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Juno at Jupiter: Extended Mission Flybys of Galilean Moons

The news that NASA will extend the InSight mission on Mars for two years, taking it through December of 2022, is not surprising, given the data trove the mission team has collected through operation of the mission seismometer. A live asset on Mars also deepens our knowledge of the planet’s atmosphere and magnetic field, all reasons enough for pushing for another two years. But the extension of the Juno mission to Jupiter deserves more attention than it’s getting, given that Juno’s remit will be expanded deep into the Jovian system.

Image: NASA has extended both the Juno mission at Jupiter through September 2025 and the InSight mission at Mars through December 2022. Credit: NASA/JPL-Caltech.

For those of us fascinated with the outer system, this is good news indeed. I’m looking over two documents, the first being a presentation based on a report submitted to NASA’ Outer Planets Assessment Group (thanks to Ashley Baldwin for passing this along). The OPAG document was produced by Scott Bolton (Southwest Research Institute); it gives the overview of what a mission extension could look like. Also on my desk this morning is the text of the 2020 Planetary Missions Senior Review (PMSR), outlining a set of three mission scenarios. The context of both analyses is the success of the mission in studying Jupiter’s interior structure, magnetic field and magnetosphere, not to mention the examination of its atmospheric dynamics, seen in such roiling imagery as that depicted with stunning complexity in many of the JunoCam images.

Launched in 2011 and operational at Jupiter since 2016, Juno’s prime missions were to have ended in July of this year, with the spacecraft having completed 34 polar orbits, each of 53 day duration. The OPAG report refers to the subsequent extended mission as “a full Jovian system explorer with close flybys of satellites and rings.” The extended mission is to last through September, 2025, with observations of the planet’s ring system, its large moons, and a series of targeted observations and close flybys of Ganymede, Europa and Io.

That last clause really got my attention, as I hadn’t seen it coming. Juno is in an elliptical orbit with a 53-day period whose perijove migrates northward. This bit from the Senior Review reveals in depth the interactions between the various mission scenarios and satellite flybys. The three scenarios mentioned offer alternatives given varying science and budget considerations:

The proposed Juno extended mission (EM) would take advantage of the natural northward progression of the periapsis of the spacecraft’s orbit and the consequent lowering of spacecraft altitudes over Jupiter’s high northern latitudes. The EM would run until the end of the mission, with an expected duration of approximately four years. Under the High and Medium Scenarios, propulsive maneuvers would be utilized not only to target Jupiter-crossing longitude and perijove altitude, as during the prime mission, but also to target close flybys of Ganymede, Europa, and Io. The flyby maneuvers would act to shorten the spacecraft orbital period, yielding more close passes of Jupiter within a given time interval, and increase the rate of northward movement of spacecraft perijove. Under the Low scenario for EM operation, the satellite gravity assists and close satellite flybys would not be attempted.

So mission scientists have a number of options to work with. The extended mission investigates the northern hemisphere and probes the region above Jupiter’s polar cap aurora. The northern adjustment in Juno’s orbit is what makes the satellite flybys possible and enables as well close analysis of its ring structures. The Juno team can look forward to 3D mapping of Jupiter’s polar cyclones and studies of the planet’s unusual dilute core, the latter an earlier Juno discovery revealing a core consisting of both rocky material and ice as well as hydrogen and helium.

Both Europa Clipper and the European Space Agency’s JUICE mission (Jupiter Icy Moons Explorer) should benefit from Juno data on the radiation environment they will operate within. At Europa, Juno will continue the search for possible plume activity while examining the ice shell and mapping surface features, while studies of Io’s magma, polar volcanoes and interactions with Jupiter’s magnetosphere will be enabled by its encounters there. At Ganymede, magnetospheric interactions and surface composition data should be produced in abundance.

In the OPAG presentation, most of the Juno flybys will be at Io, with 11 possible between mid-2022 and 2025. Two encounters are planned for Ganymede (and recall that JUICE is scheduled to orbit the huge moon), and three encounters are feasible for Europa. The actual number of flybys will, according to the Senior Review, depend upon budget choices. In that document, I find this overview of Juno’s satellite flybys:

The orbit of Juno in the EM [extended mission] would take the spacecraft through the Io and Europa plasma tori and in close proximity to Io, Europa and Ganymede. Maps of Ganymede’s surface composition would allow studies to understand the importance of radiolytic processes in surface weathering, identify changes since Voyager and Galileo, and search for new craters. Juno’s Microwave Radiometer (MWR) is particularly sensitive to the upper 10 km of Europa’s ice shell. Studies at wavelengths complementing expected results from Europa Clipper’s radar would identify regions of thick and thin ice and search for regions where shallow subsurface liquid may exist. Juno’s visible and low-light cameras would search Europa for active plumes and changes in color/albedo that may reveal eruption regions since Galileo. The fields and particles experiments would look for evidence of recent activity. Finally, the Juno EM would include a flyby of Io and search for evidence of a magma ocean.

