Nasa's Jupiter-bound spacecraft has encountered an "unknown event" shortly after using Earth as a gravity slingshot to fire it towards the outer solar system.
Spacecraft are programmed to go into safe mode when they encounter an unknown event.
Engineers are troubleshooting the problem which occurred after Juno zipped past Earth in a momentum-gathering flyby to power itself towards Jupiter.
Juno - launched in 2011 - hurtled 350 miles above the ocean off the coast of South Africa, the point of closest encounter.
Previous missions to the outer solar system have used Earth as a celestial springboard because there is no rocket powerful enough to make a direct flight. The Galileo spacecraft buzzed by Earth twice in the 1990s en route to Jupiter, the solar system's largest planet 484 million miles from the sun.
Juno flew beyond the orbit of Mars before looping back toward Earth for a flyby which boosted Juno's speed from 78,000mph relative to the sun to 87,000mph - enough momentum to cruise past the asteroid belt to Jupiter, where it should arrive in 2016.
Nasa and the European Space Agency said ground controllers in Australia and Spain picked up a signal from the spacecraft shortly after the pas, but engineers were puzzled by the low data rate and were investigating.
During the manoeuvre, the solar-powered, windmill-shaped Juno briefly slipped into Earth's shadow and emerged over India's east coast.
By space mission standards, Juno's Earth rendezvous was low-key compared with the Curiosity rover's nail-biting landing on Mars last year, mainly because such flybys have been executed before.
Despite a government shutdown that has prevented Nasa from updating its website or tweeting, the space agency's missions continue to operate. Earlier this week, Nasa's newest spacecraft, LADEE, slipped into orbit around the moon.
Since the 1970s, spacecraft have circled or flown past Jupiter including the Voyagers, Pioneers, Galileo, Ulysses, Cassini and, most recently, the New Horizons barrelling toward Pluto.
Missions have beamed back stunning views of Jupiter's trademark Great Red Spot, a raging hurricane-like storm, and its many moons.
Juno promises to inch closer to Jupiter than previous spacecraft, orbiting the planet for at least a year and studying its cloud-covered atmosphere and mysterious interior to better understand how the giant planet formed.