With planetary invasions being an Independence Day tradition, it's fitting that NASA's Juno spacecraft will enter Jupiter's orbit today, July 4, after a five-year journey to the outer solar system. Juno is the robot equivalent of Indiana Jones: a celestial archaeologist on an expedition to find Jupiter's core—and, hopefully, solve the mystery of the solar system's origin. 

HOW IT ENTERS JUPITER'S ORBIT

Juno's insertion into Jupiter's orbit will begin at 8:18 p.m. PDT on July 4, 2016. This involves a 35-minute "burn," during which time the spacecraft reorients itself and the British-built Leros 1b engine fires up so as to slow Juno's speed. (Juno will be traveling 165,000 mph on arrival.) The burn is crucial, and a failure would mean that the spacecraft zips past Jupiter and into the void. Success, however, means Juno is sufficiently slowed to be captured by Jupiter's gravity and thus enters orbit.

Juno does all of this in autopilot, the final commands having been issued by humans on June 30 and blasted to the spacecraft using NASA's Deep Space Network. During orbital insertion, the spacecraft's science instruments and all unnecessary computer features are disabled. (More features increase the likelihood of computer crashes.) Jupiter's intense radiation environment is notoriously hard on spacecraft computers, and in the event that Juno's computer is zapped by a high-energy particle, it is designed to immediately reset and resume the burn. Scientists, meanwhile, will wait anxiously for Juno to send a message to the Deep Space Network that has been compared to the "emergency broadcast signal" on television and radio. A certain tone will mean the spacecraft has achieved a successful orbital insertion.

Juno's unique design—three colossal solar panels affixed to an 11.5-ft. spacecraft at the center—is dictated by the low levels of sunlight available in the outer solar system. The sun appears 1/25 as bright at Jupiter as at Earth. The spacecraft will remain oriented to collect as many photons as possible from the Sun, and will spin like a top, twice per minute in order to maintain stability and to allow each instrument on Juno's scientific payload to collect data from Jupiter.

HOW JUNO'S INSTRUMENTS WILL STUDY JUPITER

Screengrab from NASA fact sheet. Image credit: NASA

Juno's science instruments—all but one built into the core part of the triple-bladed spacecraft—will each collect certain types of data for scientists to analyze back on Earth. The Gravity Science instrument will map the distribution of Jupiter's interior mass, and thus its gravity. The Magnetometer will meanwhile study Jupiter's magnetic field and its massive and mystifying polar magnetosphere. It will also examine Jupiter's interior dynamics. The Microwave Radiometer [PDF] will the study water content of Jupiter's deep atmosphere so as to reveal the oxygen content of Jupiter. An Ultraviolet Imaging Spectrograph and the Jovian Infrared Auroral Mapper will study Jupiter's atmosphere and auroras, while the JunoCam will take high-resolution photographs of Jupiter and its terrifying and beautiful atmosphere. (It has already returned images.)

But that's not all. The Radio and Plasma Wave Sensor and the Jovian Auroral Distribution Experiment will characterize the nature of the magnetic field and atmosphere, and auroras in particular. Lastly, the Jovian Energetic Particle Detector Instrument—JEDI—also concerns itself with Jupiter's magnetosphere, focusing on the "energy and distribution of ions, particularly hydrogen, helium, oxygen and sulphur, to see if there is any change over time." (What better than a Jedi to study energy that surrounds, penetrates, and binds?)

WHAT DON'T WE KNOW ABOUT JUPITER?

A lot. Thanks to the Galileo mission that ended in 2003, we do know much more about Jupiter and its system of moons than we did before. Among many other things, planetary scientists using Galileo data discovered giant thunderstorms along Jupiter's turbulent equator, complete with lightning strikes one thousand times more powerful than those found on Earth [PDF]. Cloudless "dry" spots of low humidity were discovered by a probe dropped into Jupiter, to its doom. The origin of the planet's rings were also worked out: They were formed from the debris left behind after meteoroid collisions with Jupiter's moons.

And yet for all we've learned, Jupiter remains a giant, terrifying mystery. Enter Juno, named after the wife of Jupiter in Roman mythology. Among the goddess's powers: the ability to see through clouds. And that power is in great demand at Jupiter, the largest known planet in the solar system. No one is completely certain what comprises Jupiter, and its oxygen content remains a mystery. Oxygen percentages might seem like snooze-level science geekery, but the answer to that question, according to NASA, is "the most important missing piece in our understanding of how our solar system formed." Moreover, it remains a mystery whether Jupiter is gas all the way down, or whether there a giant metal Earth-sized planet at its center. (Cybertron?) Just how far down do Jupiter's famous brown and tan cloud bands of clouds go? What's causing Jupiter's spectacular auroras? Juno will help us to answer these questions.

Hubble captures vivid auroras in Jupiter's atmosphere in June 2016. Image credit: Hubblesite.org

FROM DEORBITING TO DISINTEGRATION

Juno will orbit a path along Jupiter's poles [PDF], which NASA describes as "best for mapping and monitoring a planet" and the same type of orbit used by many of Earth's satellites. This means that Juno will be the first spacecraft to get a good look at Jupiter's poles. Each orbit around Jupiter will take 11 days. Because a Jupiter day is only 10 hours long, this means that Juno will have mapped and studied the entire planet in 33 orbits. These orbits will get perilously close to the tops of Jupiter's clouds—a distance of 3100 miles. NASA notes that if Jupiter were a basketball, Juno would be flying one-third of an inch from the ball's surface. 

In October 2017, the spacecraft's mission will end and it will be "deorbited," plunging beneath Jupiter's clouds, where it will ultimately disintegrate. While this might seem like an ignominious end, it is, in fact, a heroic one. By sacrificing itself in the unforgiving hell that is the interior of Jupiter, Juno spares the surrounding moons of the Jovian system the risk of Earthly contamination. Europa, to name one such moon, is thought to harbor life. When the Europa missions get underway, we will know for sure that the life discovered is not of terrestrial origin. 

You can follow the Juno mission on NASA TV or on NASA's Eyes on the Solar System application.