On July 4th, at 10:30 pm (EST), the spacecraft Juno will arrive at Jupiter after a five year and nearly two billion mile journey. It will circle the planet 37 times, collecting a variety of data, before falling into the atmosphere and destroying itself. Funded by NASA, built by Lockheed Martin in Denver, and operated by JPL in Pasadena, Juno is (quite appropriately) named for the mythical wife of Jupiter - the Roman god of sky and thunder. According to Roman mythology, Jupiter shrouded himself in clouds, and only Juno could peer beneath his veil to glimpse the god's true intentions. The Juno spacecraft will observe Jupiter like never before, flying closer and orbiting longer than any orbiter in NASA history. It carries eight instruments, some of which will be used to map the surface at many different wavelengths of light. Juno will also map the magnetic field of Jupiter and study the churning motions of the planet's surface. These measurements will help astronomers answer several important questions about the largest planet in our Solar System.
Where was Jupiter born?
Juno is tasked with acquiring data to help scientists determine the amount of water present in Jupiter's atmosphere and whether a rocky core exists (and if so, how big it is). These are two factors that will help scientists understand how Jupiter was formed in the first place. Ultimately, we want to better understand the gas giants in our Solar System in the context of the other planets we are finding in our galaxy. As we have discovered more and more of these “extrasolar planets,” we've found a surprising number of so-called “hot Jupiters.” These are planets the size of Jupiter, but orbiting their host star at distances smaller than the distance between Mercury and the Sun. Some are so close to their host stars, the planets themselves are evaporating. It is impossible to form Jupiter-size planets that close to a star, since the materials needed to form these gas giants are typically found very far from the star. The prevailing theory says that these hot Jupiters were formed far away from their host stars, and migrated inward over time.  However, that migration is only possible if there is still a significant amount of gas left in the planetary system. By now, most of the gas in our Solar System has been swept up into planets, or gobbled up by the Sun, so our Jupiter isn't going anywhere anytime soon. Why didn't our Jupiter become a hot Jupiter? By measuring the amount of light elements (such as hydrogen and oxygen) as well as the amount of heavy elements, we'll be able to better pinpoint the birthplace of Jupiter, and study how it might have migrated through our Solar System over time.
What are the complex physics at work in Jupiter's atmosphere?
What makes the Juno mission special is that the spacecraft will skim the clouds of Jupiter, coming within 3,100 miles of the planet's surface, and stay there for the duration of its mission. The last time we sent a probe to Jupiter, it was the Galileo probe, which was purposefully dropped into the planet's atmosphere. While we learned a lot from the Galileo probe, we were only able to capture 57 minutes of data before it was crushed. Juno, by comparison, will stay at Jupiter for 20 months. The combination of Juno's up-close view and the length of the mission itself mean that we will achieve better scientific precision than ever before, and will be able to track the churning motion of Jupiter's atmosphere over a long period of time. We may finally be able to understand what mechanism is responsible for Jupiter's incredible wind speeds, which can exceed 330 MPH, as well as the expected lifetime of the Great Red Spot, a hurricane-like feature on Jupiter that has been raging for over 150 years.
How strong is Jupiter's magnetic field?
The Earth has an iron core, covered by a layer of liquid iron. The motion of this liquid metal layer generates a magnetic field that encircles the Earth and protects it from the stream of charged particles, ions, that comes from the Sun. As the ions are funneled by the magnetic field lines, they glow, causing the aurorae. Jupiter also has aurorae, but they are hundreds of times more energetic than those found on Earth. By studying the aurorae, scientists can better estimate the strength of Jupiter's magnetic field. Furthermore, it appears that the glowing ions that cause the aurorae are not only coming from the Sun, but also from volcanic eruptions from Jupiter's closest moon, Io. Exactly how these ions get to Jupiter is still a mystery - one Juno hopes to solve.
The bright aurorae imply that the magnetic field of Jupiter is much stronger than Earth's magnetic field. Juno will help determine whether Jupiter has a rocky core like Earth, but the magnetic field might not require a core; the atmospheric pressure deep under the surface of Jupiter is so high that it can crush hydrogen into a liquid. This liquid hydrogen layer can conduct electricity, and will generate a magnetic field as Jupiter spins. In addition, the spinning magnetic field will also generate electricity at the poles - 10 million volts of electricity, to be exact. That corresponds to about 100 million amps being generated in Jupiter's magnetosphere. For reference the Earth's magnetic field can generate up to 1 million amps. Juno will not only take images of these aurorae in action, but will fly close enough to take physical samples of the charged particles present near the poles. The instrument responsible for sampling these particles is called the Jupiter Energetic Particle Detector Instrument, or JEDI (astronomers do love their acronyms). The strength of this radiation is one of the many challenges facing Juno during orbital insertion (see the appropriately dramatic cinematic trailer).
So this 4th of July, before you settle in to enjoy a night of fireworks and hot dogs, glance up into the sky, and reflect on the history being made on a distant planet. While incredible discoveries are being made on a monthly basis these days, do not let that diminish the importance or gravity of each accomplishment. We will be making history again next week, and you can watch it happen - live.
Catch the live-stream of the arrival of Juno at Jupiter here. Orbital insertion is scheduled for 10:30 PM (EST) on Monday, July 4th. Coverage of the event will be ongoing throughout the day. Follow the Juno mission at @NASAJuno.
Laura Haney (@LauraVican)
Co-founder and COO, Signal to Noise Magazine
PhD, Physics and Astronomy
 Dunbar, Brian. "Juno Overview." NASA. N.p., 5 May 2016. [https://www.nasa.gov/mission_pages/juno/overview/index.html]
 Masset, F. S., and J. C. B. Papaloizou. "Runaway Migration and the Formation of Hot Jupiters." The Astrophysical Journal 588 (2003): 494-508.