NASA's Juno spacecraft went into safe mode Wednesday as it flew by Earth to gain speed on its five-year journey to Jupiter, but the mission's lead scientist said the flyby achieved its objective of putting the probe on the correct course toward the solar system's largest planet.

Artist's concept of Juno flying past Earth. Credit: NASA/JPL
 
The Jupiter-bound probe flew about 350 miles over the Indian Ocean near South Africa at 3:21 p.m. EDT (1921 GMT), and all data indicate the spacecraft obtained the predicted gravity boost from the flyby, according to Scott Bolton, Juno's principal investigator from the Southwest Research Institute in San Antonio.

But the spacecraft, stretching the size of a basketball court with its solar panels extended, experienced a fault some time during the flyby, going into a safe mode to protect the probe's systems and instruments while engineers on the ground scramble to diagnose the problem.

Bolton said Juno is designed to downlink data at a slower rate than normal during a safe mode, but telemetry from the spacecraft shows all its systems and instruments are fine.

The solar-powered spacecraft zoomed over the Indian Ocean on the night side of the Earth, putting the probe's expansive solar arrays in eclipse for the first time since its launch in August 2011.

Juno also passed out of range of ground antennas around the time of closest approach, and a European Space Agency ground station in Perth, Australia, acquired the first radio signals from Juno a few minutes later.

"When we came out of the eclipse, we realized that the spacecraft was in safe mode," Bolton said. "What we do know is that all the subsystems and instruments are nominal and behaving OK."

Juno was programmed to collect data during the flyby with its science payload. The research activities - considered a bonus by the Juno science team - included gathering observations of the Earth's magnetic field and auroras and snapping a series of images of Earth with the spacecraft's primary camera.

See our flyby story for more details

"This did not affect the main purpose of the flyby, which was to put Juno on the right course to Jupiter," Bolton said.

Bolton said ground controllers see some indications Juno gathered data and images during the flyby, but it may take more time to confirm whether the craft took the images as planned. If the imagery was collected, it could take extra time recover the information from the probe's on-board computer while engineers focus their work on putting Juno back into its normal operating mode.

Juno is set to arrive in orbit around Jupiter on July 4, 2016, beginning a one-year science mission studying the gas giant's crushing atmosphere, powerful magnetic field and enigmatic core. Juno's discoveries could help scientists unravel how Jupiter, likely the solar system's oldest planet, formed and evolved in the early solar system.