Nasa's Jupiter-bound Juno spacecraft has entered the planet's magnetosphere, where the movement of particles in space is controlled by what's going on inside Jupiter.
Juno is on course to swing into orbit around Jupiter on July 4.
"We've just crossed the boundary into Jupiter's home turf. We're closing in fast on the planet itself and already gaining valuable data,"
said Scott Bolton, Juno principal investigator, from Southwest Research Institute, San Antonio, in a statement on Thursday.
Science instruments on board detected changes in the particles and fields around the spacecraft as it passed from an environment dominated by the interplanetary solar wind into Jupiter's magnetosphere.
The obstacle is Jupiter's magnetosphere, which is the largest structure in the solar system.
"If Jupiter's magnetosphere glowed in visible light, it would be twice the size of the full moon as seen from Earth," Kurth said.
Out in the solar wind a few days ago, Juno was speeding through an environment that has about 16 particles per cubic inch (one per cubic centimetre).
Once it crossed into the magnetosphere, the density was about a hundredfold less.
The density is expected to climb again, inside the magnetosphere, as the spacecraft gets closer to Jupiter itself.
The motions of these particles travelling under the control of Jupiter's magnetic field will be one type of evidence Juno examines for clues about Jupiter's deep interior.
The Juno spacecraft was launched from Cape Canaveral, Florida, on August 5, 2011,