by Duncan Lunan

For much of the 20^th century, there was little change in the images of Jupiter which were available.  

When the 200-inch reflector became operational at Mount Palomar in 1948, with twice the aperture of the previous largest in the world, in theory it should have been able to produce much sharper photographs, but the limitations of observing from the bottom of Earth’s atmosphere meant there was little improvement, even when colour photography became available by 1960  (Fig. 1).  Ed Buckley’s painting ‘Old Jupiter’ for my book New Worlds for Old features a nuclear-powered spacecraft designed by Arthur C. Clarke around 1950, but the depiction of the planet could have been from decades if not centuries earlier  (Fig. 2).  Drawings by expert observers showed more detail, and Ed Buckley imaginatively sharpened them in his 1960 painting ‘Jupiter from Europa’  (Fig. 3), but still there was little change from Donato Creti’s painting of Jupiter in an Earth sky, in 1711  (Fig. 4).  By 1970, photographs from high-altitude observatories were beginning to look more like the imagery we’re familiar with  (Fig. 5), and in 2010, when the Jeff Hawke story ‘The Time Is Out of Joint’ was reprinted in Jeff Hawke’s Cosmos  (Fig. 6), on the magazine cover Sydney Jordan took the opportunity to further update the portrayal of Jupiter when the story first appeared in 1971.

Everything changed in 1974, when the Pioneer 10 spacecraft flew past Jupiter  (Fig. 7)  and turned our mental image of the planet on its head – literally, in some cases. 

It turned out that our model of the visible surface was upside-down:  the dark bands which I grew up thinking were raised cloud belts, and are still referred to as ‘belts’ to this day, are the true surface of the planet, coloured dark brown by ammonium compounds.  The bright ‘zones’ between them are actually high-altitude cirrus-like layers of ammonia crystals, and the breaks between them correspond to the boundaries between bands of Jupiter which rotate at different speeds, due to the planet’s huge size and very rapid spinning on its axis. 

Pioneer 10  (Fig. 8)  and its near-twin Pioneer 11 were built by NASA’s Ames Research Centre in San José, and launched by Atlas-Centaur boosters  (Fig. 9)  at the then unheard-of velocity of 10 miles per second, crossing the orbit of the Moon within hours.  The spacecraft were spin-stabilised and the imaging system was one devised by John Logie Baird, scanning the image in strips through differently coloured filters and painstakingly assembling it back on Earth  (Fig. 10).  There were doubts whether the spacecraft could be slewed with sufficient accuracy to counter its velocity passing the planet, but the final results were spectacular  (Fig. 11)

It had been known for many years that Jupiter’s atmosphere contains methane and ammonia, but the Pioneer probes revealed that it’s mostly hydrogen and helium.  Jupiter also has far more internal heat than expected, generated by the slow separation of helium towards the core of the planet, still continuing more than four billion years after its formation.  Because Jupiter is so much further from the Sun than we are, it shows virtually no change in phase as it and we orbit at different speeds.  One of the jokes before the Pioneer 10 encounter was that it would turn out to be self-luminous and look much the same all the way round.  In optical wavelengths Jupiter did turn out to have a crescent phase and a nightside  (Fig. 10), but because of the heat flow from the interior, the joke turned out to be virtually true in the infrared.  Jupiter’s outer core consists of hot liquid hydrogen, within which the magnetic field is generated, and the inner core is a solid lattice of hydrogen in metallic crystalline form with a temperature of 50,000 degrees C.  Until recently most experts continue to insist that there ‘must’ be a further core within that of rock and/or metal, but none of the passing space probes or orbiters had detected it.  The Juno orbiter settled the question in 2018:  there is no rock and metal core within Jupiter, though there are denser regions within the metallic hydrogen lattice which may represent partially dissolved asteroids absorbed in the early stages of Jupiter’s formation.  (‘Jupiter’, ON September 5^th 2021).

There had been many predictions about conditions on Jupiter’s four large ‘Galilean’ moons, and many depictions of astronauts on them  (Fig. 3), but the belts of radiation trapped by the huge magnetic field turned out to be ‘supralethal’, with Extra-Vehicular Activity possible only on the outermost large moon, Callisto.  (Some possible consequences of that are discussed in the ‘Flying Factory’ section of ‘Flight in Non-Terrestrial Atmospheres’, ON December 4^th, 2022.  The onboard computer on Pioneer 10 had nearly been swamped by false commands within the radiation belts, and Pioneer 11 was directed ‘through the dragon’s teeth’, going still closer to the planet but crossing the most intense zones more rapidly, at a steeper angle  (Fig. 12).  The Pioneer probes didn’t pass close enough to any of the Galileans for sharp imagery, but a distant photo of Europa made it look like our Moon, which was how Ed Buckley depicted it  (Fig. 2).  It turned out not to look like that at all – none of the four did.  (‘Jupiter’s Moons’, ON September 12^th, 2021).

