Farewell to Jupiter, and Hello

Artist's concept of Juno at Jupiter (credit: NASA)

Artist’s concept of Juno at Jupiter (credit: NASA)

If you’ve been following the news this week, you know that Jupiter has a new moon, a man-made moon called Juno. The NASA spacecraft, bejeweled with solar cells and as big as a basketball court, entered an elongated orbit around the big planet on July 4 as it began a 20-month study of the structure of Jupiter. While Jupiter may be fading in the western sky after sunset, still visible but soon to be lost to our telescopes, it will continue to reveal many secrets to Juno during the coming months.


Juno was launched on August 5, 2011 and took a circuitous route to Jupiter, encountering Earth in 2013 for a gravity boost to pick up speed before traveling on to Jupiter.  The craft was inserted into an elongated orbit on July 4, 2016 while moving faster than any human-made object has every traveled (165,000 mph). The principal investigator of the Juno mission, Scott Bolton, said of the orbital insertion, it’s “… the hardest thing NASA’s ever done”.

What made it so hard, in part, is the withering radiation in the Jovian system that is notoriously unfriendly to the electronics that control and guide the craft. The radiation fried much of the electronics of the Galileo mission to Jupiter in the 1990s and rendered some of its instruments inoperative. Juno’s electronics are embedded in a thick titanium vault inside a tank-like hexagonal core to which is attached three long solar panel assemblies. Juno mission engineers also designed many protocols and contingency plans in case the craft endured some damage to its electronics and computers.

A major goal of the Juno mission is to map Jupiter’s immense and powerful magnetic field, a field some 20,000 times as intense as Earth’s. The entire magnetosphere of Jupiter is so large, it would appear twice the diameter of the full Moon if observed from Earth. It also causes polar auroral displays larger than our planet. But it’s uncertain how exactly the magnetic field arises, and Juno’s measurements will help scientists figure this out.

Juno will also peek under the veil of Jupiter’s clouds to help deduce what’s inside Jupiter. Like the Sun, Jupiter consists of mostly hydrogen, and deep inside the planet, an immense ocean of hydrogen may be under such intense pressure that it has turned to metal. Instruments on Juno will also measure how much water and heavier elements the clouds of the planet hold, and it will measure the winds and other weather features such as the Great Red Spot.

And then there’s Jupiter’s core. As big as the Earth but perhaps ten times as massive, the core of this big planet has so far defied detailed analysis. Some planetary scientists thought the core was a solid diamond, some thought it was dense rock, and some wonder if it’s a mixture of more hydrogen and heavier elements. As Juno orbits the planet, its position and speed will be closely monitored by an array of radio telescopes on Earth. Slight changes in the speed of the craft, presumably caused by tiny variations in the mass of Jupiter and its core, will be detected and translated into a precise map of the mass and density of the big planet and its core.

The ultimate goal of the Juno mission, as it orbits Jupiter 37 times over the next 20 months, is to figure out the structure of Jupiter and to help sort out theories of how Jupiter and the other planets in the solar system formed in the earliest days of the solar system.

Of course, the wonderful thing about astronomy is that you can wander outside and inspect the heavens for yourself. Jupiter remains one of the brightest objects in the sky in early July 2016. It lies low and fat and bright over the western horizon near the star Regulus in the constellation Leo. With a telescope in steady sky, you can inspect the belts and zones of the planet and see the Great Red Spot move across the disk of the planet in a matter of hours. And of course, you can see the four largest moons of Jupiter move from hour to hour and day to day around the planet. These are the same moons discovered by Galileo in 1610 that helped demonstrate circumstantially that Earth is not the center of the solar system.

For a whirlwind view of the moons in action, have a look at the video below of the serene Newtonian dance of Jupiter’s four largest moons as imaged by the NASA Juno team as the craft approached. It’s a mini ‘solar system’ in action:

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