A new JUPITER
Ben Evans looks at how the Juno mission has allowed us to rediscover the Solar System’s largest planet, revealing its unusual features in greater detail than ever before
For two years, a torrent of images – the surreal intricacy of which evokes the work of Salvador Dali, while their almost impressionistic juxtaposition of light and shade would delight Claude Monet – has been journeying across 870 million km of space to fascinate an Earthly audience. Since reaching Jupiter in July 2016, Juno has become the first spacecraft to enter polar orbit around the Solar System’s largest planet and the first to actively engage with the public to determine what it should be observing. In doing so, Juno has transformed Jupiter from a planet into an objet d’art.
Launched in August 2011, Juno’s 2.8 billion km journey took it into Jupiter’s gravitational clutches at a relative velocity of 265,500km/h, faster than any man-made object in history. Juno entered an elliptical 53.5-day capture orbit, passing 4,200km over the planet at its closest (‘perijove’) and sweeping outward to 8.1 million km at its farthest (‘apojove’). The original plan was for Juno to complete two capture orbits, then enter a repeating 14-day science orbit for 37 pole-to-pole laps of Jupiter. But Juno was always intended to be a voyage into the unknown, and the unknown has a habit of throwing curveballs.
One such curveball hit the mission in October 2016 soon after Juno had settled into its capture orbit. A set of sluggish helium valves forced managers to delay firing the engines that would send it into its science orbit. Matters worsened when the spacecraft put itself into safe mode, following a computer reboot. In February 2017, fearful that firing the engines might impair the mission, NASA opted to keep Juno permanently in its 53.5-day orbit.
Circling Jupiter on this looping ellipse permits global mapping of its magnetosphere, which extends 8 million km towards the Sun and spirals beyond the planet in a tadpole-like magnetotail. The hydrogen-helium atmosphere is tightly compressed by gravity and virtually impenetrable. But by flying so close to Jupiter, Juno envelops it in a ‘net’ of observations, yielding insights into
its magnetic field, it score and an electric ally conducting inner‘ shell’ of metallic hydrogen.
In May 2017, Juno showed the magnetic field to be strangely ‘lumpy’ – stronger in some places, weaker in others – but still many times more powerful than the strongest fields on Earth. “This uneven distribution,” says deputy principal investigator Jack Connerney, “suggests that the field might be generated by dynamo action closer to the surface, above the layer of metallic hydrogen.”
Juno’s close perijove orbit coincidentally also enabled the discovery of a new equatorial radiation belt, characterised by energetic hydrogen, oxygen and sulphur ions. “We only found it because Juno’s unique orbit allows it to get really close to the cloudtops,” explains physicist Heidi Becker, “and we literally flew through it.”
It is thought that the particles derive from energetic neutral atoms from the moons Io and Europa, which are stripped of their electrons by interaction with Jupiter’s upper atmosphere.
No dive to destruction… yet
The capture orbit also benefits Juno’s longevity. A 14-day science orbit would have exposed it to significant radiation and an elevated risk of hardware failure, as well as endangering the moons Europa, Ganymede and Callisto, which might possess subsurface oceans. Balancing this life-limiting radiation dosage against the need to safeguard potentially life-bearing moons, the mission was targeted to end in February 2018 with a destructive dive into Jupiter’s atmosphere. However, the 53.5-day orbit carries Juno through more benign radiation, allowing it to endure until July 2018 and possibly longer.
“Every 53 days,” says principal investigator Scott Bolton, “we go screaming by Jupiter, getting doused by a fire-hose of Jovian science and there is always something new.”
Key findings include polar aurorae that function quite differently to our Northern and Southern Lights. Ultraviolet and energetic-particle data showed that the signatures of powerful electric currents, aligned with the Jovian field, accelerate electrons at energies up to 400,000 electron-volts, some 10-30 times higher than Earth’s aurorae. As the power density strengthens, the process destabilises and other mechanisms take over. Radio emissions, shifted into the audio range, have even allowed the public to hear Jupiter’s ghostly ‘voice’.
And on this mission, the public has a ringside seat. Thanks to JunoCam, its visible-light camera, atmospheric features can be imaged in colour, at resolutions as fine as 2.9km per pixel, by ‘citizen scientists’. In January 2017, NASA initiated an online voting campaign to pick regions for Juno to photograph during successive perijoves, as it completed each two-hour, north-to-south sweep.
Before Juno, Jupiter’s poles had never been seen. “It’s bluer up there and there are a lot of storms,” explains Bolton. “There is no sign of the latitudinal bands or zones and belts that we are used to.”
Both of the poles feature a huge central vortex surrounded by swirling groups of cyclones – eight in the north, five in the south – with winds peaking at 350km/h. The northern cyclones measure up to 4,600km across, whilst their southern cousins are even larger, reaching almost 7,000km in diameter.
These cyclones are enduring features, having been observed continuously by Juno for many months. Despite being densely packed together,