The two spacecraft which totally changed our ideas about the Galilean moons were originally designated ‘Mariner Jupiter-Saturn’, continuing the Jet Propulsion Laboratory’s line of Mariner Venus and Mars missions in the 1960s and early 70s.  There was some controversy when the 1976 Mars landers were renamed ‘Viking’, and there was more when the MJS probes were named ‘Voyager’, because that had been the name for much larger Mars and Venus landers to be launched by Saturn I, cancelled along with the rest of the post-Apollo programme.  At launch in 1977 the logos showed only the MJS objectives  (Fig. 13), though extended missions to Uranus, Neptune and Pluto were possible, and the first two of those later took place.

The Voyager spacecraft carried television cameras, and when I visited JPL and Ames Research Centre in 1979, I was surprised to learn that there was considerable rivalry between the two imaging teams.  Pioneer 11 was then approaching Saturn as a pathfinder for the Voyagers in 1980 and 1981, while the Voyagers themselves were fast approaching Jupiter, and each side maintained that the other would get no usable imagery from the respective encounters.  In the event, both were entirely successful.  Among the many discoveries of the Voyager-Jupiter flybys were confirmation of Jupiter’s rings, along with aurorae and lightning on the nightside of the planet;  the volcanoes of Io, and the subsurface ocean of Europa.  It became clear that Jupiter’s major moons were like a Solar System in miniature, not just dynamically  (see ‘Jupiter’s Moons’)  but in appearance and composition.  If Io with its multiple active volcanoes resembles Venus  (and there’s increasing evidence that some at least of Venus’s volcanoes are active), and Europa with its water cover resembles Earth, while Ganymede with its geologically active past resembles Mars, then Callisto may be telling us what the legendary ‘fifth planet’ would have been like, where the Asteroid Belt is now, if Jupiter’s migration through the early Solar System hadn’t prevented its formation  (Fig. 14).  Furthermore, the densities of the Galilean moons decrease with distance, so it seems that Jupiter must have been self-luminous, shining like a star, during their formation.  

On my return visit to JPL in 1984, I was shown a display of spacecraft cameras used over 20 years of planetary exploration to date, and although they became less cumbersome with time, they all seemed surprisingly chunky to me.  It was explained to me that because cameras impose specific requirements on guidance systems, attitude control, power supplies etc, they tend to get ‘frozen’ into the design at a comparatively early stage;  and rather than use state-of-the-art equipment which might have hidden flaws, NASA prefers to use systems which have been well tried on less expensive spacecraft, in Earth orbit, before risking them on long-duration planetary missions, so they could easily be ten years or more behind the most recent designs.

My 1979 visit had been a quick in-and-out, escorted to a meeting with the head of the Public Affairs Office to negotiate the use of high-quality imagery in the High Frontier exhibition  (‘Eyewitness to History -2’, ON July 31^st, 2022).  But in 1984 I had just been keynote speaker at the ‘View from Earth, 1984’ seminar, (‘Eyewitness to History – 3’, ON August 7^th, 2022), and I was invited to a VIP tour of JPL by two of the other speakers, Mike Urban  (a team controller on Voyager 2, Fig. 15)  and Warren James, of the Galileo Project  (Fig. 16).  I had mentioned Waverider in my talk at Big Bear Lake, and I was asked to lecture on it while I was at JPL, particularly for the benefit of Dr. Jim Randolph, the head of Advanced Mission Planning.  I must admit, my first thought was to decline – what business had I, with my Arts degree, lecturing on aerodynamics to the world’s foremost experts?  But then I thought, I’m a science writer, self-employed, I have no job or academic status to lose:  even if I make a fool of myself, they can’t do any worse to me than show me the door.  In the event, it went off perfectly well, and when I was asked a question I couldn’t answer, I simply replied, “You’d have to ask Professor Nonweiler about that”, or “Gordon Ross” as the case might be.  Dr. Randolph’s subsequent support for ASTRA’s Waverider project was invaluable to us through the rest of the decade. 

At the start of my visit I was left on my own in the Theodore Von Kármán Auditorium, where the press are briefed on planetary flybys, to watch a film about JPL and another about the Voyager missions.  Prominent signs said ‘No Photography’, but I’m afraid that, being alone, I took full advantage.  The room was dominated by the engineering test model of a Voyager spacecraft  (Fig. 17), and less conspicuously, along the opposite wall there was a display of models of previous US space probes, all to the same scale  (Fig. 18, for instance).  One outside attraction was a Corporal missile like the one I had seen at South Uist in 1959  (‘Visitor at Uist’, ON September 9^th, 2022), and an indoor one was the gas gun which had been used for supersonic tests of X and Y-wing Waverider models, in 1962.

