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.