The First 150 Days of Juno


Back in July, there was much fanfare surrounding the successful arrival of the Juno spacecraft at Jupiter.  A near-perfect engine firing had placed the solar-powered spacecraft into just the right orbit, with the promise of great things to come.  Juno’s science goals are fourfold:  explore the origins, interior structure, atmosphere and magnetosphere of this archetypal giant planet.  If all had gone to plan, we would now have had 6 or 7 close flybys of Jupiter (known as the perijoves), where the vast majority of Juno’s high impact science would be taking place.  In reality, we have only had a single science-intensive flyby so far in August (Perijove 1), with another expected this month (December, Perijove 3), and many more to come in 2017.  It’s important to note that Juno will still achieve its scientific potential, but we scientists are having to be more patient than we’d originally planned.




The reasons for this are two-fold.  Juno was originally injected into a 53 day orbit around Jupiter, with the plan to complete two of those while all the instruments and spacecraft were being checked, before firing the engine again (the period reduction maneuover, or PRM) in October to move the spacecraft into its 14-day science orbits.  However, shortly before the PRM, mission managers reported that two helium valves – which play a vital role in firing the main engine – weren’t operating properly.  Instead of opening in a few seconds, telemetry indicated that they were taking several minutes.  So instead of risking the spacecraft by firing the engine in October, the Juno managers decided to wait and analyse the issue in more depth.   That’s not to say that Juno will never reach the 14-day orbit, but it’s been pushed back so that we now expect to stay in this 53-day orbit for at least the first half of next year.  From a science perspective, that just means we’ll be taking data more slowly.  We could stay in this orbit indefinitely and still achieve all the mission goals.  We don’t get any extra radiation exposure by doing this, as Juno only really gets exposed during these close flybys.  Plus it’s always better having a healthy, working spacecraft than an uncontrollable one!


Now, with the PRM postponed, Juno’s scientific payload was scheduled to provide complete coverage of Perijove 2 on October 19th.  But Juno unexpectedly went into ‘safe mode’ just  13 hours before the flyby – the onboard computer went through a reboot – which means all the science instruments were off while the software went through a wide variety of diagnostics and awaited further commands from Earth.  Safe modes are designed into the software should the computer encounter any unusual conditions – everything non-essential is turned off, and the spacecraft makes sure its solar panels are pointed at the Sun to maximise its power.   It came out of safe mode on October 24th, and mission managers are now being cautious about the next perijoves in the hope that this doesn’t happen again.


So the new plan is to remain in 53 day orbits for the foreseeable future.  The next perijoves are therefore December 11th, February 2nd, March 27th, May 18th, July 11th, and so-on, and a world wide campaign of ground-based imaging is still underway to support those encounters.  The full list can be found here:

In fact, we do hope that we can trim the orbit before the 20th perijove in 2019.  Otherwise the spacecraft will enter long eclipses that it was never designed for, depleting the batteries and ultimately ending the mission before it can complete the 32 perijoves that were originally planned.  But there’s still plenty of time to resolve these issues.



Science so far


Despite these mishaps, Juno has already provided unprecedented new views of Jupiter that have only served to maximise our impatience, and to what our appetite for what’s still to come when Juno gets into its groove.


During the first orbit Juno was moving through parts of Jupiter’s magnetosphere that have never been probed before, measuring the magnetic field and plasma environment to understand the structure and dynamics of the extensive magnetosphere.   At the same time Juno was collecting a whole series of images that have been assembled into the ‘marble movie’, allowing us to ride along with this robotic explorer, watching the dance of the Galilean moons and the spinning of Jupiter’s dynamic globe.  To me, the incredible thing about these images is the vantage point:  from Earth, we only ever see the jovian disc in full illumination, but Juno can provide a view that only a robot can – a crescent Jupiter, with the terminator shadow separating jovian day from jovian night.  The Marble movie covers July 10th to October 14th:


On August 27th, Juno came within 2500 miles of the cloud tops, providing  an unprecedented view of the giant planet.  The visible images of the northern and southern poles were striking.  Rather than the banded atmospheric structure that we’re all familiar with, the poles look completely different.  There are no belts and zones up here, but  multitude of small-scale storm systems – giant swirling cyclones with pinwheel structures that presumably wander about the polar atmosphere over time.  This is rather different to Saturn, where we clear evidence of planet-encircling jet streams all the way to the poles, including the northern jet stream that meanders to form a hexagon when viewed from above.  It’s quite clear from these early images that there’s no such hexagon at either of Jupiter’s poles.  The images have also shown clouds towering high at the terminator regions, catching the Sun’s rays even though they’re really on the nightside, suggesting they’re rising tens of kilometers above the cloud decks, rather like clouds catching the last rays of sunset before night.  That’ll tell us a lot about what these clouds are made of and the physics about how they form.



Jupiter’s north pole from Juno.


But Juno has more than just visible light observations.  The JIRAM instrument from Italy has mapped the entire planet in the infrared, allowing us to see Jupiter’s glowing internal heat and silhouetted clouds in more detail than we’ve ever been able to obtain from the Earth.


First infrared views of Jupiter from the JIRAM instrument, showing aurora at 3 microns and Jupiter's internal glow at 5 microns.

First infrared views of Jupiter from the JIRAM instrument, showing aurora at 3 microns and Jupiter’s internal glow at 5 microns.


The unique vantage point allows JIRAM to see Jupiter’s aurora, glowing hot due to emission from H3+ ions in the upper atmosphere as they’re bombarded by electrons moving along the magnetic field lines.  The bright, structured appearance of the southern aurora is breathtaking, like gazing into the eye of Sauron, and is really the first of its kind.

Jupiter's southern aurora glimpsed by JIRAM.

Jupiter’s southern aurora glimpsed by JIRAM.


Not only can Juno see the aurora, but it can also listen to them.  A radio wave detector can hear the emissions of the energetic particles that form the aurora, some of the strongest emissions in the solar system.  You can hear how the intensities change with time in this audio clip, which condenses 13 hours and begins immediately after closest approach.  It gives you an impression of the structure of the plasma environment as Juno hurtles through the jovian system.


Listening to Jupiter's plasma wave activity after the first perijove.

Listening to Jupiter’s plasma wave activity after the first perijove.


Among the most hotly-anticipated results are those from the Microwave Radiometer, which is able to peer deeper inside Jupiter than ever before, probing 250 miles below the topmost cloud decks to reveal the inner workings of the giant planet’s atmosphere.   We were all excited to see that Jupiter continues to exhibit some kind of banded structure all the way down to these deep levels.  But the suggestion so far that is that it’s not the same all the way down, it changes as a function of depth.  Like seeing only the tips of icebergs above water, the bands that we see at the cloud tops are just the very top of a fascinating, variable layer that we’ll explore in great depth with the coming perijoves.


Slicing through Jupiter's sub-cloud atmosphere with the microwave radiometer.

Slicing through Jupiter’s sub-cloud atmosphere with the microwave radiometer.

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