National Geographic Daily News
An illustration of the Juno spacecraft.

An artist's rendering shows the Juno spacecraft in orbit above Jupiter.

Illustration courtesy NASA/Caltech

Brian Handwerk

for National Geographic News

Published August 4, 2011

This Friday a NASA spacecraft is slated to launch on a five-year journey to Jupiter.

When it arrives, the craft will probe deeper into the gas giant planet than any previous mission, searching for the unseen core hidden below the thick atmosphere. It will also endure the solar system's strongest radiation zone to study the origins of the giant auroras that dance across Jupiter's poles.

The probe—dubbed Juno—will blast off from Florida aboard an Atlas V rocket, starting a 400-million-mile (644-million-kilometer) trek.

When it arrives at Jupiter in 2016, the spacecraft will spend about one Earth year making 33 elliptical polar orbits, skimming as close as 3,100 miles (5,000 kilometers) above the clouds.

Watch NASA video about the Juno mission.

Carrying a suite of eight main science instruments, Juno will collect data on Jupiter's atmosphere that may be key to understanding the birth of our cosmic neighborhood. (Also see "New Model of Jupiter's Core Ignites Planet Birth Debate.")

"We're really trying to understand the origins of Jupiter—how it formed, the role it played in the formation of the rest of our solar system, and what that can tell us about the solar systems that we're discovering around other stars now," said Juno's principal investigator, Scott Bolton, of the Southwest Research Institute (SwRI) in Colorado.

By delving far beneath the colorful zones and belts of Jupiter's high clouds, the Juno mission also aims to answer some fundamental questions about the planet's mysterious inner workings.

"It's as exciting as learning about the deep ocean," said Timothy Dowling, a planetary-atmosphere expert at the University of Louisville in Kentucky who's not part of the Juno team.

"It has that same feel. All that we see on Jupiter—the light and dark bands, the jets, the enormous storms—we'll explore the foundation underneath all of that."

(Related: "Jupiter Loses Big Belt; Great Spot Left Hanging.")

Jupiter Water May Offer Clues to Planet Birth

Jupiter may already seem well studied, since spacecraft headed elsewhere in the solar system have taken countless pictures as they swung close to the planet to use its gravity like a slingshot. (Related: "Lightning Strikes, Changing Climate Revealed on Jupiter.")

But until now only one other probe, Galileo, has orbited the giant world and attempted to study its composition and activity.

As the largest planet in our solar system, Jupiter could help resolve theories for the process of planet formation.

Current theory states that the planets formed from a dusty disk of material that surrounded the newborn sun roughly 4.6 billion years ago. (Related: "Newborn Planet Found Orbiting Young Star.")

Scientists know that Jupiter is primarily made of hydrogen and helium, like the sun and most of the universe. But the planet is also enriched with heavier elements such as carbon and nitrogen—elements that became the building blocks for not only the rocky planets such as Earth and Mars but also for life.

One theory to explain the heavy elements in Jupiter is that water, in the form of ice, was one of the first multielement molecules to form inside the protoplanetary disk.

This ice clumped together and trapped heavy elements in the dust to make dirty snowballs called planetesimals. In Jupiter's case, an ice ball may have attracted gases as it swept through the disk, building up the planet's mass.

Juno will help test that theory using a device called a passive microwave radiometer, which will measure how much water is in Jupiter's deep atmosphere. In this region, gravity is so intense that much of the primordial solar material that formed the planets is probably still trapped.

"I think it's a very fundamental mission, in the sense that we're tying to go back and look at the first step in the solar system's history after the sun formed and investigate why the planets are the way they are," SwRI's Bolton said.

Juno Probing Jupiter's Depths

Other instruments on Juno will map the circulation, composition, temperature, and other features deeper in Jupiter's atmosphere than any previous experiments.

"Essentially we'll be unraveling basic aspects of how meteorology works in an alien environment, thereby extending our understanding of atmospheric circulation and climate beyond the confines of Earth," said Adam Showman, of the University of Arizona's Lunar and Planetary Laboratory.

"I think that's pretty exciting, especially because gas giants like Jupiter are among the most common planetary environments in the universe."

(Related: "New Planet Found; Star's Fourth World Stumps Astronomers.")

And while Jupiter is nearly all atmosphere, the planet should have some kind of solid core far beneath the swirling clouds, where atmospheric pressures are millions of times greater than at sea level on Earth.

By precisely measuring the way Juno gets pulled and pushed by the massive planet's gravity field, scientists may finally be able to detect and measure the planet's core.

"This mission is very unique," said the University of Louisville's Dowling. "We've never tried to probe into the heart of a planet like Jupiter. It's basically the first time we'll see an MRI of a gas giant."

Juno Needs "Radiation Vault" to Survive

In addition to peering far below Jupiter's surface, Juno will be gazing high above the clouds, studying the origins of the planet's intense magnetic field and how it interacts with the Jovian atmosphere.

The biggest and strongest magnetic dynamo in the solar system, Jupiter's magnetic field creates spectacular "hyperauroras," which Juno will see in unprecedented detail.

(See "Jupiter Auroras Fed by Largest Moon's Magnetic 'Bubble.'")

Juno will also sample the charged particles that create the auroras and will observe them in ultraviolet light. In addition, the spacecraft will capture color photos of the poles with its JunoCam—sure to be one of the mission's most popular features.

"That camera is really for the public, and the data will be immediately made available for the public—but I want to see a picture of the poles too," SwRI's Bolton said.

But the magnetic field that drives Jupiter's auroras also traps charged particles in a bubble around the planet, creating the strongest radiation zone in the solar system, something that would fricassee the spacecraft without special precautions.

"We basically have a radiation vault—a titanium box in the middle of the spacecraft—and the sensitive electronics are inside that box, like an armored tank going to Jupiter," Bolton said.

At Mission's End, Juno to Plunge Into Jupiter

In the end, Jupiter's radiation will degrade the solar cells on Juno's arrays, putting limits on the lifetime of the mission.

Unlike previous deep-space missions, which used nuclear power, Juno will operate entirely on sunlight. When it reaches Jupiter, it will be the most distant solar-powered spacecraft yet launched.

The craft's three superefficient arrays—each 9 by 30 feet (2.7 by 9.1 meters)—will allow it to work in sunlight that's about 25 times weaker than the light here on Earth.

When Juno's mission ends, NASA scientists will instruct the craft to deorbit over Jupiter so that Juno burns up in the giant planet's atmosphere. This way, the defunct spacecraft won't be left to wander the Jovian system and potentially crash on one of the planet's many moons, contaminating worlds such as Europa with watery habitats that possibly host life.

Ultimately, the Juno mission will provide scientists with a wealth of data for understanding not just our own solar system but also the scores of gas giants scattered across the galaxy.

"Jupiter is our prototype giant planet," the University of Arizona's Showman said. "So the understanding we gain from Juno about Jupiter will lend great insights to our understanding of the hundreds of giant planets being discovered around other stars."

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