What an interesting development Juno’s extended mission turns out to be! Continuing science operations with existing equipment far undercuts the cost of new missions while extending long-duration datasets and, in the case of Juno, enabling a set of exciting new targets. We have the option here of a series of Galilean moon flybys that were never in Juno’s original mission, observations that could inform later choices made for Europa Clipper and JUICE. All told, Juno’s unanticipated extended mission is a heartening contribution to outer system science.

{ 16 comments… add one }
  • Enzo January 11, 2021, 16:05

    This article:

    Talks about Ganymede flyby starting mid 2021 at ~ 1,000 Km, Europa from late 2022 at ~320 Km, resolution 1-2 Km, and Io from 2024 at ~1,500 Km

    • Ljk January 12, 2021, 12:16

      And to think the Juno team seriously considered NOT putting a camera on the probe!

      Too many engineers. :^)

      • Ashley Baldwin January 12, 2021, 20:11

        Too little cash unfortunately. The team had already managed to squeeze 8 instruments into the tight budget envelope – almost a flagship payload – and a lot with just a starting power of 435 watts from its solar array. Expected to drop quickly as the radiation belts took their toll.

        JunoCam was only included as a cut price PR student project afterthought – based on the Mars Science Laboratory ‘Mardi’ camera/telescope. What the team would give for the New Horizons LORRI telescope now !

        Unfortunately JunoCam has a very low visual resolution which will be inferior to even the old Galileo pictures . The infrared spectrographic imager, JIRAM will give the best images for Io’s volcanoes and maybe with luck the UV spectrometer might catch a further glimpse of Europa’s elusive plumes.

  • Geoffrey Hillend January 11, 2021, 16:43

    Great!. Juno’s already discovered a lot. Now it will get even more unexpected and not previously planned information.

  • Brett January 11, 2021, 17:34

    It’d be pretty neat if JUNO found unexpectedly thin ice when it does the flyby.

  • ljk January 11, 2021, 18:31

    I know that Callisto is the farthest out of the four Galilean moons and the least exotic among those moons – if such a thing can be said about any world that is actually alien – but seeing as it will probably be the first target of human bases in the Jovian system since it is the only major satellite outside the gas giant’s deadly radiation belts and the most geologically stable, Callisto should probably receive its fair share of exploratory attention, too.

    There is this NASA slideshow on the concept:


    And this related document:


    Some relevant references:



  • Alex Tolley January 11, 2021, 20:05

    The value of these longer, extended missions suggests to me that long term, permanent data acquisition of these bodies is where we should end up. As instruments improve, we need cheap platforms and propulsion systems to deliver these instruments to theit targets.

    I favor electric engines and solar sails for the task, with as small as possible payload mass, such as a CubeSat chassis dedicated to the instrument. Beamed energy for either propulsion method should reduce the transit time. We then end up with swarms of these probes constantly sending data home and building up a detailed picture of these bodies.

    We have had examples of successful CubeSat missions, both to Earth orbit and and beyond. They needn’t be super-robust, but rather cheap enough to be replaceable if they fail, and upgradeable with new software and hardware as technology evolves.

    Obviously, not all types of instruments can be low mass and suitable for a small platform, but many can be, either alone or as bundles where appropriate.

  • Ashley Baldwin January 12, 2021, 11:13

    Thanks Paul and good points Alex.

    A real left field and ingenious move by the Juno team. A four year Jupiter moons tour . Wow !

    We also see here the benefits of both Flagship missions and the growing number of smaller Discovery and New Frontiers programmes in driving forward technological as well as scientific progress . All three are able to introduce new instruments – though particularly flagship class. Between Europa Clipper, New Horizons and all the comet/asteroid missions, a whole gamut of versatile new technology has been able to gain the necessary high technological readiness levels ( ‘TRL’ with TRL 6 and beyond admissible for use in New Frontiers and Discovery missions ) that enable cheap adoption for future missions. Or the use of free backups even . Maybe not always absolute cutting edge ,but not far short , potent – and much cheaper. Juno has an
    Impressive payload for a non flagship mission.

    The latest round of New Frontiers short list concepts illustrate this ( though the Juno extension, with all its Io flybys , has likely sounded the death knell for IVO) . It’s this cheap instrumentation that allows cash strapped NASA to still consider TWO successful missions for its next New Frontiers round . Possibly DaVinci and Trident.

