Overall, NASA’s Juno spacecraft in orbit of Jupiter is in good health and good condition as the probe heads toward its two-Earth year anniversary in orbit of the giant planet this July. The spacecraft’s good health bodes well in terms of NASA’s upcoming decision of whether to end the mission this summer or extend it, a decision that is largely understood to be related to how the spacecraft holds up to Jupiter’s intense radiation field.
Following a flawless launch and cruise to Jupiter, the Juno spacecraft entered orbit of the gas giant on 4 July 2016, entering a 53 day checkout orbit. Juno was supposed to complete two of these checkout orbits before performing an engine burn to reduce the spacecraft’s orbital period and apojove (farthest point in the craft’s orbit of Jupiter) to its planned 14 day science orbit.
This engine burn, called the Period Reduction Maneuver, was planned for 18 October 2016. However, during the second 53 day checkout orbit and just four days prior to the Period Reduction Maneuver as NASA worked through tests of Juno’s primary engine, engineers saw something in the data that gave them cause for concern.
At the time, Rick Nybakken, Juno project manager at NASA’s Jet Propulsion Laboratory said, “Telemetry indicates that two helium check valves that play an important role in the firing of the spacecraft’s main engine did not operate as expected during a command sequence that was initiated yesterday. The valves should have opened in a few seconds, but it took several minutes.”
The helium valve issue was highly reminiscent of a failure on the Japan Aerospace Exploration Agency’s (JAXA’s) Akatsuki spacecraft which suffered an incomplete engine burn due to faulty helium valves while trying to enter orbit of Venus in 2010.
Ultimately, the data on Juno’s helium valves indicated that it was too risky to attempt ignition of the engine, and NASA decided to leave Juno in its 53 day orbit, significantly changing the mission outlook and timing of science operations.
Juno’s 14 day science orbit had been designed to allow the craft to meet its minimum mission success, defined by Juno’s pre-launch criteria as 12 science gathering dives close to Jupiter’s atmosphere, within six months of entering the science orbit. Now, a year-and-a-half after entering orbit of Jupiter, Juno has completed just nine science dives (11 perijoves – time of closest approach – overall).
Under the current mission timeline, Juno will not achieve its 12th science dive – assuming every science instrument continues to function – until 16 July 2018 during the 14th overall perijove, a far cry from the planned 34 science dive perijoves that were planned to be completed by this month (February 2018) had Juno entered its proper 14 day science orbit.
Current status and mission extension decision:
Under the originally planned mission timeline, Juno would have completed 37 total orbits of Jupiter, two in the 53 day checkout orbit, 34 science orbits, and one final orbit to send the craft into Jupiter’s atmosphere for a destructive end-of-mission plunge to protect the planet’s potentially life harboring moons.
At the time of launch and through its cruise to Jupiter, mission managers stated that there was no possibility for extending the Juno mission beyond February 2018 because of the radiation environment expected while the craft was in its 14 day science orbit.
However, at a post Jupiter orbit insertion news conference on 4/5 July, the Juno team acknowledged that there was a potential to extend the Juno mission if the radiation environment experienced by the craft was less than expected and if all the craft’s science instruments and control systems were still in good condition by February 2018.
But since Juno has remained in a 53 day orbit and has swung well outside Jupiter’s radiation belts during those orbits, the radiation environment the craft has been exposed to has indeed been less than expected.
“We have found Jupiter’s radiation environment to be less extreme than expected and that has been beneficial for the spacecraft and instruments,” stated NASA in a response to inquiries regarding Juno’s status made by NASASpaceflight. “Everything is currently operating nominally and we expect that to continue for the foreseeable future.”
In fact, the craft is quite healthy, with NASA noting that “The Juno spacecraft and instruments are continuing to operate in orbit around Jupiter and are providing us with fascinating science data and images. We have learned that Jupiter is more complex than we anticipated and have been genuinely surprised by some of our findings.”
In short, Juno has returned amazing data that has surprised scientists and enabled them to learn more about the composition of the largest planet in our solar system despite the craft not being in its intended science orbit. Moreover, Juno’s mission to date can be classed as successful, with the craft anticipated to meet minimum mission success in July 2018 without issue.
But whether the mission will end or be allowed to continue beyond its 12th perijove science dive in July is currently unknown, with NASA needing to make that decision in the coming months.
“NASA will make and announce the decision on the continuation of the Juno mission within the next few months. The factors being considered include the health of the spacecraft and instruments, the potential to obtain the anticipated science data, and any challenges and benefits from the longer, 53-day orbit,” noted NASA.
Should the decision be made to end Juno’s mission this year, the craft will, as planned, complete its 12th science perijove, 14th overall perijove, on 16 July 2018 before embarking on its final orbit that will align the spacecraft for a destructive entry into Jupiter’s atmosphere 53 days later.
Should the mission receive permission – and funding – to continue, Juno will remain in its 53 day current orbit, completing additional perijove science dives and returning that data and images to its science teams back on Earth… all while ensuring spacecraft health for the all-too-important end of mission destructive dive into Jupiter’s atmosphere to protect the planet’s moons.
