Secrets of the Juno spacecraft

This illustration depicts NASA's Juno spacecraft at Jupiter (NASA/JPL-Caltech)

This illustration depicts NASA’s Juno spacecraft at Jupiter (NASA/JPL-Caltech)

Last week the space headlines were all about Jupiter. A billion-dollar spacecraft named Juno successfully fired its engine and went into orbit around the giant planet. NASA released its usual hype: that this mission may reveal the birth of the solar system!

They always do that. They know that many people will question spending one billion dollars to learn about a distant [1]world. So the press release says that the mission will teach us about Earth, or the secrets of life’s origins, or about the birth of the planets. The truth is much simpler: Juno will feed us more information about Jupiter. Actually, that’s plenty good enough, and we’re all very excited.

It’s interesting to compare Juno to the previous Jupiter orbiter, Galileo, which completed its mission 13 years ago. Galileo had operated for 14 years, whereas Juno will be sent crashing into Jupiter to its destruction just two years from now.

Galileo had marvelous telescopic cameras that operated until the very end. By contrast, the “Junocam” was almost an afterthought. Radiation is expected to destroy it after just seven or eight orbits, and then no more snapshots. Moreover, it is incapable of imaging Jupiter’s giant moons, whereas Galileo focused much of its energy on those mysterious planet-sized satellites, at least one of which has a high chance of having life.

Instead, Juno will focus on the invisible parts of the spectrum. Its instruments will probe the ultraviolet, the infrared, the changing level of gravity and the giant planet’s awesome magnetic fields: all geeky stuff invisible to the eye. They will measure the pressure density below the cloud tops, and reveal the composition of its gases’ minority components. It may reveal whether the core of Jupiter is a small rocky ball or merely highly compressed hydrogen: more geeky stuff.

The cameras on the two Voyager spacecraft were steerable. But the single Junocam is bolted to the spacecraft itself. If you want a picture of something, you’ve got to fire a thruster and move the entire 12-by-12-foot spacecraft. And those images have only somewhat better sharpness than the ones we get right here from Hubble. Specifically, we’ll get a resolution of ten miles per pixel.  During each two-week orbit around Jupiter, the Junocam’s total image capability is just 40 MB. This isn’t a holiday picture-taking expedition.

A very unusual aspect is Juno’s solar panels, which have never been used this far away from the Sun, since sunlight is only four percent as strong there as it is here. Nonetheless the increased efficiency of solar technology this past decade has allowed the three panels, each measuring nine-by-29 feet, to generate 486 watts. Thus, for the first time, we have not used plutonium as the electricity-generating source. This is the first eco-friendly spacecraft!

Another novel aspect of this mission is the astonishing amount of radiation that the craft must endure. During each two-week orbit, Juno will zoom just 2,600 miles above Jupiter’s cloud tops, and do this 37 times. Every two years, you and I receive less than one Sievert of radiation. But in the same two years, Juno will be hit with 20 million Sieverts. It will be interesting to see whether the shielding for its electronics holds up.

No, Juno doesn’t have the capability to give us the pretty pictures that Galileo did, or reveal new secrets about its fascinating moons. Nonetheless, this should be a very interesting two years.


Want to know more? To read Bob’s previous “Night Sky” columns, visit our Almanac Weekly website at

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