JUNO: Seeing Jupiter Through New Eyes

• Juno at Jupiter Orbital Insertion (JOI). Image credit Lockheed Martin.

When spacecraft encounter distant worlds, we are mesmerized by the fantastic images that come back to Earth… the ruddy vistas of Mars, the deep aquamarine of Neptune, or the cracked and crinkled patchwork of Io. But some stories aren’t best told with pictures, and in the case of the recent rendezvous of NASA’s Juno spacecraft with giant Jupiter, it’s not what scientists will see that’s important… it’s what they will, in effect, hear.

On Independence Day, July 4, 2016, Juno will complete a 1.4 billion mile (2.2 billion km.) journey to the largest planet in our solar system. Only once before has Jupiter been extensively studied by an orbiting probe, NASA’s Galileo orbiter, and Juno is on its way to complete the job begun by that mission. But the mission team won’t be relying on cameras to do the job--they’ll be listening to radio signals.

“The questions we’re trying to answer won’t be resolved by taking pictures,” says Steven Levin, JPL’s Project Scientist for the Juno mission. “To understand the origins of the solar system, we need to know more about the composition of Jupiter. Determining what lies underneath those clouds is what’s important—we want to know how big the core is, how much water is present, and what the structure of the planet looks like as you go down deep inside.”

The answers to these questions lie far below the swirling, hypnotically beautiful clouds, and will be uncovered by the measurement of electromagnetic waves. Radio waves translated into plot-points and squiggly lines will tell the story.

Juno will measure the water content in the atmosphere to a depth of about 370 miles (600 km.) using an instrument called a microwave radiometer, which can measure invisible signals emanating from deep inside Jupiter at six different wavelengths. Each of these correlates with temperatures at specific depths in the atmosphere, and by combining these readings, researchers can build a layered picture of the distribution of water (and other elements, such as ammonia) throughout the upper layers of the planet. This water distribution model will help to pin-down Jupiter’s place in the early solar system.

But to truly understand the gas giant, we also want to know more about what lies even deeper within. While the bulk of the Jupiter is composed of hydrogen and helium, it may possess a small, dense core of heavy elements. This is probably surrounded by a much larger mass of hydrogen that is so highly compressed that it has been stripped of its electrons, becoming a bizarre metallic and electrically-conductive fluid. But short of descending to the inhospitable depths of the planet, how can this be measured?

“We currently have no direct evidence that a dense inner core exists, so we are going to take a gravity measurement of the whole planet and figure out how big that core is, and try to determine how much of those heavier elements are in the planet,” Levin says. But understanding the gravitational field of Jupiter, and learning about its core, is more involved than reading data from onboard instruments. The solution is to precisely track the spacecraft’s radio signals as it orbits the planet.

When Juno arrives at Jupiter, it will enter orbit over the poles, instead of around the equator as Galileo did, circling the planet once every 14 days over the course of 18 months. This orbit will be face-on to Earth, and by carefully measuring tiny changes in the speed of the spacecraft, scientists can map the gravitational pull of Jupiter.

William Folkner, the Gravity Science Investigation Lead for Juno, thinks of it like studying spinning golf balls. “There are lots of different kinds of golf balls, each with a different structure inside,” he says. The golf balls have the same mass, but will often spin differently, revealing the makeup of their interiors. “You can see the differences in how they spin. So, like a gigantic golf ball, we hope to measure changes in the direction of Jupiter’s spin axis as a function of time, by looking for variations in Juno’s acceleration as it orbits Jupiter.” This will tell Folkner and his colleagues the overall shape, size and density of Jupiter’s core.

This measurement must be precise—very precise. How accurately can the spacecraft’s signals be tracked at this distance, about 558 million miles when the mission begins? “We expect to measure the spacecraft velocity, the component in direction towards Earth, with an accuracy of 10 microns per second, every minute. That’s comes out to 0.0004 inches per second,” Folkner says. This is an astoundingly small increment, considering not only the distance the radio signals must travel, but also that they are being collected by radio dishes on Earth, affected by such variables as weather, electromagnetic interference, and of course our planet’s own motions relative to Jupiter and the spacecraft. This is radio science at its most exacting.

In the past decade, theories about the solar system’s formation have stagnated somewhat for want of more data. By the end of Juno’s 18-month mission, researchers will have a much better understanding of the history of the gas giant as well as the early solar system. They are eager to understand why Jupiter, when compared to other planetary systems, seems to occupy a unique place within our own, and how this relates to the formation of the earth.

You can track the progress of Juno on the mission website at [https://www.nasa.gov/mission_pages/juno/main/index.html]