NASA is celebrating Independence Day this year by putting a spacecraft into orbit around Jupiter. The space agency’s Juno mission is slated to arrive at the massive planet on the night of July 4th, after having traveled across more than 1.7 billion miles of space over the past five years. Once Juno arrives, the probe’s main engine will fire, slowing the spacecraft down and placing it into orbit around Jupiter. It’s an important event for the mission, especially since NASA has only one shot at getting it right. If Juno flies past Jupiter, the mission will be blown.
Juno will eventually fly closer to the gas giant than any other spacecraft before
If all goes as planned, Juno will eventually fly closer to the gas giant than any other spacecraft before, allowing NASA to figure out what’s going on underneath all of Jupiter’s thick clouds. "We have sent spacecraft to the Jovian system before, but they all kept their distance from Jupiter," Steve Levin, a Juno project scientist at NASA’s Jet Propulsion Laboratory, told The Verge. "[Juno’s] orbit actually enables a lot. It’s a key part of doing the science we want to do."
That science involves studying the amount of water in Jupiter’s atmosphere, as well as mapping the planet’s huge magnetic field. Juno will also study Jupiter’s gravity to figure out if a dense core lurks deep underneath. All of that information will help NASA deconstruct the origins and history of the solar system’s largest planet. Scientists have a lot of theories about how Jupiter formed and how it got into its current orbit, but Juno’s data will help researchers strengthen our understanding of where Jupiter came from. And that will ultimately tell us how the rest of the planets formed—including our own.
An artist's rendering of the Juno spacecraft firing its main engine. (NASA)
In order for NASA to get these answers, Juno has to get into the right orbit around Jupiter first. And it’s not going to be a simple, circular path like the Moon takes around Earth. Juno’s orbit will be highly elliptical, taking the spacecraft super close to Jupiter’s poles for just a few hours and then way out into space for two weeks at a time. It may seem like an inefficient way to orbit the planet, but it’s crucial for keeping the spacecraft alive.
Why the crazy orbit?
The area around Jupiter isn’t a very inviting place. The planet is surrounded by a large magnetic field, thought to be generated by a swirling ocean of liquid metallic hydrogen located deep underneath the planet’s surface. The motion of this conductive material produces an electric current, ultimately creating a magnetic field known as the magnetosphere that’s 20,000 times stronger than the magnetic field of Earth. This magnetosphere extends out millions of miles around Jupiter, making it the "the biggest entity in the solar system," according to Scott Bolton, Juno’s principal investigator.
The inner radiation belts of Jupiter. (NASA)
This magnetic field is always interacting with solar wind — charged particles constantly spewing from the Sun that carve out the shape of the magnetosphere. These charged particles get trapped inside the field, creating huge, powerful belts of radiation that surround Jupiter. Those belts make orbiting the planet difficult, because they can "fry the electronics on the spacecraft," according to Levin. Juno has been equipped with special shielding and armor to reduce exposure, but the spacecraft will still get a healthy dose of radiation. "Over the life of the mission, Juno will be exposed to the equivalent of over 100 million dental X-rays," Rick Nybakken, Juno's project manager from NASA's Jet Propulsion Laboratory, said in a statement. "But we are ready. We designed an orbit around Jupiter that minimizes exposure to Jupiter's harsh radiation environment."
Juno’s unusual orbit will basically help the spacecraft avoid getting fried
Juno’s unusual orbit will basically help the spacecraft avoid getting fried. The radiation belts are shaped a bit like giant donuts stemming from Jupiter’s equator, but they’re not as strong at the planet’s poles. So each time Juno approaches the planet during orbit, the spacecraft will come in over the north pole and then dive in really close to Jupiter at the equator—going in between the radiation belts and the planet. Juno will then swoop away from Jupiter over the south pole and continue on out into space for a while before looping back toward the planet again. This close approach to Jupiter is known as a Perijove pass.
An artist's rendering of Juno's Perijove pass. (NASA)
Juno will spend most of its orbiting time far away from Jupiter, and will only collect data for about six hours during each Perijove pass. It’s not a super long time, but this path brings Juno within about 2,600 miles of Jupiter. That’s pretty close, considering the planet itself is more than 86,000 miles in diameter.
