Juno Now Orbiting Jupiter

Artist illustration of Juno as it approaches Jupiter. NASA
Artist illustration of Juno as it approaches Jupiter. (NASA)

It took a while — almost five years since launch — but the Juno spacecraft is now at Jupiter and orbiting the giant planet. A 35-minute rocket burn to slow Juno down from its record-breaking 130,000 mph entry speed led to a successful insertion into orbit just minutes before midnight, making it another July 4th NASA spectacular.

During its mission, Juno will orbit the planet 37 times, dipping as low as 2,600 miles above the planet’s upper clouds of ammonia and water.  Primary goals of the mission are to determine whether Jupiter has a solid rocky core or is made up of gases all the way through, to learn about its extraordinarily powerful magnetic forces, and to determine better the components of those upper clouds and what might lie beneath them.

The overriding purpose is to better understand how Jupiter — the first planet formed in our solar system — came to be, and consequently how our solar system was formed. Considering that Jupiter contains more matter than the rest of the solar system planets, moons, asteroids and comets combined, it clearly is the place to look to understand the origins of the solar system.

But another goal, and a significant one at that, is to learn about the big gas giant as a way to learn about similar planets orbiting other stars.  Woven into the Juno mission from the beginning was a requirement that the two years of orbiting be designed and operated with distant solar systems and exo-Jupiters in mind.

I had the opportunity to speak with Juno principal investigator Scott Bolton just the day before Juno’s arrival, and he made clear that providing information and insights that will help understand exo-Jupiters is a high priority, indeed.

Scott Bolton, principal investigator of the Juno mission. (NASA)
Scott Bolton, principal investigator of the Juno mission. (NASA)

“We know that our Jupiter is quite different from many of the other Jupiter-sized planets found, and so there will be differences,” he said.  “But the dynamics we find, the presence of a rocky core or not, the water abundances, the structure of the planet — I think that will all be extremely useful to exoplanet modelers and theorists.”

He also made the intriguing observation that there may well be links between Juno discoveries and the search for Earth-size planets around other stars.

“It may be that finding a system with a Jupiter of a size like ours,  and in  a location {in its solar system} similar to ours, would be a strong signal that there is also an Earth-sized planet in the system.”

Many Worlds carried a column about Juno, Jupiter and exo-Jupiters a few weeks ago, and you can find it here.

But I wanted to also celebrate the spacecraft’s arrival, as well as share more of the conversation with Bolton.

Image of Jupiter and its moons taken during the approach. (NASA)
Image of Jupiter and its moons taken during the approach. (NASA)

First a little more about Juno:  Its body is 11.5 feet tall and 11.5 feet in diameter. But with its three solar panels open, it spans about 66 feet — more than two-thirds of the distance of an NBA basketball court.

The spacecraft will pass as close as 2,900 miles from the upper levels of the Jovian atmosphere during some orbits.  The previous record for spacecraft proximity to Jupiter was 27,000 miles from the atmosphere’s top when Pioneer 11 passed by in 1974.

The only other spacecraft to orbit Jupiter was the Galileo, which arrived in 1995 and was intentionally directed into the planet in 2003.  (NASA is concerned that a spacecraft potentially contaminated with Earthly life could hit Europa or one of the other moons considered possibly habitable, and so the agency ends missions with these death plunges.) While Galileo traveled out from Jupiter to some of its moons, Juno will be all Jupiter all the time.

Unlike Galileo, Juno’s orbit will take it over the planet’s poles, providing a first close look at those volatile regions — where radiation and magnetic forces are especially strong.  In terms of radiation, for instance, the background level on Earth is .4 rads.  During its mission, Juno will be exposed to 20 million rads.

Repeated close passes over the poles and the cloud tops are expected to provide answers to unresolved questions regarding its core – is it solid or gaseous – and the abundance of water at different levels.  The water abundances are expected to provide answers to when in astronomical time and where in the solar system Jupiter was formed.

