Juno’s epic journey to probe Jupiter’s secrets

Jupiter, king of the Roman gods, wanted to hide a few indiscretions. So, the story goes, he shrouded himself in cloudy veils “to hide his mischief” from his wife Juno (who also happened to be his sister).

But, to Jupiter’s dismay, it didn’t work – Juno was able to see through those veils to her husband/brother’s true nature.

Like its namesake, NASA’s Juno spacecraft, due to arrive at Jupiter on 4 July, aims to peer through the planet’s clouds and uncover the secrets within – along with how the solar system formed.

The fifth planet from the sun, Jupiter is the most massive non-sun object in the solar system. Take out the sun, and you can fit everything else in the solar system into Jupiter – all the planets, dwarf planets, comets, asteroids and dust.

It can even be thought of as its own mini solar system. Italian astronomer Galileo Galilei discovered four of the gas giant’s moons in the 17th century. We now know of 64 confirmed moons and a whole host of comets captured by its intense gravitational pull.

In fact, Jupiter is made of the same stuff as the sun – mostly hydrogen and helium – and if it was 80 times more massive, it would have ignited and started blazing as a star.

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Jupiter and its familiar Great Red Spot, a storm that’s raged for more than 400 years.

Some planetary scientists think Jupiter and other gas giants, such as Saturn, helped “shepherd” the rocky planets into the inner solar system – and maybe knocked a few into the sun.

But despite its immense size, we know very little about Jupiter. What’s deep inside? When did it form – and how has it moved around the solar system? What drives its rich tapestry of white, brown and orange flurries that course around the planet every nine hours? 

Enter: Juno

While Jupiter has been visited by eight NASA missions – from Pioneer 10, which flew past in December 1973, to New Horizons, which hung around for five months in 2007 – Juno will circle the gas giant for around a year and a half, diligently mapping its entire surface over more than 30 orbits. 

Its suite of 29 sensors will feed nine instruments:

A gravity/radio science system (Gravity Science)
A six-wavelength microwave radiometer for atmospheric sounding and composition (MWR)
A vector magnetometer (MAG)
Plasma and energetic particle detectors (JADE and JEDI)
A radio/plasma wave experiment (Waves)
An ultraviolet imager/spectrometer (UVS)
An infrared imager/spectrometer (JIRAM)
Colour JunoCam.

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The planet’s gravitational field will give clues to its internal composition. Does it have a rocky centre? And if so, how big is it?

Radio waves will penetrate the optically opaque ammonia clouds to determine what lies beneath. (Radio telescopes on Earth which do something similar but on a planet-wide scale will be observing Jupiter at the same time.)

The particles, electric field and plasma that shrouds the planet will determine how its magnetic field is connected to its atmosphere to produce incredible northern and southern lights. These auroras will be probed by ultraviolet and infrared cameras to ascertain the chemical reactions going on in the gases.

And of course, a high-resolution camera will send back the most detailed views of Jupiter yet.

The journey counts too

When Juno slots itself into Jupiter orbit on 4 July, it marks the end of an extraordinary 2.8-billion-kilometre journey through the solar system.

The spacecraft launched from Cape Canaveral Air Force Station in Florida, US, by an Atlas V551 rocket in August 2011.

When in space, the 3.5 x 3.5-metre spacecraft unfurled its nine-metre solar panels like petals to begin its self-powered journey to Jupiter, which orbits the sun around four times farther out than Earth.

Instead of heading in the gas giant’s direction, though, it took off towards the sun, looping behind it, and heading back through the inner solar system towards Earth.

Why? For a speed boost. 

Two years after launch, Juno slipped behind Earth in a “gravitational slingshot” manoeuvre. As we hurled around the sun at around 30 kilometres per second, Juno was dragged along. At just the right moment it flung out of Earth’s gravitational grasp but retained its momentum, boosting its speed by nearly four kilometres per second.

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How a gravity assist works. Earth’s point of view is represented on the left, and the sun’s point of view is on the right.

Just after launch as it headed away from Earth, Juno’s trio of solar panels generated about 14 kilowatts of power. But Jupiter, which lies five times further from the sun as Earth, only receives around 4% of Earth’s sunshine. 

So Juno’s solar panels will only pump out a measly 486 watts of power when it reaches its destination. Its engines, though, will spring to life and thrusters will control orientation and trajectory. 

Two lithium-ion batteries will keep it going when Juno passes on the dark side of Jupiter (and won’t see any sunlight at all).

Planet scanner

Juno won’t circle Jupiter in a regular, circular orbit. The planet’s immense magnetic field, probably due to a massive ocean of highly pressurised electricity-conducting “metallic” hydrogen deep inside, clings to electrons and ions to form an intense radiation belt.

This radiation balloons up to three million kilometres towards the sun and tapers into a tail extending more than a billion kilometres behind – as far as Saturn’s orbit. 

If Juno spent much time in the worst of the radiation, it would spell the end of the spacecraft’s instruments.

So Juno will take a highly elliptical route – skirting as close as 4,300 kilometres from Jupiter’s cloud tops, then flying past the moon Callisto’s orbit of 1.9 million kilometres, minimising the amount of time spent in the radiation danger zone. 

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Juno’s highly elliptical orbits will keep it out of the worst of Jupiter’s radiation.

The spacecraft’s centimetre-thick titanium walls will also help shield its instruments. And despite each orbit’s huge distance, it will need only 14 days to complete a circuit.

Like the Pioneer spacecraft before it, Juno will spin as it makes its loops around Jupiter. On the two-hour trip from pole to pole, the spacecraft will rotate around 400 times. Not only does this stabilise the spacecraft and make it easier to control, it allows the sensors equal time to sweep across their field of view and take measurements of the planet below. 

One-way ticket

Unfortunately, all good things must come to an end. Juno, once it’s completed its mapping task, will plunge into its two-year companion to be crushed by Jupiter’s immense pressure.

The gas giant will have a bit of a wait until its next visitor, though. The European Space Agency’s Jupiter Icy moons Explorer (JUICE) mission is due to launch in 2022 and arrive in 2030. 

But it will spend around three years observing the planet and its three largest moons Ganymede, Callisto and Europa.

And so Jupiter’s cloudy veil will be pulled back further.

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