Nasa's Juno spacecraft loops into Jupiter orbit

Nasa's Juno spacecraft capped a five-year journey to Jupiter late Monday with a do-or-die engine burn to sling itself into orbit, setting the stage for a 20-month dance around the biggest planet in the solar system to learn how and where it formed.

"We're there. We're in orbit. We conquered Jupiter," lead mission scientist Scott Bolton, with the Southwest Research Institute in San Antonio, told reporters on Tuesday. "Now the fun begins."

Juno will spend the next three months getting into position to begin studying what lies beneath Jupiter's thick clouds and mapping the planet's gargantuan magnetic fields.

Flying in egg-shaped orbits, each one lasting 14 days, Juno also will look for evidence that Jupiter has a dense inner core and measure how much water is in the atmosphere, a key yardstick for figuring out how far away from the sun the gas giant formed. Jupiter's origins, in turn, affected the development and position of the rest of the planets, including Earth and its fortuitous location conducive to the evolution of life.

"The question I've had my whole life that I'm hoping we get an answer to is 'How'd we get here?' That's really pretty fundamental to me," Bolton said.

Jupiter orbits five times farther from the sun than Earth, but it may have started out elsewhere and migrated, jostling its smaller sibling planets as it moved. Jupiter's immense gravity also diverts many asteroids and comets from potentially catastrophic collisions with Earth and the rest of the inner solar system.

Launched from Florida nearly five years ago, Juno needed to be precisely positioned, ignite its main engine at exactly the right time and keep it firing for 35 minutes to become only the second spacecraft to orbit Jupiter. If anything had gone even slightly awry, Juno would have sailed helplessly past Jupiter, unable to complete a $1-billion mission.

Only one other spacecraft, Galileo, has ever circled Jupiter, which is itself orbited by 67 known moons. Bolton said Juno is likely to discover even more.