After a 50-minute delay, an Atlas rocket carrying the four-ton spacecraft arced into the sky from Kennedy Space Center in Florida on Friday. Just under an hour after launch, controllers confirmed that the craft was performing flawlessly, its three enormous solar panels fully extended to support the first solar-powered spacecraft to visit an outer planet.
Past NASA missions have flown by Jupiter or, in the case of the Galileo mission, orbited the giant planet and toured its moons. But this is the first mission to focus its instruments full-time on the planet itself.
Despite the attention scientists have heaped on Jupiter to date, basic questions remain unanswered. For instance, no one knows how deep the clouds go. The dynamo that generates the planet's powerful, expansive magnetic field is a mystery.
Beyond understating the planet's structure and behavior, scientists are interested in a more-detailed look at Jupiter because it is widely believed to have been the first planet that formed after the sun did, some 4.6 billion years ago.
Starting with a rocky core roughly 10 times the mass of Earth, Jupiter quickly swept up gases, blossoming into the giant that astronomers see today, according to current ideas about Jupiter's formation.
As a result, the planet is thought to contain, beneath its swirling cloud tops, a virtually pristine record of the chemical composition of the disk of dust and gas that surrounded the newborn sun, as the planets formed.
The data Juno gathers has the potential to make a significant contribution to scientists' understanding of the solar system's evolution, Dr. Bolton said during a post-launch briefing today.
The transition from a sun with a disc to a sun with its first planet "is a big step," he says. "We don't really understand what happened – what the elements and volatiles were doing early in the solar system." Volatiles are elements that readily shift from solids to gases at low temperatures.
A planet that has 318 times Earth's mass and whose dominant ingredient is hydrogen, with a little helium and dashes of a few other gases mixed in, Jupiter is expected to open a unique window on this transition.
But that window won't open for five years. The mission plan calls for Juno to begin orbiting Jupiter's poles on July 4, 2016.
Between now and then, Juno faces a 1.7 billion-mile cruise that will take it around the sun once, then bring it past Earth in 2013 for a boost from Earth's gravity for the final leg of its trek to Jupiter.
During this period, the science team and the controllers will put Juno through some preparatory paces, calibrating instruments as well as getting to know the new craft's "quirks and idiosyncrasies," says Jan Chodas, Juno's project manager.