Juno receives party-like welcome on the most unwelcoming planet in the solar system

As Nasa scientists celebrate the arrival of their lorry-sized probe Juno in orbit around Jupiter, the planet itself seems in party mood. For months it has been putting on spectacular light shows of shimmering crowns around its north pole. They are Jupiter’s version of the Northern Lights, triggered by the impact of high-speed particles with its atmosphere. Like everything else about Jupiter, their scale is truly out of this world. They span an area far greater than that of our entire planet and are sculpted by radiation and magnetic fields of incredible intensity. As such, they serve as a celestial warning sign, cautioning all-comers that they are entering a realm of fearsome forces. With a mass more than double that of all the other planets in our Solar System combined, and a size sufficient to swallow more than 1,000 Earths, Jupiter is second only to the Sun in its influence on the Solar System. Made up chiefly of the same gases as the Sun – hydrogen and helium – it is less a planet like the Earth than a failed star. It even emits its own heat, powered by the brute strength of its gravity. All of this has made Jupiter a prime target for interplanetary missions – but an exceptionally challenging one. Simply getting there is tough enough. Because Jupiter is more than five times as far from the Sun as the Earth is, it took Juno five years to get there on a 2.8 billion-kilometre trajectory, travelling at more than 250,000 kilometres an hour – the highest speed yet reached by a space probe. While most previous missions to the planet have simply raced past, Juno is only the second to attempt to orbit Jupiter and find out what it can. On Monday its main engine slowed the 3.6-tonne probe to the point where it became snagged in the planet’s gravitational field. Now it can settle down to its job of observing the giant planet, and gleaning insights into its many mysteries. For despite centuries of study by Earth-based observatories, the true nature of Jupiter has remained largely unknown, veiled by an atmosphere at least 1,000km thick. Back in the mid-1990s, the only previous orbiting mission, Galileo, fired a probe down towards whatever lurks beneath the clouds. It survived less than an hour before being crushed by colossal atmospheric pressure, Jupiter’s surface still undetected. Juno 's designers hope to do better by being smarter. The probe is scheduled to carry out dozens of swoops towards the planet, during which Nasa analysts will look for any tell-tale changes in speed. These will reveal changes in the pull of gravity from Jupiter’s hidden surface – and thus its physical shape. That, in turn, should cast light on one of the biggest mysteries about Jupiter: how it came to exist in the first place. For years the textbook answer was that the Sun and planets all formed from the collapse of a giant cloud of gas, dust and debris under its own gravity about 4.5 billion years ago. But now astronomers suspect the primordial Solar System was far more violent, with the planets being formed from chunks of ice and rock crashing into each other, their gravity flinging them around like celestial cue-balls. As a result, Jupiter and the other planets may have formed in places quite different from where they are today. Data sent back from Juno about the internal shape and composition of Jupiter is expected to give new insights about where and how it formed. And that will give new clues into the formation of other solar systems around other stars – some of which may harbour life. Gleaning these insights will impose a terrible toll on Juno , however. Each swoop towards Jupiter will expose the probe to the planet's intense radiation field, the equivalent of millions of medical X-rays. Juno 's designers have tried to shield its delicate electronics by surrounding them in centimetre-thick titanium. But in the end, its repeated close encounters will prove its undoing. Some time in 2018, Juno will be sent on a final, suicide mission down into Jupiter's clouds. Until then, the billion-dollar probe is in a race against time to find out what it can about this unfriendly cosmic giant with its shimmering crown. Robert Matthews is visiting professor of science at Aston University, Birmingham.

Juno receives party-like welcome on the most unwelcoming planet in the solar system

After a five-year, 2.8 billion-km journey, Nasa’s probe entered Jupiter’s orbit to set about telling us as much as it can about the gas giant before a final, suicidal, fact-finding descent through 1,000km of inhospitable atmosphere.