    The big problem of course is the large costs associated with missions beyond Jupiter – that require expensive and limited availability and radioisotope power sources. These eat into the strictly cost capped PI budget – as with Dragonfly. Juno utilises relatively cheap solar power – so allowing a much bigger instrument payload and to have enough ‘consumables’ ( thruster fuel) to allow a theoretical operation through 2025). How ironic that it’s main engine malfunction led to the radiation avoiding orbit that allowed its continued function for so long. It wasn’t expected to last even two years originally !

    Solar power is evolving though and it’s likely to reach a level whereby it can power future Saturn/Enceladus missions too – but not to the Ice giants beyond. To this end I’m aware that NASA are looking at ‘sub Flagship’ missions with budgets in between the $850 million cap for New Frontiers and the $2 billion plus of a Europa Clipper ‘cheap’ flagship .

    The use of cheaper heavy lift vehicles like the Falcon Heavy ( now an option for the Clipper too given Congress’ recent backdown on mandating SLS for its launch ) and New Glenn will help too . As will the possible use of solar electric ‘ staging ‘ with ion drives like NEXT to reduce the decade long ( or longer) transit times to Uranus ( most likely) and Neptune. Given their exoplanet proxy like nature I’m sure the next Decadal study will finally prioritise the ice giants. Especially with key Jupiter gravity assist launch windows opening between 2029 and 2031.

    As far as I am aware all JUNO’s instrument payload is still functioning fine – even straw man , JunoCam – with no reason in its new tilted radiation avoiding orbit that the extension shouldn’t last through to its Sept 2025 conclusion . With an extra 40 plus close Jupiter encounters thrown in too !

    It might yet be about to welcome Clipper on its arrival . Who would have thought ?

  • Ljk January 12, 2021, 12:18

    So yesterday I commented here that Callisto should not be neglected die to its multiple values as a future human base and such.

    Then today this just showed up….


    • James Jason Wentworth January 13, 2021, 11:11

      I’m glad that both of the Chinese’s proposed missions will also examine Jupiter’s irregular satellites, as well as Callisto and/or the other Galilean moons. They certainly have the necessary lifting power, and they also plan to use a Galileo-type VEEGA (Venus-Earth-Earth Gravity Assist) to obtain the velocity to get out to Jupiter.

  • Andrei January 12, 2021, 12:34

    Insight can provide nice data, as long as the lander last. Too bad the mole only got a small distance into the ground. It’s too shallow to provide data.
    And Juno can also provide both images and data, becoming a precursor mission for those who will arrive there later. Laplace-P is somewhat in doubt though, already in 2016 the launch date got moved up to 2026 .

  • Gary Fowler January 12, 2021, 14:55

    i remember the original thought that this was a Jupiter only mission and would notf be visiting the moons, thank god for failed main engine. Now we have a Ganymede and europa mission 10 years ahead of schedule, truly exciting

    • Mike Serfas January 12, 2021, 21:31

      The main engine never really “failed”; rather, two check valves took longer than expected to open. https://www.nasa.gov/press-release/nasa-s-juno-mission-to-remain-in-current-orbit-at-jupiter This isn’t my field and I’m in no position to speculate, but I was surprised that no one could come up with any sort of work-around to alter the temperature or release the pressure and try again until the valves were more limber. Considering that the 53-day orbit allowed “bonus science” in the far magnetosphere and moons, reduced radiation exposure, conserved fuel, and allowed more time for transmission of images from JunoCam — and any orbital maneuver can be risky — I don’t understand a situation where firing the engine would have been worth the risk to the mission, even if the valves had functioned perfectly. This was a solar powered spacecraft with a short term mission, equipped with a splendid camera only as an afterthought for outreach to the public .. what good luck such features are on hand now! Some very smart people work at NASA, and now they are working quite a bit longer and on a more interesting range of topics than originally proposed. What a shame that is. :)

  • Edwin January 12, 2021, 18:55


    Thanks Paul and Everyone for the interesting article and comments,

    I’m sure looking forward to the results of this one

    Cheers Edwin

  • Edwin January 13, 2021, 20:16
    • Michelangelo Pantaleoni January 15, 2021, 4:22

      The important part of this presa release: “The satellite encounters begin with a low-altitude flyby of Ganymede on June 7, 2021 (PJ34), which reduces the orbital period from about 53 days to 43 days. That flyby sets up a close flyby of Europa on Sept. 29, 2022 (PJ45), reducing the orbital period further to 38 days. A pair of close Io flybys, on Dec. 30, 2023 (PJ57), and Feb. 3, 2024 (PJ58), combine to reduce the orbital period to 33 days.”

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