Five hundred and eighty eight million kilometres away sits a cloud of gas so large that it weighs more than Saturn, Uranus and Neptune combined. It's so powerful that it's been accused of slinging entire planets into the sun, and so ancient it could hold the key to the origin of Earth. Image: NASA
I'm talking about Jupiter; a tempestuous beast of a world that has inspired fear and awe for centuries. After decades of scrutinising Jupiter's swirling cloud tops and glowering red eye from afar, we're finally going to pull back the curtain and learn what lies beneath.
When NASA's Juno spacecraft -- a 181kg box of reinforced titanium strapped to a 20m-long solar panel -- arrives in orbit around Jupiter on July 4, humans will be able to peer beneath the monstrous gas giant's upper atmosphere for the first time, into the layers of poisonous thunder clouds packed below. Over the course of a year, Juno will map the entire surface of Jupiter, all the while using nine scientific instruments to probe the gas giant's interior composition and powerful magnetic field. "Nobody's ever seen Jupiter the way we will," Juno mission director Scott Bolton told Gizmodo.
By the time the spacecraft makes its suicide plunge into Jupiter's atmosphere, we'll have a detailed understanding of the gas giant's inner workings, and a better picture of how our solar system formed.
“Nobody’s ever seen Jupiter the way we will”
One might ask why, as self-interested beings with a world's worth of problems here on Earth, we should bother studying a planet as hostile as Jupiter. Shouldn't we focus on getting people to Mars, or mining asteroids for scarce metals, or discovering habitable worlds beyond our solar system in case we screw this one up beyond repair? No doubt, these are all important goals. But so is the study of planets in our solar system where life could never survive.
And it's arguable that no other planet can tell us as much about Earth's humble beginnings as Jupiter. "Jupiter represents the first and maybe the most important step in the transition between forming a star and forming a solar system," Bolton said. "It's so big, it used up more than half the leftovers that were there after the formation of our sun."
After ballooning in size, Jupiter used its hefty gravity to shape the solar system around it, clearing out ancient comets, asteroids and perhaps even planets. Many theorists suspect that Jupiter's dust-busting activity set the stage for the formation of Mercury, Venus, Earth and Mars.
But 4.5 billion years has made the details fuzzy. To understand exactly how the planet-forming palooza went down, we need to know what elements were available in the early solar system.
Jupiter, like our sun, is composed primarily of hydrogen and helium. But it's also enriched in heavy elements that are critical for forming terrestrial planets. Using microwave sounding, Juno will determine the global abundance of two key elements -- nitrogen in the form of ammonia, and oxygen in the form of water -- throughout Jupiter's atmosphere.
"The stuff Jupiter has more of is the stuff we're all made of," Bolton said. "We're looking at the history of the volatiles that formed the Earth by going back and seeing how much Jupiter has."
Artist's concept of Juno sweeping through Jupiter's powerful magnetic field. Image: NASA/JPL-Caltech
Juno will also shed light on the nature of Jupiter's core, which features temperatures hotter than the surface of the sun and pressures tens of millions of times greater than that of our atmosphere. Some believe the core to be a solid hunk of metal larger than our planet. Others say it's a high pressure sea of hydrogen and helium gas. Discerning between these wild possibilities will not only constrain models for the evolution of the solar system, it will help us understand the origin of the gas giant's enormous magnetic field.
Which brings us to Juno's next big goal: Studying Jupiter's magnetosphere.
Jupiter's magnetic field is nearly 20,000 times as powerful as our own, and the region it influences, called the magnetosphere, is the single largest "object" in our solar system. The magnetosphere has a tadpole shape, ballooning one to three million kilometres toward the sun and stretching all the way out to Saturn some one billion kilometres distant. Within this region, swarms of charged particles are trapped and pulled in toward Jupiter, where they cause powerful geomagnetic storms. Jupiter's poles are alive with never-ending northern light displays that span regions larger than the Earth itself.
“It’s not unlike flying through the eye of a hurricane.”
By passing over Jupiter's poles and taking careful measurements of charged particles and magnetic field strength, Juno will offer insights into the fundamental processes underlying space weather, including how it forms and how powerful it can get.
"Juno will be a game-changer for auroral science," said William Dunn, an astrophysicist studying space weather on Jupiter at the University College London. "It is going to give us unprecedented access to Jupiter's very extreme polar environment. We've never had a spacecraft doing the daredevil maneuvers that Juno will be doing to explore this dangerous and largely unknown region."
Dangerous is a key word: To do all this science, Juno is going to be flying into the most intense radiation environment in the solar system. On Earth, we're exposed to a background radiation of about 0.3 rad. During a series of low altitude passes that bring it within 5000km of Jupiter's cloud tops, Juno will be exposed to a radiation environment of approximately 20 million rad.
"Jupiter is, in a way, constantly firing at our spacecraft," said Rick Nybakken, Juno's project manager at NASA's Jet Propulsion Laboratory. "Charged particles travelling at the speed of light will be coming at us from all angles. It's not unlike flying through the eye of a hurricane."
Mission orbits diagram. Image: NASA/JPL-Caltech
To keep the spacecraft from frying, the flight computer and the cores of most of its scientific instruments are being stored inside a first-of-its-kind radiation vault -- a hefty titanium box that reduces radiation exposure approximately 800-fold. The solar cells lining the wings of the spacecraft have a special radiation coverglass, and the orbit was also carefully designed to minimise exposure to Jupiter's intense radiation belts.
Even with all of these precautions, the spacecraft will accrue radiation damage over time. But Nybakken feels confident that the mission will survive long enough to accomplish its key science goals.
"We're going into orbit in a region nobody's been before, so yes, there is going to be some uncertainty," he said. "But in a sense, we're standing on the shoulders of giants. Not only does the spacecraft build on other solar system missions, but on what we learned from Galileo," he said, referring to first space probe NASA sent into orbit around Jupiter, in the 1990s.
For Nybakken and the engineering team, the nail-biting moment will come on July 4, when a 35-minute main engine burn slows the spacecraft down from a rip-roaring 241,400km per hour so that it can be captured by the gas giant's gravity. If this manoeuvre doesn't go off perfectly, Juno won't enter Jupiter's orbit, and there's no mission. There's no way of issuing a course correction en route, because it takes 49 minutes for a signal to reach the spacecraft from Earth.
"I'm not really going to relax until that main engine burn is complete," Nybakken said. "That is the big milestone."
The next major step comes when Juno establishes its "science orbit" in late October, before commencing a series of 33 two-week passes -- its main data-collection phase -- in November. Just over a year later, Juno will plunge suicidally into Jupiter's atmosphere, its atoms dispersing to join those of Galileo before it. The mission is short-lived by design: NASA wants to avoid contaminating Europa at all costs, and that means not leaving any spent probes kicking about the ice moon's backyard. (Eventually, the space agency is planning send a dedicated astrobiology mission to Europa to search for signs of alien life.)
In the meantime, we can expect to learn a wealth of information about Jupiter's inner workings in the months and years to come. And while there's little chance of find life in its noxious ammonia thunderclouds, the opportunity to explore the strange, hostile planet that shaped a tiny dollop of starstuff into the solar system we know and love isn't a bad alternative. In discovering Jupiter, we'll discover a part of ourselves.