Juno flies in an elliptical orbit around Jupiter, getting close to the planet once every 53.5 days.
Juno's closest flyby of Jupiter lasts several hours and is called a perijove. During that time, the probe flies over Jupiter's north pole and exits with views of the south pole.
Juno passes within thousands of miles of the planet's surface during a perijove, enabling the probe to make detailed and unprecedented measurements of gravity, magnetism, and plasma fields using a suite of high-tech instruments. It also beams back stunning images.
Until Juno's arrival at Jupiter in July 2016, no top-down or bottom-up images had ever been taken of Jupiter's poles.
The probe flew over Jupiter's poles for the first time and sent back data that blew scientists away.
Observatories like NASA's Hubble Space Telescope had recorded auroras at Jupiter's poles, but nobody had ever seen the enigmatic regions in the detail Juno revealed.
"[I]t looks like nothing we have seen or imagined before," Scott Bolton, a planetary scientist at the Southwest Research Institute in San Antonio and the Juno mission's leader, said in a NASA press release in September 2016.
"It's bluer in color up there than other parts of the planet, and there are a lot of storms," he added. "There is no sign of the latitudinal bands or zone and belts that we are used to — this image is hardly recognizable as Jupiter."
The planet's weather goes much deeper than anyone thought.
Juno's first ultra-close pass helped researchers peel back Jupiter's atmospheric layers like an onion and see hundreds of miles deep into the planet's hydrogen, helium, and other gases.
"The structure of the zones and belts still exists deep down," Bolton said during a news conference in 2016. "So whatever's making those colors, whatever's making those stripes, is still existing pretty far down into Jupiter. That came as a surprise to many of the scientists."
Later studies revealed that the appearance of belts, bands, and storms may originate more than 1,900 miles below the planet's gassy surface.
Surprisingly symmetric clusters of storms exist at Jupiter's poles.
In April, NASA released eye-catching images of symmetric clusters of storms.
The images were created from multiple photos taken in infrared light (a wavelength that's normally invisible to us). Next, scientists merged the pictures together made them 3D to emphasize brightness and temperature.
Scientists are still trying to understand what, exactly, creates these stormy super-cells that stretch thousands of miles wide.
Most lightning is concentrated at Jupiter's north pole.
During Juno's first eight perijoves, or flybys, the spacecraft detected 377 lightning discharges via their crackle of radio emissions. When researchers mapped the locations of the lightning, they found it concentrated at the planet's poles, not its equator.
Researchers think this happens because when the sun warms Jupiter's atmosphere, the heat stabilizes the planet's mid-areas against upwelling storms. But at the poles, where it's much cooler, big cells of warm inner gases can punch to the surface and create lightning.
Weirdly, most of the lightning was recorded at Jupiter's north pole — something researchers have yet to explain.
The planet's gravity is kind of lopsided.
Juno has a special instrument that allows it to precisely map Jupiter's gravitational field. The first major results from that data show that field is slightly lopsided, with a bit more "pull" from its northern hemisphere.
Researchers think this is tied to Jupiter's deep-running weather systems, which can push huge masses of its atmospheric gases to certain parts of the planet.
Its magnetic field is also lopsided — and stronger than scientists thought.
Jupiter's magnetic field is the strongest in the solar system and drives all sorts of important behaviors, including polar auroras, plasma fields around the planet, and radiation that can threaten space missions. So it's important to understand in detail. Luckily, magnetic instruments on Juno can continuously map the planet's magnetic field.
Those readings have surprised researchers in multiple ways. They showed Jupiter's magnetic field strength is about two times more powerful than researchers previously thought.
The data also revealed that the entire magnetic field, like the planet's gravity field, is slightly lopsided. The reasons for that that have yet to be explained.
Jupiter's Great Red Spot suggests the planet is rich with water.
Many moons around Jupiter hold water and may even harbor vast oceans that are habitable to aliens. But Jupiter looks pretty dry on the surface, and shoring up evidence for water molecules inside the planet itself has proven difficult.
Juno is confronting this problem with its suite of sensitive instruments and ultra-close flybys. Researchers combined Juno's data with telescope data gathered about the Great Red Spot, which affords a deeper view of Jupiter's atmosphere. They discovered that the world may have two to nine times more oxygen than the sun — and oxygen is a key component of water.
The planet may have a "fuzzy" core.
"There seems to be a fuzzy core, and it may be much larger than anybody had anticipated," Bolton said during a NASA press conference in May 2017.
Hydrogen in the depths of Jupiter exists in a semi-gaseous, semi-dissolved — hence, "fuzzy" —state. Earth, by comparison, has outer liquid that moves around a solid core.
Jupiter's fuzzy core may be what drives its powerful dynamo and magnetic fields, sort of like the metallic dynamo at Earth's core. The previous thinking was that hydrogen — normally a gas as we know it — compressed into a liquid-metal state was the culprit for Jupiter's magnetic fields.
Something strange powers Jupiter's auroras.
Auroras start with a planet's magnetic field. That field grabs charged solar particles blown into space, then races them toward the planet's poles. When the particles slam into atoms of gas in the atmosphere, they create a light show.
But Juno researchers have discovered that Jupiter's polar auroras have 10-30 times more energy than expected, and they give off that energy mostly in ultraviolet light, which is invisible to human eyes. What's more, the auroras dim when the planet rotates into darkness.
Researchers have yet to figure out how auroras work at Jupiter, though they suspect it has something to do with an energetic field of plasma around the planet, according to Space.com.
Juno found a possible new volcano on Jupiter's moon Io.
Juno was designed to study Jupiter in detail, but the planet's dozens of moons make for interesting targets, too.
The planet's moon Io — a hellish, sulfur-choked moon that's about as wide as North America and littered with volcanoes — is one example. Scientists got just a few glances at the moon decades ago, using spacecraft like Galileo and Voyager.
But in December 2017, Juno's infrared-seeing camera found what looks like a previously undetected volcano on the moon. According to NASA's Jet Propulsion Laboratory, the world has 150 active volcanoes — yet 250 more await discovery, based on how much material is spewing off the planet's surface and into space.