But the highlight, unglamorous though it was, was the Galileo spacecraft in the clean room.  Galileo had begun life as Jupiter Orbiter Project, aka Pioneer Jupiter Orbiter, using backup hardware from the Pioneer 10 and 11 missions.  It grew into a much more sophisticated Mariner-based spacecraft against determined opposition from senators and congressmen representing the farming states. Seeking to halt what they called the ‘boondoggle’ of space exploration.  As well as the US supersonic airliner, they had successfully cancelled NASA’s Search for Extraterrestrial Intelligence, shut down the scientific stations left on the Moon by the astronauts, halted analysis of the moonrock they’d brought back, done their best to prevent NASA from saving the Skylab space station, and passed bills preventing NASA from using Martin Marietta’s Aft Cargo Carrier, or any other Shuttle applications which involved making use of the External Tank.  For a time, one could buy car bumper stickers at space events, reading ‘Anyone who buys Wisconsin cheese is a traitor to the human race’.  But when it came to Pioneer Jupiter Orbiter, the space enthusiasts fought back.  Organisations like the Planetary Society, the L5 Society, the National Space Institute, and the American Institute for Aeronautics and Astronautics, formed ‘phone trees’ to lobby their own congressmen and senators, insisting they stay for the votes on space projects rather than going home for the weekends.  The JOP mission was saved, and after the trials it had been through, it seemed only right to call it ‘Galileo’.

By 1984, however, the mission was in different trouble.  Galileo was to have been launched by the Space Shuttle, but the window had been missed and now it was to be launched by a Titan-Centaur combination.  When I was there the spacecraft had been disassembled for the necessary modifications, in the usual surgical conditions.  Looking down on it from the glass-panelled gallery, I could see the complete engineering test model of the spacecraft at the rear, with its communications antenna folded  (Fig. 19).  The main body of the spacecraft (the ‘bus’)  was to the right, with the scientific instruments spread on benches around it  (Fig. 20).  Directly ahead of me was the high-gain antenna, fully open  (Fig. 21).  It was of the type used on the Tracking & Data Relay Satellites which had been maintaining communications with the Space Shuttle since its first launch in 1981.

I had a rare opportunity to see the other side of the conversion, as well.  At the View from Earth symposium, an elderly gentleman had invited me to tour the Atlas-Convair plant at San Diego, where the Centaur boosters were made.  He had been brought of retirement because his outdated speciality was suddenly in demand.  What he used to do was to build wooden ‘dynamic models’ of spacecraft, which could be subjected to simulated vibrations of launch and separation from the boosters, without breaking the spacecraft itself if there was a problem.  As Galileo had never been intended to launch on Titan-Centaur, he’d been called on to exercise that special skill one more time.  What he had built was like a misshapen wooden hut, looking nothing like the spacecraft I’d seen at JPL, but he assured me that on a vibrating table, it would behave just like the Galileo would during the stresses of launch.  The Atlas booster was still at the forefront of US military launches, outdone in throw-weight only by the Titan III at the time, so no photography was allowed within the plant and I’m afraid you’ll have to take my word for the remarkable construction I saw there.

Still it wasn’t the launch vibrations, which were so carefully allowed for, which were to give the mission its biggest problem.  During the Voyager 2 flyby of Saturn, the camera platform had jammed due to overwork.  Its liquid lubricant had spun out of the bearing and took 24 hours to seep back in, after which all went well.  But there were backup plans to get what coverage they could, by slewing the entire spacecraft, if the problem recurred during the Uranus or Neptune encounters.  Galileo’s high-gain antenna was not to be deployed until it was well away from the Sun, and its liquid lubricant had been replaced by graphite.  Unfortunately, all that unscheduled travel by road and rail adapting the spacecraft for different boosters had shaken the graphite out of the bearings, and because it wasn’t a liquid, it didn’t seep back in.  Three of the 18 ribs failed to open, making the antenna unusable, and data and images could only be returned using the low-gain antenna at much slower transmission rates  (Fig. 22).  Nevertheless, Galileo sent back the highest-resolution images of the planet and moons to date, and returned a huge amount of data during its eight years in orbit.  New discoveries are still being made from it today.

Overlapping with Galileo, there were three more Jupiter flybys.  The one most often forgotten is Ulysses, which was intended as a two-spacecraft mission to pass simultaneously over the poles of the Sun, and went ahead via Jupiter slingshot in 1992, although the US component of the mission was cancelled.  Ulysses carried no cameras, so its contribution to space exploration is often overlooked. 