Selected science returns thus far:
At the forefront of Juno’s successive perijove encounters with Jupiter are the stunning images JunoCam has captured and returned to Earth. But the images, while captivating, are not the primary purpose for the craft’s mission. In fact, most of the scientific data returned by Juno thus far is still being analyzed by mission scientists.
However, early science results have revealed Jupiter’s complex, gigantic, turbulent environment, with Earth-sized polar cyclones, plunging storm systems that travel deep into the heart of the gas giant, and a mammoth, lumpy magnetic field that may indicate it was generated closer to the planet’s surface than previously thought.
“We knew going in that Jupiter would throw us some curves,” said Scott Bolton, Juno principal investigator from the Southwest Research Institute. “There is so much going on here that we didn’t expect that we have had to take a step back and begin to rethink of this as a whole new Jupiter.”
Among the findings that have so far challenged assumptions are those provided by Juno’s imager, JunoCam. The images show both of Jupiter’s poles are covered in Earth-sized swirling storms that are densely clustered and rubbing together.
“We’re puzzled as to how they could be formed, how stable the configuration is, and why Jupiter’s north pole doesn’t look like the south pole,” said Bolton. “We’re questioning whether this is a dynamic system, and are we seeing just one stage, and over the next year, we’re going to watch it disappear, or is this a stable configuration and these storms are circulating around one another?”
Another surprise has come from Juno’s Microwave Radiometer, which samples the thermal microwave radiation from Jupiter’s atmosphere from the top of the ammonia clouds to deep within its atmosphere. Data from the Microwave Radiometer indicates that Jupiter’s iconic atmospheric belts near the equator have ammonia that penetrates as far down as the Microwave Radiometer can see (a few hundred miles/kilometers) while belts and zones at other latitudes seem to evolve into other structures.
Additionally, measurements of Jupiter’s magnetosphere from Juno’s magnetometer investigation indicate that the planet’s magnetic field is even stronger than models expected and more irregular in shape. The magnetometer data indicates that the magnetic field greatly exceeds expectations at 7.766 Gauss, about 10 times greater than the strongest magnetic field found on Earth.
“Already we see that the magnetic field looks lumpy: it is stronger in some places and weaker in others,” said Jack Connerney, Juno deputy principal investigator and the lead for the mission’s magnetic field investigation at NASA’s Goddard Space Flight Center. “This uneven distribution suggests that the field might be generated by dynamo action closer to the surface, above the layer of metallic hydrogen.”
Continuing the investigation of the planet itself, Juno has also returned information on Jupiter’s iconic Great Red Spot – a raging anticyclonic atmospheric storm 1.3 times the width of Earth. In July 2017, Juno flew directly over the Great Red Spot, returning numerous scientific data points on the storm, which indicates that the feature penetrates well below the visible cloud layer.
“Juno found that the Great Red Spot’s roots go 50 to 100 times deeper than Earth’s oceans (200 miles or 300 kilometers) and are warmer at the base than they are at the top,” said Andy Ingersoll, professor of planetary science at Caltech and a Juno co-investigator. “Winds are associated with differences in temperature, and the warmth of the spot’s base explains the ferocious winds we see at the top of the atmosphere.”
In addition to discoveries regarding the Great Red Spot, Juno has also detected a new radiation zone around Jupiter’s equator. “The closer you get to Jupiter, the weirder it gets,” said Heidi Becker, Juno’s radiation monitoring investigation lead at JPL.
— NASA's Juno Mission (@NASAJuno) December 11, 2017
“We knew the radiation would probably surprise us, but we didn’t think we’d find a new radiation zone that close to the planet. We only found it because Juno’s unique orbit around Jupiter allows it to get really close to the cloud tops during science collection flybys, and we literally flew through it.”
The new zone was identified by the Jupiter Energetic Particle Detector Instrument investigation. The particles are believed to be derived from energetic neutral atoms (fast-moving ions with no electric charge) created in the gas around the Jupiter moons Io and Europa. The neutral atoms then become ions as their electrons are stripped away by interaction with the upper atmosphere of Jupiter.
Moving slightly away from the planet, Juno has also returned exciting data regarding Jupiter’s auroral displays. Presently, Juno scientists have observed massive amounts of energy swirling over Jupiter’s polar regions that contribute to the giant planet’s powerful auroras – only not in ways researchers expected.
Examining data collected by the ultraviolet spectrograph and energetic-particle detector instruments aboard Juno, a team led by Barry Mauk of the Johns Hopkins University Applied Physics Laboratory, observed signatures of powerful electric potentials, aligned with Jupiter’s magnetic field, that accelerate electrons toward the Jovian atmosphere at energies up to 400,000 electron volts.
This is 10 to 30 times higher than the largest auroral potentials observed at Earth. Jupiter has the most powerful auroras in the solar system, so the team was not surprised that electric potentials play a role in their generation. What’s puzzling is that despite the magnitudes of these potentials at Jupiter, they are observed only sometimes and are not the source of the most intense auroras, as they are at Earth.
“At Jupiter, the brightest auroras are caused by some kind of turbulent acceleration process that we do not understand very well,” said Mauk. “There are hints in our latest data indicating that as the power density of the auroral generation becomes stronger and stronger, the process becomes unstable and a new acceleration process takes over. But we’ll have to keep looking at the data.”