That’s the upside of Juno’s close encounters.  The downside is that the extreme radiation present at the poles and elsewhere around Jupiter will stress the spacecraft enormously.  To minimize the radiation risk, a 400-pound titanium vault at the heart of the spacecraft protects the computers and most essential components of the instruments onboard.

Because of the unprecedented and extreme conditions Juno will face close in to Jupiter, it is expected to have an orbiting lifetime of 20 months, significantly less than Galileo.


ASA's Hubble Space Telescope captured images of Jupiter's auroras on the poles of the gas giant. The observations were supported by measurements taken by Juno. (NASA)
Jupiter’s enormous polar auroras — created by its intense magnetic fields — as captured by NASA’s Hubble Space Telescope.  While in transit, Juno collected data that supported and deepened knowledge of what was occurring when the images were collected. (NASA)

The potential usefulness for exoplanet research of the data from a mission within our solar system data is not unique to Juno, although the role of exo-Jupiters of of particular importance.  Shawn Domogal-Goldman, a research space scientist at the Goddard Space Flight Center who is active in planning exoplanet exploration as well as being part of it, said in an email that “we should now view every solar system mission as a close-up encounter with an exoplanet. We can make measurements on Jupiter that we will likely never (at least in our lifetimes or that of our children or grandchildren) make for exoplanets.”

He is especially interested in what Juno learns about Jupiter’s center.

“We have lots of theories on how planets form, and are beginning to gain an understanding of how planets of different sizes form and then migrate to produce the systems we see today,” he wrote. “But a lot of those theories hinge upon core formation for gas giants. Detecting a core of Jupiter would provide major support of those theories, and do so in a way no exoplanet mission ever could.

Many of the Jupiters discovered thus far are quite close to their suns, and so are very different from our Jupiter.  Some are also much larger — nearing the mass of a star.  So few, if any, Jupiters particularly like ours have been found.

But that’s not because they’re not there, Bolton said.  They’re just harder to find because of the limits of our current methods and technology for observing.  But like our Jupiter, many doubtless loom large in the formation of their solar systems.

Bolton likes to talk about Jupiter, and its role as our solar system’s first planet, in terms of a recipe.  While the sun is almost all hydrogen and helium, Jupiter has carbon, oxygen, sulfur and other “heavy” elements (that is to say, heavier than hydrogen or helium)  at levels much higher than the sun.  How did that happen?

“We have to constrain the recipe by understanding what elements are there below the top clouds, and how abundant they are.  We know some of this, but there’s a long way to go.”

“This is an essential first step, but then we still have to form the rest of the solar system from there… Keep in mind that the process that led to life on Earth really begins at Jupiter.”

By piecing together the story of how Jupiters form, he said, scientists will inevitably gain essential knowledge about how solar systems form, our own and those billions more very far away.




Juno, Jupiter and Exo-Jupiters

Artist rendering of Juno spacecraft in orbit around Jupiter. NASA
Artist rendering of Juno spacecraft in orbit around Jupiter. NASA

The last NASA mission to orbit Jupiter, the Galileo, was designed, flown and its data analyzed as if it was circling the only Jupiter in the sky.

This is hardly surprising since the spacecraft launched in 1989, before the exoplanet era had arrived.  Ironically, Galileo entered its Jupiter orbit in late 1995,  just a few months after the first exoplanet was detected.

That planet, 51 Pegasi b, was a Jupiter-sized planet shockingly close to its host star, and its location and white-hot temperatures turned upside down many then-current theories about gas giant planets and their roles in the formation of solar system.  Scientists are still struggling to make sense of what 51 Pegasi b, and the 250 or so Jupiters found after it, are telling us.

So the Juno mission, which is scheduled to begin orbiting Jupiter on July 4, will arrive at a planet understood quite differently than when Galileo made its appearance.  Juno was built first and foremost to unravel some of the enduring mysteries of the planet:  When and where was it formed?  Does it have rocky core?  Is there water deep in the atmosphere?

