[UPDATE: Tuesday, July 5, 1.05pm]
Juno fired its main engine beginning at 11:18 p.m. EDT/0318 Tuesday GMT, slowing the spacecraft so it could be captured by the planet’s gravity.
[UPDATE: Tuesday, July 5, 11.00am]
Juno, the US$1.1 billion Nasa spacecraft now approaching its orbit of Jupiter will have to dodge debris and extreme radiation on its high-stakes mission to probe the origin of the solar system.
Juno has arrived near the largest planet in our cosmic neighbourhood, five years after the unmanned solar-powered observatory launched from Cape Canaveral, Florida.
"We are barreling down on Jupiter really quick," said Scott Bolton, Juno principal investigator from the Southwest Research Institute in San Antonio, Texas.
As Nasa counted down to Juno’s arrival, he admitted to being nervous about the fate of the spacecraft, which is travelling at a speed of more than 209,200 k/ph towards what he called "the king of the solar system".
5 Facts about Jupiter you need to know about
A key concern is that the spacecraft must survive radiation levels as high as one hundred million X-rays in the course of a year, explained Heidi Becker, senior engineer on radiation effects at Nasa’s Jet Propulsion Laboratory.
Those high-energy electrons, moving at the speed of light, "will go right through a spacecraft and strip the atoms apart inside your electronics and fry your brain if you don’t do anything about it," she said.
"So we did a lot about it," she added, describing the half-inch-thick layer of titanium that protects the electronics in a vault to bring the radiation dose down.
Still, she described the close approach as going "into the scariest part of the scariest place... part of Jupiter's radiation environment where nobody has ever been."
Juno must also avoid debris as it speeds through a belt of dust and meteorites surrounding Jupiter.
"If it gets hit - even by a big piece of dust, even by a small piece of dust - it can do very serious damage," Bolton said.
On approach, the engine doors are open, leaving the nozzle vulnerable, he said.
"If any dust is in our way and hits that nozzle, it will knock a hole right through the coating that protects that nozzle and allows the engine to burn uninterrupted," he told reporters. "That is one of the big gambles."
After that, a tricky, automatic move must go well as the engine fires to slow it down enough to be captured by Jupiter’s orbit.
This “burn,” or orbit insertion, begins at 11:18 pm (0318 GMT) and should last around 35 minutes. The spacecraft must then re-orient itself toward the Sun in order to power the solar arrays.
If it fails to enter orbit, Juno may shoot past Jupiter, bringing a mission 15 years in the making to a swift end some 540 million miles (869 million kilometers) from Earth.
Scientists hope to find out more about how much water Jupiter holds and the makeup of its core in order to figure out how the planet - and others in the neighborhood, including Earth - formed billions of years ago.
Although Juno will not be the first spacecraft to circle Jupiter, Nasa says its orbit will bring it closer than its predecessor, Galileo, which launched in 1989.
Galileo found evidence of subsurface saltwater on Jupiter’s moons Europa, Ganymede and Callisto before making a final plunge into Jupiter in 2003.
Nasa says Juno should be able to get closer than Galileo - this time within 5,000 km above the cloud tops.
"We have done everything humanly possible to make this mission a success," said Nasa's director of planetary science, Jim Green. "But, it is still a cliffhanger for me, too."