Scientists are now counting down to the moment that NASA spacecraft Juno arrives at Jupiter late this afternoon (NZT), following a five-year trek across the Solar System.

Juno's mission: To peer through Jupiter's cloud-covered atmosphere and map the interior from a unique vantage point above the poles. Among the lingering questions: How much water exists? Is there a solid core? Why are Jupiter's southern and northern lights the brightest in the solar system?

At around 3.15pm NZT today, the solar-powered spacecraft will fire its main rocket engine to slow itself down from a speed of 250,000 km/h and slip into orbit around Jupiter.

The spacecraft is travelling through a hostile radiation environment and rings of debris and dust, "making for very serious hazards," Juno chief scientist Scott Bolton said during a briefing.


But Juno should be able to withstand the harsh conditions because it's "built like an armored tank," he said.

With its billowy clouds and colourful stripes, Jupiter is an extreme world that likely formed first, shortly after the sun.

Unlocking its history may hold clues to understanding how Earth and the rest of the solar system developed.

"We know much about what it looks like from the outside, but know almost nothing about what's on the inside - only what's been inferred by experts," said Dr Grant Christie, an astronomer at Auckland's Stardome Observatory.

The spacecraft's camera and other instruments were switched off for arrival, so there won't be any pictures at the moment it reaches its destination.

Jupiter was been fundamental to the evolution of our solar system, weighing twice as much as all of the other planets, asteroids and comets within it combined.

"It's probably why we are here," Dr Christie said.

The trek to Jupiter, spanning nearly five years and 2.8 billion km, took Juno on a tour of the inner solar system followed by a swing past Earth that catapulted it beyond the asteroid belt between Mars and Jupiter.

Along the way, Juno became the first spacecraft to cruise such a distance while powered by the sun, beating Europe's comet-chasing Rosetta spacecraft.

A trio of massive solar wings sticks out from Juno like blades from a windmill, generating 500 watts of power to run its nine instruments.

Juno, built by Lockheed Martin, is an armoured spacecraft - its computer and electronics are locked in a titanium vault to shield them from harmful radiation.

Even so, Juno is expected to get blasted with radiation equal to more than 100 million dental X-rays during the mission.

Like Galileo before it, Juno will meet its demise in 2018 when it deliberately dives into Jupiter's atmosphere and disintegrates - a necessary sacrifice to prevent any chance of accidentally crashing into the planet's potentially habitable moons.

- AP, NZ Herald