But the spacecraft and its instruments will do their unraveling within our current, very different galactic context, where exoplanet scientists will be waiting for results with nearly as much eagerness and anticipation as solar system and planetary scientists.  And the findings from Juno may well have as much impact on the subsequent study of the many, many Jupiter-like planets known to exist in other solar systems as it does on the study of our solar system and its formation history.

Scott Bolton, principal investigator for Juno, recently told a NASA gathering that one of the primarily goals of Juno is to learn, through exploration of Jupiter, “the recipe” for the formation of our planets, our solar system, and those solar systems and planets well beyond Earth.

This is possible because Jupiter was the first planet formed after our sun, which is made almost entirely of hydrogen and helium.  Jupiter is also largely made up of those two elements, but it does have some additional heavy elements that somehow got there — carbon, nitrogen, phosphorus, important gases.

“We don’t know exactly how that happened, but we know that it’s really important,” Bolton said.  “That’s because the stuff that Jupiter has more of is what we’re all made of made of, and is what Earth is made out of, and what life comes from.  So really learning about that history is critical if we’re going to figure out how we got here…and how we find other systems like the Earth elsewhere.”

Artist rendering of the formation of a solar system in its early stages. (NASA/JPL-Caltech
Artist rendering of the formation of a solar system in its early stages. (NASA/JPL-Caltech

Jonathan Lunine, Director of the Cornell Center for Astrophysics and Planetary Sciences,at Cornell University, is a Juno team member with a background in planet and exoplanet formation.  He said that while Juno was not designed “with exoplanets in mind, per se,” its findings will have inherent and significant relevance for exo-Jupiters elsewhere.

“Juno was designed to tell us of the origin and evolution of Jupiter,” he said. “But, clearly, one should think of our solar system’s giant planets as the touchstone to be used, with exoplanet observations, to understand how planetary systems form in general.”

One of those scientists who will be looking to Juno for insight into Jupiter-like exoplanets is Hannah Wakeford, a fellow at Goddard Space Flight Center who studies the atmospheres of hot Jupiters like 51 Pegasi b.

“Juno may well answer some of the outstanding big questions about Jupiter, and that new information will be enormously helpful in studying other gas giants similar to Jupiter,” she said.  “What Juno finds certainly won’t apply directly to all Jupiter-mass planets,  but it will give real world data that can go into our models and very much help limit the possible explanations.”

One of the most important goals of the Juno mission is to determine whether Jupiter has a rocky/icy core or is gaseous, or mostly gaseous, all the way through.  This issue has been hotly debated for years, and Juno should provide data to settle the issue.

Hannah Wakeford, a research fellow at Goddard specializing in the atmospheres of hot Jupiters.
Hannah Wakeford, a research fellow at Goddard specializing in the atmospheres of hot Jupiters.

Then there is the effort to measure how much water and oxygen Jupiter has in its lower atmosphere, below the thick top layer of clouds.   The spacecraft has instruments that can tease out some answers, and they, too, will become central to future Jupiter science.

The same issues loom large when it comes to extra solar gas giants. Understanding water abundances and the presence (or absence) of a solid core is considered essential to characterizing exo-Jupiters, and to learning about their histories.

As Wakeford explained it, the question of a core is key to understanding how and where a Jupiter-sized planet formed.  If there is a rocky core, then the planet most likely began as a small planetesimal and was in the right place in the protoplanetary disk to pull in and keep massive amounts of gas and dust.

But if there is no solid or rocky material detected — by measuring the gravitational and magnetic fields of the interior — then Jupiter would be more like a failed star that formed through a gravitational collapse that didn’t have the mass to become a star.

“That information {about whether there is a core or not} would give us one data point for understanding other Jupiter-sized planets, and there are definitely problems with that.  But that data point would be one more than we have now,” Wakeford said.

