Juno is the second of the new, medium-cost "New Frontiers" Solar System missions -- the first one being the New Horizons Pluto probe which has just successfully completed its gravity-assist flyby of Jupiter to catapult itself onwards to Pluto in 2015. Juno is one of those space missions that may not be very interesting to laymen, but is considered very important by many scientists.

It will make virtually no observations of the planet's fascinating moons, and the one onboard camera planned for it until now is more for PR purposes than for science (although it will still send useful full-color images of Jupiter's polar cloud patterns).

However, in October 2016 it will enter an elongated polar orbit around Jupiter -- farther away than the planet's moon Callisto at its farthest point, but swooping only 4300 km above Jupiter's cloud tops at its periapsis.

This will let it "thread the needle" through Jupiter's most intense doughnut-shaped radiation belts -- enabling it to survive the planet's powerful radiation at short range for over a year, and even to use solar panels for its electrical power despite their sensitivity to radiation-- and it will also allow it to study some important questions which the "Galileo" orbiter could not answer from its much more distant equatorial orbit around Jupiter.

Besides the "Junocam" camera (provided by Malin Space Science Systems), its experiments include

* Extremely precise tracking of its orbit to allow precise mapping of Jupiter's gravity field, which in turn will allow us to determine how big a central core of rock and ice lies within its inner layers of super-compressed hydrogen and helium. There are two radically different theories of how the giant planets formed in the first place -- in one, the rock-ice core formed first, and finally grew to a size enabling its gravity to start hauling in more and more hydrogen and helium gas in a runaway manner from the nebula out of which the Solar System was forming; while in the other theory, "density waves" crammed rock and ice particles together with gas simultaneously in a mixed concentration. The central rock-ice core should be bigger if the first theory is true.

* Microwave measurments of the precise amount of water vapor and ammonia in Jupiter's atmosphere. The entry probe that the Galileo craft actually dropped into Jupiter in 1995 was spectacularly successful, and one of its discoveries was that Jupiter has a concentration of many of the gaseous elements heavier than helium that is about three times higher than that in the Sun. This was a surprise.

It indicates that the chunks of water ice that became part of Jupiter contained more trapped gases than thought -- either because they were in a different chemical form ("clathrates") that had been thought, or because the ice that formed Jupiter actually drifted in from the coldest outermost parts of the Solar System.

In fact, the possibility can't be quite ruled out that Jupiter and the other three giant planets themselves originally formed much farther from the Sun and then slowly migrated inwards before coming to a stop, just as we now know that many giant planets orbiting other stars have actually spiralled all the way inwards to within only a few million kilometers of their suns.

But the Galileo probe had trouble making good measurements of water vapor and ammonia -- the latter being particularly important to decide whether the clathrate or the distant-origin theory of Jupiter's component ice is correct.

This is mostly because, by pure bad luck, it happened to parachute down into one of the temporary "dark spots" that cover only about 8 percent of Jupiter's equatorial belt, where strong downdrafts blow dry air from the upper stratosphere down into the deeper atmosphere, removing clouds and water vapor from the air that the Galileo entry probe analyzed. Juno's microwave-radiometry maps will decisively settle the question of just how common water and ammonia realy are in its air.

* A magnetometer to make very detailed studies of the nature of the planet's magnetic field, and thus of the dynamo of super-compressed, electrically charged liquid metallic hydrogen in its deep interior that produces the field. This process is still pretty mysterious even for our own Earth and the magnetic field generated by its own core dynamo of molten iron.

* Four more instruments -- a mapping ultraviolet spectrometer, plasma analyzer, high-energy radiation detector, and plasma wave detector -- which, along with the magnetometer, will study Jupiter's complex magnetosphere in the polar regions where its intense charged-particle radiation actually plunges into the planet's atmosphere and produces powerful auroras.

These eight American experiments by themselves would make a good, high-priority scientific planetary mission, despite its relative lack of charisma for nonscientists.

But in 2005, ASI (the Italian Space Agency) proposed adding two more instruments to Juno:another visible-light camera ("Itacam") oriented both toward PR and cloud-pattern studies; and "JIRAM" -- a combined near-infrared camera and spectrometer that would both make further studies of Jupiter's auroras, and examine its weather patterns and storms (including those same "dark spots") in new detail.

Juno's visible-light camera and microwave radiometer would also provide further studies of the planet's weather and wind patterns. And its sensitive gravity-field map may allow detection of winds -- or, more accurately for material at such great depths, currents -- as much as one-fifth of the way in from the planet's outer cloud layers to its core.) Italy also proposed to build the high-frequency Ka-band transponder which is an important part of the sensitive gravity tracking experiment.

Italy has considerable experience in building radio and infrared instruments for space missions. The "VIMS" visible and near-infrared mapping spectrometer on the current Cassini Saturn orbiter is Italian, as is part of the craft's Titan-mapping radar system.

NASA therefore took Italy's proposal very seriously, but not until this Feb. 12 did it officially decide to allow the team to incorporate both "JIRAM" and the Italian Ka-band transponder into Juno's payload, if their final technical checks confirm the feasibility of these instruments. If not, the US will have to build the Ka-band beacon itself. ("Itacam" was rejected, being redundant to the American "Junocam").

This means that Juno -- even though it will virtually ignore Jupiter's four big and fascinating major moons, which must instead be studied by the next mission to the Jupiter system -- will be able to make an even more detailed study of the planet itself than had been previously hoped.

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Related Links
Juno at University of Wisconsin-Madison
Jupiter and its Moons

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