"Welcome to Jupiter."

With those words, just before 4pm, came confirmation that NASA's unmanned Juno probe completed its crucial bid to slip into Jupiter's orbit.

The Juno spacecraft had killed its engines in a bid to slow down enough to be dragged into Jupiter's gravitational pull, with staff in NASA's California-based Jet Propulsion Laboratory watching tensely.

Learning the radio-tracked mission had been successful, the control room erupted into cheers, hoots and applause.


It was the moment of truth after what had been a five-year trek across the solar system to reach the point where the solar-powered craft's main rocket engine slowed down from 250,000 km/h to enter the orbit of the gas giant.

Despite its robust, protective design - Juno was "built like an armoured tank", NASA said - it was never a certainty whether the probe would have made it through the hostile radiation and rings of debris and dust surrounding Jupiter.

Although the powered-down probe didn't capture any images of the moment it entered orbit, NASA has promised close-up views of the planet when Juno skims the cloud tops during the 20-month, $1.5 billion mission, later this year.

Now that it's locked into orbit, Juno will be on track to peer through Jupiter's cloud-socked atmosphere and map the interior from a unique vantage point above the poles.

Among the lingering questions are how much water exists in it, whether there's a solid core, and what it is that makes Jupiter's southern and northern lights the brightest in the solar system.

Named after Jupiter's cloud-piercing wife in Roman mythology, Juno is only the second mission designed to spend time at Jupiter.

Now that the mission has gone to plan, here's what we can expect to learn from the mission the Juno spacecraft spends the next 20 months orbiting the biggest planet in our solar system.

What is Jupiter's core?
Jupiter's composition is more of a mystery than anything else. Scientists currently believe the gaseous planet has a dense central core that may be surrounded by a layer of metallic hydrogen, with another layer of molecular hydrogen on top.

As NASA scientist Glenn Orton put it during a Q&A this morning: "The general question is what is the interior structure of Jupiter like, and that's the main focus (of the mission)."

The big query is whether or not Jupiter's core is solid. If that does prove to be the case, it will have a slight and measurable impact on Juno's orbit.

A constant radio signal being sent to Juno and relayed back to Earth will give scientists a picture of the internal makeup of the planet.

What it can teach us about Earth
Jupiter is surrounded by a thick and tumultuous layer of gas and cloud.

Juno will get so close to Jupiter's inhospitable environment that it will be able to study its atmosphere giving unprecedented insight into its origins as well as other planets in our solar system.

By better understanding Jupiter's chemistry we will understand "what our solar system was like billions of years ago," NASA's Dr. Michelle Thaller said.

The huge gas planet was likely the first planet formed and had a major impact on the formation of other planets.

Like our sun, Jupiter is composed primarily of hydrogen and helium but is also imbued with other heavy elements fundamental to the creation of terrestrial planets.

The spacecraft will hunt for oxygen in the form of water in Jupiter's atmosphere, which may help explain how Earth got its water.

It will also use microwave sounding to look for the abundance of another highly important element: nitrogen, in the form of ammonia.

"The stuff Jupiter has more of is the stuff we're all made of," Juno's principal investigator Dr Scott Bolton told Gizmodo.

"We're looking at the history of the volatiles that formed the Earth by going back and seeing how much Jupiter has."

The mystery of the Great Red Spot
The most prominent feature of the gaseous planet is the Great Red Spot, which is a giant spinning storm in Jupiter's atmosphere larger than Earth that has lasted for centuries.

However in recent years the spot has been mysteriously shrinking.

It was once an oval about 41,036km wide in the late 1880s but when observed in 2014 it had shrunk to its smallest known size of 16,495km across.

Juno will explore how deep into the atmosphere the Great Red Spot extends to give researchers a better understanding of its puzzling nature.

The greatest light show in space

Jupiter's aurora lights are the brightest in the solar system, making Earth's northern and southern lights look dim by comparison. The spectacular phenomena can stretch for tens of thousands of kilometres around the poles of the gaseous planet.

"Auroras that are bigger than the entire planet Earth," NASA's Dr Thaller said.

Juno will use sensors to identify the electrons and types of positive ions that produce the magical light show in order to allow researchers to better understand it.

Jupiter's intense magnetic field
Jupiter has a very dangerous magnetic field that creates a lot of high energy radiation.

The planet's magnetic field is nearly 20,000 times as powerful as our own - although it weakens with distance and at the cloud tops is only 20 times more intense than Earth's surface field.

The Juno mission should give us greater understanding of Jupiter's intense magnetic field.
Earth's magnetic field is created by a combination of a liquid layer of iron, nickel and other metals around its inner solid core where the flow of liquid iron becomes electrically charged.

On Jupiter it's believed the planet's complex magnetic field is produced by electrical currents in the what is likely the rapidly spinning metallic hydrogen interior.