A spacecraft has skimmed the clouds of Jupiter in a record-breaking close approach to the giant planet.
Juno activated its whole suite of nine instruments as it began its journey to reach 4,000km above Jupiter's swirling cloud tops at 209,000km/h today.
Right on schedule, the agency tweeted: "@NASAJuno soared close to the cloud tops of Jupiter this morning. Scientists await results."
Mission controllers at NASA expect to capture stunning images and a wealth of scientific data from the approach.
Principal investigator Dr Scott Bolton, of the Southwest Research Institute in San Antonio, Texas said: "This is our first opportunity to really take a close-up look at the king of our solar system and begin to figure out how he works."
It will take some days for the images and information gathered by Juno to be downloaded on Earth.
NASA hopes to release a handful of close-up images from JunoCam, the probe's panoramic colour camera, during the later part of this week. They should include the first detailed pictures of Jupiter's north and south poles.
In total, 35 more close fly-bys are planned during Juno's primary mission, scheduled to end in February 2018.
No previous spacecraft has flown so near to Jupiter before.
The previous record for a close approach to the planet was set by Nasa's Pioneer 11 spacecraft, which passed at a distance of 43,000km in 1974.
Only one other spacecraft, Galileo, which visited Jupiter and its moons from 1995 to 2003, has orbited the planet. Although it was deliberately crashed onto Jupiter at the end of its mission, it orbited from much further out than Juno.
Powered by three huge solar panels, Juno was launched into space by an Atlas V rocket from Cape Canaveral, Florida, on August 5, 2011.
It took five years to complete the 1.8 billion-mile journey from Earth.
The probe has had to be specially strengthened to withstand the circuit-frying radiation around Jupiter. Its vital flight computer is housed in an armoured vault made of titanium.
At the end of its 20-month mission, Juno will follow in the footsteps of Galileo by making a one-way plunge into the planet's thick atmosphere.
