Juno’s risky rendezvous with Jupiter

Scientists are biting their nails as NASA’s $US1.1 billion Juno spacecraft prepares to slot into orbit around Jupiter. 

The most anxious moments, says Scott Bolton, the mission’s principal investigator and a researcher at the Southwest Research Institute in San Antonio, Texas, will be a 35-minute main engine burn scheduled to begin shortly after 8 pm PDT on 4 July (3am UTC on 5 July).

It’s a critical manoeuvre because, at the time, the spacecraft will be hurtling by Jupiter at a speed of about 250,000 kilometres per hour relative to Earth, making it the fastest-moving spacecraft humans have ever created. 

“The rocket has to burn at the right thrust at the right time in the right direction for the right length of time,” Bolton says. “If that doesn’t happen, we fly past Jupiter.

“The whole thing’s riding on this burn.”

Once the spacecraft is safely in orbit, the drama continues as it begins a series of highly elongated orbits that, at their closest points, come within 5,000 kilometres of Jupiter’s cloud tops, and at their farthest loop out millions of kilometres.  

These close flybys allow scientists to bring to bear a suite of instruments that will not only provide spectacular views of Jupiter’s multihued clouds but peer deep into its interior. 

One instrument is a microwave radiometer that uses several frequencies of microwave emissions to scan beneath the clouds for water. “Each channel sees a different depth into the atmosphere,” says Steve Levin of NASA’s Jet Propulsion Laboratory in Pasadena, California. By scanning from multiple angles, he says, the instrument can do something akin to a CT scan, producing a 3-D picture of Jupiter’s atmospheric structures. 

“We will see the roots of the Great Red Spot,” Bolton says, referring to a giant storm that has persisted since it was first spotted by Galileo 400 years ago.

Without shielding, instruments would be exposed to 20 million Rads, ‘which is like a human having 100 million dental X-rays in a little over a year’.

Other instruments will peer even deeper, making careful maps of Jupiter’s magnetic and gravity fields to determine the planet’s structure all the way to its centre. One of the big questions, says Jack Connerney, an astrophysicist at NASA’s Goddard Space Flight Centre in Greenbelt Maryland, is whether Jupiter has a rocky core, or if it is composed simply of highly compressed hydrogen and helium gases all the way to its centre.

Passing this close to Jupiter doesn’t come without danger. Not only is there the risk that the braking manoeuvre won’t come off as planned, but Jupiter’s extreme magnetic field produces enough radiation to ruin conventional spacecraft instruments. 

When electrons hit an instrument, says Heidi Becker, a Jet Propulsion Laboratory physicist, they create “a spray of radiation, like bullets”. 

Without shielding, she says, instruments would be exposed to 20 million Rads, “which is like a human having 100 million dental X-rays in a little over a year”.

For this reason, she says, the spacecraft’s vital components are protected in what she calls a suit of armour: “a titanium vault one-half inch [1.3 centimetres] thick. The vault weighs almost 400 pounds [180 kilograms] and brings down the dose by a factor of about 800.”

Bolton compares it to an armoured tank. “We’re shielded and ready,” he says. 

But, he admits, there’s still risk. “Jupiter is a planet on steroids,” he says. “Everything about it is extreme … and we’re going right up next to it.”

Further reading:
Juno’s epic journey to probe Jupiter’s secrets
Unravelling Jupiter’s giant storms
Peeling back Jupiter’s skin to find ammonia within