'Hope and crossed fingers' as Juno enters Jupiter's orbit

Juno above Jupiter, as illustrated by NASA (NASA)
Juno above Jupiter, as illustrated by NASA (NASA)

NASA is only hours away from putting its Juno spacecraft into orbit around Jupiter, following a five-year, 2.8 billion kilometre flight.

At 3:18pm (NZ time) the $1.5 billion probe will begin firing its brakes for 35 minutes, slowing into a polar orbit around the gas giant. In doing so, it'll burn through almost eight tonnes of fuel.

Scientists on Earth won't know if it's worked or not until 48 minutes after its done - that's how long it takes information to get from Jupiter back to Earth.

"There's a lot of hope and crossed fingers involved, but a lot of calculation as well," space commentator Matthew Pavletich told Paul Henry on Tuesday.

"After so many years of sending spacecraft into deep space to the planets, they're getting pretty good at it."

NASA scientists are excited, but nervous.

"Juno is going into the scariest part of the scariest place," engineer Heidi Becker said yesterday. "It's part of Jupiter's radiation environment where there are electrons moving at the speed of light that will go right through the spacecraft and strip the atoms away and fry your brain."

'Hope and crossed fingers' as Juno enters Jupiter's orbit

Mission Control for Juno (Getty)

Its instruments are encased within a titanium vault. Whether it's strong enough to withstand the extreme conditions, she said "it better be".

NASA only gets one shot at putting Juno into orbit. It's currently going 266,000km/h, fast enough to orbit the Earth in nine minutes. Initially it will take 53 days to loop around the planet, but in October NASA plans to move it in closer so it only takes 14 days.

If it's successful, Juno will spent about a year orbiting the largest planet in our solar system, the first to do so since Galileo in the 1990s.

Galileo spent eight years hanging out with Jupiter, completing 35 orbits. Juno's expected lifespan is only a fraction of that because it will be getting in much, much closer - possibly only a couple of thousand kilometres from the planet's cloudtops.

"It's not coming back. After doing something like 20 orbits of Jupiter… the camera's electronics are going to be fried by the extreme radiation," says Mr Pavletich.

"Early in 2018 the plan is to nosedive it into Jupiter's atmosphere, because they don't want it to possibly contaminate one of the moons that might harbour life underneath a thick ice crust."

Those moons are Europa, Ganymede and Callisto, which are believed to have liquid oceans beneath their icy exteriors. NASA knows of 67 moons orbiting Jupiter, four of them bigger than our own, but expects to find more.

"Jupiter is just a fascinating mini solar system in its own right," says Mr Pavletich.

As for the $1.5 billion price tag, Mr Pavletich says it's equivalent to spending one cent of a $600 weekly wage.

"Loose change down the back of America's budget."