How the Juno spacecraft uncovered Jupiter’s secrets

A flexible flight plan

On Aug. 27, 2016, Juno experienced its first close approach of Jupiter, or perijove, sweeping just 2,600 miles (4,200 km) above Jupiter’s clouds, closer than any spacecraft in history. This flyby provided our first ever high-latitude glimpse of the north pole. It took almost two days to download the 6 megabytes of data from perijove 1, but the results proved well worth the wait. “It’s bluer in color up there than other parts of the planet, and there are a lot of storms,” noted Bolton. “There is no sign of the latitudinal bands or zones and belts that we are used to — this image is hardly recognizable as Jupiter.”

Juno revealed the poles are dominated by densely packed cyclones, all jostling for position. In the north resides a central cyclone, encircled by eight others, all around 2,000 miles (3,200 km) in diameter, as wide as the contiguous U.S. Clinging to the periphery of the pole, as if itching for admission, are other tumultuous weather systems, including a 5,000-mile-wide (8,000 km) behemoth known as the North North Temperate Little Red Spot 1, the third-largest anticyclonic oval storm on Jupiter.

The planet’s deep south proved no less tempestuous, harboring a pentagon of five cyclones around a central sixth. But while the northern storms remained stable over time, scientists were astonished in November 2019 when another cyclone — smaller than the rest, yet still the size of Texas — muscled its way into the southern group. The gatecrasher’s petulance did not go unpunished and within months, it had been pushed out and vanished. To Bolton, “it almost appeared like the polar cyclones were part of a private club that seemed to resist new members.”

As well as spotlighting the storminess of the poles, Juno’s first perijove showed the planet’s magnetic fields and aurorae are much more powerful and extensive than previously thought. It revealed the field is irregularly shaped, lumpy in places, and — at 7.766 Gauss — more than 10 times as powerful as Earth’s magnetic field at its strongest.

But despite a promising start, all was not well. The PRM burn in October 2016 was postponed due to worries about helium valves in the main engine’s pressurization system. Although the valves opened on command, they did so more sluggishly than expected. Matters were compounded further when an unrelated computer reboot threw Juno into safe mode for a few days in late October.

The most efficient time to perform the PRM was during the risky perijove passage, but an increasingly nervous NASA chose to weigh its options before trying again. “There was concern that another engine burn could result in a less-than-desirable orbit,” said Project Manager Rick Nybakken of the Jet Propulsion Laboratory in a statement. “The bottom line is a burn represented a risk to completion of Juno’s science objectives.”

This risk ultimately decided the issue. In February 2017, NASA announced that Juno would remain in its 53-day orbit for the rest of the mission. Researchers did not expect the longer orbit to impair the science, as the probe’s altitude during each perijove would remain the same as it would have during its planned 14-day orbit. Indeed, the 53-day orbit meant that Juno would actually spend less time in the regions of most intensive radiation. “This is significant,” said Bolton, “because radiation has been the main life-limiting factor for Juno.”