jupiter’s ring system was acquired by NASA’s Galileo spacecraft when the Sun was behind the planet, and the spacecraft was in Jupiter’s shadow peering back toward the Sun. Credit: NASA/JPL/Cornell University



The next spacecraft to see the giant in up close was actually destined for its ringed neighbor. En route to Saturn, the Cassini-Huygens mission made a flyby in 2000 and took over 26,000 images. Cassini compiled the most detailed image of Jupiter made to date, which resolves features as small as 40 miles across. That’s a lot considering the planet spans more than 86,000 miles in diameter. Many of Cassini’s images caused scientists to rethink their theories about Jupiter’s cloud bands. Based on how clouds work on Earth, it was originally believed the white bands were zones of upwelling, and the dark bands were areas of sinking. But with Cassini’s new data, it became clear that the opposite was true; storms in the dark bands were causing the upwelling, which meant that the white bands were actually zones of sinking.


The most detailed global color portrait of Jupiter ever produced; taken by Cassini. NASA/JPL/Space Science Institute




Images from NASA’s Cassini spacecraft using three different filters reveal cloud structures and movements at different depths in the atmosphere around Jupiter’s south pole. Credit: NASA/JPL/University of Arizona

New Horizons

Believe it or not, even New Horizons, the first spacecraft sent to Pluto, had a date with Jupiter back in 2007 on its way to the Kuiper Belt. This planetary rendezvous was vital to New Horizon’s mission since it required a significant gravity assist from Jupiter in order to get up enough speed to set it on its final trajectory. The probe received a 9,000 mph boost from the encounter, but before it sped away, it managed to take some stunning images. One composite image the New Horizon’s team released shows Jupiter in infrared (the Great Red Spot is in white) and Io in the foreground, caught in the act of erupting.


A Jupiter-Io Montage from New Horizons. Credit: NASA, Johns Hopkins U. APL, SWRI



Jupiter’s clouds from New Horizons. Credit: NASA, Johns Hopkins U. APL, SWRI



What will Jupiter look like after nine years of flying solo? What old theories planetary theories will Juno overturn? What new surprises lie in store for us in the Jovian system? Only time (and science) will tell, so tune in to NASA on Monday evening to find out.

I leave you with this unbelievable juxtaposition of what Galileo Galilei observed and recorded in his notebook in 1610 and Juno’s glimpse of the same view taken 406 years later. And if it doesn’t blow your mind, I don’t even want to know you.


Galileo’s 1610 observations of Jupiter’s moons



NASA’s Juno spacecraft obtained this color view on June 21, 2016, at a distance of 6.8 million miles (10.9 million kilometers) from Jupiter. Credit: NASA/JPL-Caltech/SwRI/MSSS