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Why NASA’s Juno Jupiter orbiter is a big deal

The gas giant is still a mystery. That’s why NASA is so excited it got a satellite into the planet’s orbit.

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Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

NASA has shot humans to the moon, sent robots to Mars, and even sent a probe beyond the borders of our solar system. All these have been herculean efforts. But this July 4, when the Juno spacecraft successfully entered orbit around Jupiter, Scott Bolton — Juno’s principal investigator — proclaimed, “We just did the hardest thing NASA has ever done.”

A true ranking of the difficultly of NASA feats is impossible. But Bolton’s comments were not hyperbole. Inserting Juno into orbit around Jupiter was a huge, risky $1.1 billion gamble for the space agency — even more treacherous than another notable recent feat, the New Horizons mission to Pluto.

Here’s why.

First off: Why even send a probe to Jupiter?

Even though Jupiter is the largest planet in the solar system, scientists know remarkably little about it.

Basic questions about the planet they’d like to answer include:

  • Does Jupiter have a solid core?
  • How does it generate such extreme levels of radiation?
  • How did Jupiter form and evolve?

Juno is equipped with nine scientific instruments, including sensors that can measure gravity, probe deep into Jupiter’s atmosphere, and test the planet’s magnetic fields, as well as various cameras to capture the planet across a range of the electromagnetic spectrum.

Jupiter is made up of the same basic ingredients as the sun — mainly hydrogen and helium. Scientists are hoping a close-up investigation of its surface can reveal some history of the origin of our solar system. What’s more, the galaxy may be littered with other gas giants we haven’t yet discovered.

“Juno is not only going to help us better understand Jupiter, it’s going to help us better understand the universe around us and our place in it,” Barry Mauk, of the Johns Hopkins University Applied Physics Laboratory, said in a press statement.

Juno’s 1.7-billion-mile journey to Jupiter began in August 2011. The GIF below traces the path Juno took on its journey. You’ll see that about two years into its flight, the orbiter passed back around the Earth for a gravity assist. That propelled Juno to around 165,000 mph, making it the fastest man-made object ever built, and gave it the energy to reach Jupiter within three years.


When Juno approached Jupiter, it hit the brakes and slowed to around 130,000 mph, a record speed for a craft being inserted into orbit.

Jupiter can fry electronics to a cinder

There’s a reason NASA has never sent a probe to fly so close to the surface of Jupiter: The planet has a magnetic field that can fry electronics to a cinder.

Just like Earth has a protective bubble of radiation (a magnetosphere, which is what creates the aurora light shows around the poles), Jupiter has one too. But Jupiter’s is much, much more massive and more powerful. “Its magnetic field extends so far into space that, if it glowed in visible light, Jupiter would appear to be twice the size of the full moon in our night sky,” the New Yorker explains.

Juno will study what inside Jupiter creates such a massive field of magnetism. But doing so puts the probe in danger.

This field can send electrons flying at nearly the speed of light. At that velocity, they can “go right through a spacecraft and strip the atoms apart inside of electronics,” Heidi Becker, Juno's radiation monitoring team lead, said at a July 4 press conference.


For Juno to get close to the surface of Jupiter, it had to sneak in where the magnetism is weakest: near the poles. Once inserted near the poles, Juno can orbit underneath the most intense areas of radiation and protect its sensitive electronics.

Here’s NASA’s visualization of the maneuver. It’s like threading a needle in the deep reaches of space.


In order for this needle threading to work, Juno’s thrusters had to fire for more than 35 minutes to slow it down. It had to be incredibly precise, as Juno was aiming for an opening “that’s tens of kilometers wide,” Rick Nybakken, Juno’s project manager said at the July 4 press conference. “We’re going to hit that within 1.2 seconds, after a journey of 1.7 billion miles.”

Juno will repeat this careful approach for 37 orbits — the length of its mission before it plunges into the surface of the planet. To continually sneak in at the radiation opening at the poles, Juno has been put on a highly eccentric, elliptical orbit around the planet. For the first part of the mission, each revolution around Jupiter will take 53.5 days.


Here’s an illustration of what that looks like from Juno’s perspective.

Even with this sneaky orbit precaution, Juno will be subjected to the radiation equivalent of receiving “a hundred million X-rays in less than a year,” Becker said. To protect the orbiter’s sensitive electronics, NASA engineered a 400-pound radiation vault made of titanium (an appropriate metal considering Jupiter’s mythical namesake).

Jupiter has a poorly understood ring that could have destroyed Juno

Radiation was a known obstacle. But perhaps the biggest potential risk of the Juno mission was less well-known: Jupiter’s rings.

The Galileo mission in the 1990s made some observations of the rings of dust and debris that surround Jupiter, but NASA scientists still don’t know how close to the surface these get to the planet, nor do they know the distribution of material within them (some of the debris may be from meteorites captured in orbit around the planet).

Since Juno was flying at around 130,000 mph, collisions with this debris would be catastrophic. “And if it gets hit, even by a big piece of dust, a small piece of dust, it can do very serious damage,” Bolton said at the July 4 press conference. “That’s one of the big gambles.” Juno has been reinforced to survive some impacts, but it was especially vulnerable in its approach to Jupiter, when it had to expose its relatively fragile main engine.

Jupiter’s rings as seen by the Galileo mission in 1996.
NASA/JPL/Cornell University

When will we begin to see some data from Juno?

For the approach into orbit, NASA had to turn off most of Juno’s scientific instruments. They have to be turned back on. According to Science Juno should be able to start making scientific observations after its first 53-day orbit, which will be around the end of August. Around October 19, Juno will fire up its thrusters again, and will accelerate its 53-day orbit into a 14-day orbit to gather data more quickly.

It takes around 48 minutes to transmit data from Jupiter to Earth. Bit by bit, we’re going to learn the secrets of this gas giant — whether Jupiter likes it or not.

Watch: Nasa’s 3-billion-mile journey to Pluto, explained

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