NASA's Juno spacecraft to Jupiter lifted off from Cape Canaveral Air Force Station in Florida on Friday afternoon, after about a half-hour delay as engineers tested the helium system of the unmanned rocket's upper stage.
Juno is solar powered, with three huge panels, a first for a spacecraft intended to roam so far from the sun.
The four-tonne spacecraft will orbit Jupiter — believed to be the oldest planet in the solar system — for about a year while it probes how much water is in the atmosphere and studies the planet's magnetic and gravity fields.
NASA expects the probe's camera will provide close-up images of Jupiter, including the first detailed glimpse of its poles.
The last probe to Jupiter, Galileo, was launched in 1989. Juno will orbit far closer to Jupiter than Galileo, which was deliberately destroyed in a plunge into Jupiter's atmosphere in 2003. Juno's closer approach means the spacecraft will be able to get far better measurements of the planet's interior.