Juno should answer the long-debated question of whether or not Jupiter has a rocky, solid core. If it does, the implications for understanding the planet -- and many exo-Jupiters of similar sizes -- are great.
Juno should answer the long-debated question of whether or not Jupiter has a rocky, solid core. If it does, the implications for understanding the planet — and many exo-Jupiters of similar sizes — are great. (NASA)

The issue of water abundance is also key.  Juno has a microwave instrument that can see deep inside the planet, piercing through the many layers of clouds.  The amount of water present (likely in crystal or vapor form) provides an important clue about where the planet was formed in its disk —  inside the ice line of the solar system, or outside.

Morever, the abundance of water has implications for Jupiter and exo-Jupiter migrations. Wakeford said that if Jupiter turns out to have significantly more, or significantly less, water than what is predicted for a planet that formed at its current location in the solar system, that would suggest the planet migrated to that orbit at some point in its history.  And if that method succeeds in nailing whether or not Jupiter has migrated significantly during the eons, then it could be used for exo-Jupiters, too.

For Lunine, the issue of water abundance is particularly compelling.   He said that in the years ahead, he plans to use some of his dedicated time on the James Webb Space Telescope (to be launched in 2018) to observe and analyze exo-Jupiter atmospheres from the perspective of whether they, like our Jupiter, have increased amounts of oxygen (from water) and carbon compounds relative to their host stars.  The information has the potential to explain a lot about the planet’s formation and history.  He said others in the field will surely be focused on this potentially revealing exo-Jupiter enrichment as well.

Although many of the early exoplanets detected were Jupiter size and larger, with a fair number of those “hot” Jupiters orbiting surprisingly close to their host stars, that turned out to be an artifact of the observing techniques.  These large bodies orbiting close to their stars simply were the easiest to detect.

Scott Bolton, principal investigator of the Juno mission. (NASA)
Scott Bolton, principal investigator of the Juno mission. (NASA)

But the Kepler Space Telescope and other planet-finding instruments have identified more than 3,000 planets smaller than the Jupiters, and the expectation is that future discovery methods will show that Jupiter-size exoplanets are relatively rare and that planets smaller than any detected so far are most common.  Nonetheless, Jupiters will remain central to exoplanet research because, as Juno principal investigator Bolton said, they contain that astrophysical and chemical recipe for all that came later.

Adding to the interest (and challenge), Jupiter is, as Bolton described it, a “planet on steroids.”  It 300 times more massive than Earth, and at the planet’s center the temperature is several times hotter than the surface of the sun.  The pressure is tens of millions times the air pressure of Earth.  In this environment, scientists have concluded that the abundant hydrogen is in a liquid metallic form.

In addition to its focus on the formation and evolution of the planet (through the search for a core and measure that water (oxygen), the Juno mission will also study Jupiter’s magnetic fields, which is 20,000 times more powerful than Earth’s and by far the strongest in the solar system.  The extreme magnetism is a function, scientists believe, of the presence of that metallic hydrogen and the speedy rotation of the planet, which is day is but 10 hours long.

Outer jets and belts composed largely of ammonia and hydrogen sulfide gas can block study of the inner atmosphere. Winds blow the cloud regions in different directions. (NASA)
Outer jets and belts composed largely of ammonia, ammonium hydrosulfide and water clouds can block study of the inner atmosphere. Winds blow the cloud regions in different directions. (NASA)

The Juno mission will include 37 passes closer to the Jupiter than any previous spacecraft — 2,600 miles above the upper clouds.  Those layers are made up of largely ammonia and hydrogen sulfide, with the H2O clouds much deeper in the atmosphere.  Working out ways to see through or around those upper clouds into the far more scientifically important atmospheres is another high priority task for Juno.

Opaque clouds and hazes are common to exoplanets, too, and especially the larger ones.  Some hot Jupiters, for instance, have clouds of iron oxides surrounding them, blocking efforts to look into their far more important atmospheres.  Developing techniques to pierce through those outer layers of hazes and clouds will be another potential Juno boon to exoplanet study.