Cassini made a distant pass by Jupiter in 2000, at a distance of nearly 10 million km., on its way to Saturn.  It was a good test for the spacecraft’s instruments, and returned some useful images of Io in transit across the face of Jupiter.  In 2007 New Horizons passed Jupiter on its way to Pluto, and among its photographic targets were the Little Red Spot, then prominent in the planet’s southern hemisphere, and studies of the erupting volcanoes on Io.

The most recent Jupiter mission is Juno, still in orbit around the planet after its arrival in July 2016.  Initially it entered polar orbit with a period of 53 days  (Fig. 23), and after three passes over the polar regions, it was intended to lower it to a 14-day ‘science orbit’ which would allow 34 close passes before ending on the 37^th pass in February 2018  (Fig. 24).  However a problem developed in the propulsion system and the mission has now been extended to 79 orbits at least, continuing to 2025.  This extension has allowed much more time for photography, which was originally called as a ‘citizen science’ project and has produced much excellent work.  Jupiter’s polar regions, which had been thought to be placid, have instead turned out to have huge and apparently permanent hurricanes about the size of the Earth.  Most of Jupiter’s storms go down about 60 miles into the atmosphere, but the Great Red Spot’s roots are at least 200 miles down and there’s a lot about it we still don’t understand, including why it’s been slowly shrinking since the 19^th century, and is now down to twice the size of the Earth. 

As predicted in ‘Solar System Exploration’  (ON 5^th March 2023), Juno is making successively closer approaches to Io  (Fig. 25).  On its 49^th orbit, on September 29^th 2022, Juno took a series of images of Io from distances between 52,515 and 64,944 km.  It’s the closest pass by any spacecraft since New Horizons in 2006, and changes due to volcanic activity can be seen even at that distance.  (Nancy Atkinson, ‘Just Dropped:  New Close-up Images of Io from Juno, With More to Come’, Universe Today, online, March 4^th 2023.)  The next Io pass will be on the 51^st orbit, on May 16^th, at a distance of 35,000 km..  On February 3^rd 2024, the closest pass will be at just 1,500 km, and the images should be spectacular. 

Meanwhile, as noted in ‘The Sky Above You’ for March, the James Webb Space Telescope, the Keck telescope on Hawaii and others have obtained new data on Io’s aurora  (long known), and confirmed that Europa, Ganymede and Callisto have aurorae as well.  They’re being explained as the product of interaction with Jupiter’s magnetic field, which rotates faster than the orbital speeds of all four.  But since Callisto is the only one outside Jupiter’s radiation belts, does this mean it too has subsurface water, like Europa and Ganymede?  Another big discovery from the Juno flybys, in relation to the Galileo ones, is that Europa’s ice crust is not bound to the surface below, but floating freely and rotating separately.  It supports the possibility that the ocean below the ice is very deep, possible as much as 100 miles, containing hundreds of times as much water as Earth’s oceans even though Europa is only a little larger than our Moon.

Meanwhile, Europe’s Jupiter Icy Moons Explorer  (JUICE)  launched successfully on April 14^th, on the second-last of the Ariane V boosters  (famous most recently for the successful launch of the James Webb Space Telescope in December 2021).  It had another demanding task to perform, because the launch window for this mission had to be precise to the second, in order to achieve a flyby of Earth and Moon which will be the first of four course corrections, before a Venus flyby and two Earth passes.  JUICE will reach Jupiter in 2031 and study Ganymede, Europa and Callisto, before entering orbit around Ganymede in 2032.  This will be the first launch to the outer planets by anyone except the USA, and the first time any satellite has gone into orbit around around any moon except our own.  JUICE is concentrating on Ganymede because NASA’s Europa Clipper, due for launch next year, will focus on Europa.  Some critics have remarked that it’s a waste of effort because although Ganymede has a subsurface ocean, it’s too far below ice and rock for traces of life to reach the surface.  As Ganymede is bigger than the planet Mercury, and will be studied in detail from orbit, they’re neglecting the possibility of major new discoveries – which are very likely, in the light of past experience.  And at the end of the mission in 2035  (Fig. 26), JUICE can be crashed on to the surface without the worries about contaminating it which dogged the Galileo and Cassini missions for a time, though in the end they burned up in the atmospheres of their respective planets.

Recommended reading

This summary of Jupiter system exploration has barely covered even the edited highlights.  For much fuller information, look for:

Richard O. Fimmel, William Swindell, Eric Burgess, Pioneer Odyssey, (revised edition), NASA SP-349, US Government Printing Office, 1974.

David Morrison and Jane Samz, Voyage to Jupiter, NASA SP-439, US Government Printing Office, 1980

Michael Hanlon, The Worlds of Galileo, Constable, 2001.