The largest planet in the solar system may finally give up its secrets thanks to a new probe

A few days from now, a US spacecraft carrying a 200kg titanium vault crammed with delicate electronic equipment and fitted with a vast array of solar panels will sweep over the poles of Jupiter before entering into orbit around the giant planet. The craft, named Juno, has travelled almost three billion kilometres since it was launched in 2011. For the next two years, the huge spaceship will skim over Jupiter’s thick atmosphere while trying to avoid the planet’s huge belts of deadly radiation in a bid to uncover the secrets of this mysterious, remote world.

The $1.1bn (£757,837m) mission is designed to peer deep into the thick layers of gases that make up the planet’s atmosphere and return data that could be crucial to understanding the birth not just of Jupiter but of all the planets in our solar system, including Earth.

New radio map of Jupiter reveals ammonia plumes beneath clouds Read more

It will be an extraordinarily delicate task, however. Juno will have to travel extremely close to Jupiter’s powerful magnetic field and radiation belts, and these could easily destroy its instruments. “That is why we have fitted the spacecraft with a titanium vault – to protect the probe’s electronics,” said Juno’s principal investigator, Scott Bolton of the Southwest Research Institute in San Antonio, Texas. “This is a mission with fantastic potential,” he added. “The trouble is that Jupiter is a planet on steroids. Everything about it is massive and that poses real problems for Juno. It will have to weave its way round Jupiter on a very, very delicate path.”

Jupiter is more massive than all the other objects – planets, moons, asteroids and comets – in the solar system, with the exception of the Sun itself. It is also believed to be the first planet created when the solar system formed 4.6 billion years ago. “It appeared first and was then able to scoop up most of the loose material that was sweeping round the Sun then. That is how it got to be so big,” said Bolton.

Understanding Jupiter’s creation is therefore crucial to understanding the formation of the rest of the solar system. However, the planet, which is 317 times bigger than the Earth, is not easy to study. Its magnetic field is the most powerful of any known planet and catches high-energy particles as they fly through space, trapping them in two vast radiation belts that surround it. These are similar to Earth’s Van Allen belts but are many millions of times more intense. Any satellite that strayed into them would find its electronic circuits fried.

“That makes life very difficult for Juno,” said Bolton. “We need to get close to Jupiter to be able to probe its atmosphere – but that means getting close to its radiation belts. We are going to have to fly very carefully or its instruments will be destroyed by radiation.”

This is only one of the many headaches that the spaceship’s designers have had to overcome, however. Juno will also have to carry out its tricky interplanetary manoeuvres on an incredibly tight power budget: the craft’s solar panels will be able to generate a few hundred watts of electricity, sufficient to power a couple of electric light bulbs, and not much more.

In the past, spacecraft sent to study planets far from the Sun have used generators powered by radioactive isotopes. “However, when we were designing and building Juno in the early 21st century, Nasa had temporarily run out of isotopes and so we have had to rely on solar power,” said Bolton.

And given Jupiter’s distance, that is a tricky issue. The planet orbits at 780m km from the Sun. The Earth, by comparison, orbits a mere 150m km from the Sun, which means that sunlight here is 25 times more powerful than it is at Jupiter.

Just after launch, when Juno was still close to the Earth, its giant solar panels – some of the biggest fitted to a planetary probe – could generate around 12 kilowatts of power. Now, as it approaches deep space near Jupiter, that has dropped to just over 400 watts.

Nevertheless, Bolton said he was confident that Juno could operate at such power levels. “The spacecraft is extremely efficient. Its instruments have been designed to run on that kind of power,” he said.

As to those instruments, they have been built to study Jupiter’s atmosphere in unprecedented detail, and to solve several key puzzles about the giant planet.

In 1995 Galileo – the only spacecraft that has orbited Jupiter to date – dropped a 339kg probe into the planet’s atmosphere. As it descended, the probe transmitted details about the various elements it encountered – and found several unexpected features. In particular, it found that most of the elements, such as carbon and nitrogen, in Jupiter’s atmosphere exist at enriched levels with one key exception – oxygen was found to be at surprisingly low levels.

“We had expected to see oxygen, in the form of water which contains oxygen, but saw very little,” said Bolton. “There are two explanations for this. One is that we simply do not understand Jupiter properly at all. The alternative explanation is that we were really, really unlucky and picked the one part of the planet that had no water in it. We’d hit Jupiter’s Sahara desert – through sheer bad luck.”

Juno is designed to discover which of these ideas is correct. “We need to find what the rest of Jupiter has got in it. That is what Juno is about,” added Bolton. “We are going to look all over the place and at much greater depths than we did in 1995.”

This task will be achieved not by dropping a probe but by relying on Juno’s microwave antennae. “Jupiter emits microwaves. It glows with them,” said Bolton.

“And, crucially, microwaves interact with water. That is why your microwave oven heats up wet spaghetti. Similarly, in Jupiter’s atmosphere microwaves are interacting with water and by studying what microwaves are being emitted, we can work out how much water – and oxygen – exists and at what depths.”

Juno is designed to make 37 orbits of Jupiter over its scheduled 20-month lifetime and will skim within 3,000 miles of the top of the planet’s atmosphere while trying to dodge its radiation belts, and all on a power budget of a few hundred watts.

“It is going to be a very risky mission but a worthwhile one,” added Bolton. “Frankly, if Juno survives two years of that kind of treatment before its electronics get burned up we will be very happy.”

Juno is now on the home stretch on its route to Jupiter. “For the rest of the mission, we project that Jupiter’s gravity will dominate as the trajectory-perturbing effects of other celestial bodies are reduced to insignificant roles,” said Rick Nybakken, Juno’s project manager.

On 4 July the spacecraft will finally reach its target. “And that is when the really tricky stuff starts,” added Bolton.