Getting into orbit
Juno will be traveling at more than 40 miles per second when it reaches Jupiter on the Fourth. At that point, the spacecraft’s main engine will burn for about 35 minutes — long enough to slow the vehicle down by 1,200 miles per hour and allow it to be captured by Jupiter’s gravitational pull. This will put Juno into an initial orbit that lasts 53 days.
NASA is taking extra special precautions to ensure Juno gets into that orbit, too. During the engine burn, all of the spacecraft’s scientific instruments will be turned off, and anything that’s not needed to make the main engine fire will be powered down. That way nothing onboard the spacecraft will interfere with the burn. "Because this is a critical event, it has to work and it has to work at the right time," said Levin. "If we fire the main engine and something goes wrong, and we said, ‘Okay we’ll try again in a day,’ it’s too late. We’ve blown past Jupiter, and we would lose the whole mission."
The spacecraft will be all on its own out there
Plus, the spacecraft will be all on its own out there. It takes about an hour to get a radio signal from Jupiter to Earth, and Juno has only about 20 to 30 minutes to insert itself into orbit. The spacecraft has backup systems to help it restart if something goes wrong, but if there is a failure, NASA won’t have enough time to send any corrective signals from Earth. "So [orbital insertion] is kind of a nail biter for us, just because it’s so important to us, and the spacecraft has to do it all on its own," said Levin.
NASA will receive confirmation that Juno’s engine has started burning at 11:18PM ET on July 4th, and then know if the burn has been successful just before midnight. Juno won’t do much science during its initial 53-day orbit, but at the end of its first loop on August 27th, Juno will make its close approach to Jupiter and get its first good look at the planet with all of its instruments on. That’s also when the first close-up pictures will be taken from the probe’s onboard camera, JunoCam. After that, the spacecraft will make another 53-day orbit and conduct another Perijove pass starting on October 19th. That same day the spacecraft’s engine will burn again, helping to place the probe into a shorter two-week orbit. Overall, Juno will complete 32 science orbits around Jupiter before its mission is over.
The paths of Juno's two-week orbits. (NASA)
Deconstructing Jupiter’s history
Juno is in a constant state of spin, rotating three times per minute. That way, the spacecraft’s nine instruments will point at Jupiter 400 times as Juno flies from pole to pole, peering inside the planet and learning more about its early history.
A leading theory is that Jupiter formed from huge chunks of ice and other debris super far away from where the planet is now at the fringes of the solar system. These chunks clumped together to become a dense core that had enough gravity to pull in gases and other heavier elements. Then, the planet eventually migrated toward the Sun to settle where it is now. Another theory says that Jupiter formed exactly where it is now from the gas and dust leftover from the formation of the Sun. These gases may have clumped together, becoming dense enough to pull in other gases and materials to form into a planet. Both origin stories are plausible, but Juno will help NASA determine which theory makes the most sense.
A way of doing that is studying how much water is in Jupiter’s atmosphere. Microwave radiometers on board will be able to look for the amount of H20 inside the planet’s hydrogen and helium clouds, telling researchers a lot about how and where Jupiter formed. "If it formed really far from the Sun where it’s very cold," said Levin, "then the other materials that were carried in with the water will have carried in more stuff. If it formed closer to the Sun, where the water is not as cold, [then] other materials did not stick to it as well." And if Jupiter didn’t form from pieces of ice at all, then Juno won’t find as much water as predicted.
A big point of study is Jupiter’s potential core
Another big point of study is Jupiter’s potential core. Juno will be measuring the planet’s magnetic field and gravitational field to determine if a dense core lies underneath all of Jupiter’s circulating gases and liquids. It’s possible that such a core is rocky—or made of materials that make up rock—but it won’t look like any rocks here on Earth. "It’s not going to look like a piece of granite," said Levin. "Remember that it’s going to be at tens of millions of times the pressure here on the Earth. We don’t really know how materials behave at those very high pressures." If a core is present, that supports the idea that the planet formed from large ice chunks and indicates there are a lot of heavier elements inside.
All of these details can help NASA paint a picture of Jupiter’s formation. And once we know how Jupiter fits in our solar system, we’ll know more about how the rest of the planets came to be. "Because Jupiter is so big, and because Jupiter formed first, learning how Jupiter formed is a key piece of information—perhaps the key piece of information—to understand how did the solar system form," said Levin. "How did the planets get here? Where do we come from?"