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NASA celebrates the Fourth of July with the arrival of its next-gen Jupiter probe

In Roman mythology, there’s a story about Jupiter, first among the Gods, drawing a veil of mist and clouds around himself, to help obscure himself. Only his wife could see through these clouds to see his true nature. Her name was Juno. Thus was born the name for NASA’s Jupiter-exploring space probe, which just reached the gas giant this Fourth of July. If all goes according to plan, it will pierce Jupiter’s veil of clouds and reveal its secrets — and when it comes to Jupiter, there are a whole lot of secrets to reveal.

The main goal of Juno is to better understand the evolution of Jupiter, and to use that to better understand the evolution of the solar system. It recently approached the planet to begin its many elliptical orbits, coming in for close fly-bys of just a few thousand kilometers at the poles, then swinging out for long-distance shots in between. It will be able to peer over 500 kilometers below the gaseous surface of the planet, providing a deeper look than its predecessor, the Galileo probe, could ever manage.

Jupiter is an enticing target for a number of reasons, and not just that it’s the biggest and coolest planet (though that’s certainly part of it). For one, it’s the closest planet to the Sun that’s beyond the “frost line,” the line within which the ancient Sun provided too much heat to allow condensation of certain light elements, and thus provided a strong bias toward rocky, metal-heavy planets like Mercury, Earth, and Mars. Beyond that line, gas giants become far more possible, and Jupiter is the first such example in our neighborhood.

One major ongoing mystery about Jupiter’s modern composition is the form and distribution of the water it contains. We know that Jupiter contains the majority of all water in the solar system, so understanding how it condensed over time could easily help understand the distribution of water on planets like Mars, and even Earth while it was forming. The specialized microwave antennae will use the only known wavelengths of light that can penetrate the Jovian atmosphere, determining not just the distribution but the temperature profile of Jupiter’s water, and how it circulates through the lower atmosphere.

That’s the sort of insight we’ve never had about Jupiter — the vast majority of what we “know” about its internal structure and dynamics has been derived from models, or inferred from indirect evidence like the shape of the gravitational field.

Diagram of Jupiter, from Wikipedia

Gas giants are also the biggest and most visible planets in the universe, and thus remain the most likely to produce the largest number of high-quality exo-planet readings for the foreseeable future. Properly interpreting those readings will require a more and more nuanced understanding of how gas giants form and progress over their lifetimes. Sometimes astronomers use the larger-scale context provided by distant planets to better understand the small sample size of planets in our solar system, other times they study the nearby planets of our system to better calibrate the hunt for distant equivalents.

Juno launched from Earth in 2011, and soon after the probe portion detached from the launch stages, and used a short-term burst of energy to spin up to several rotations per minute. It lost most of this rotation as it extended the arms of its solar panels and moved its moment of inertia outward, but the left-over angular momentum has kept the spacecraft steady on its almost five-year journey to Jupiter’s pole. The spin will also ensure that all of Juno’s instruments will be able to observe the gas giant, regardless of where they’re situated on the probe itself.

Starting from its arrival this past Independence Day, NASA will get about two years of orbital readings out of Juno before it is forced to execute a controlled de-orbit, sending back information as long as it can before being obscured by clouds and crashed by the planet’s incredible pressure and gravitational power.

Juno could spark a storm of activity in planetary science, clearing up long-standing questions and providing the concrete basis for the next generation of speculation, which will then inform the next generation of experiments and launches. It took about 20 years to transition from Galileo to Juno, the second-ever visit to Jupiter, but the results should be more than worth the wait.

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