A Nasa spacecraft bound for Jupiter will swing by Earth to get the boost it needs to arrive at the giant gas planet in 2016.

Using Earth as a gravitational slingshot is a common trick since there is not a rocket powerful enough to catapult a spacecraft directly to the outer solar system.

Launched in 2011, the Juno spacecraft first zipped past Mars, Earth's closest planetary neighbour.

It looped back and will make a quick pass by Earth on Wednesday to gather momentum for its trip to Jupiter, located 484 million miles from the sun.

During the manoeuvre, the solar-powered, windmill-shaped Juno will briefly pass into Earth's shadow and emerge over India's east coast.

At closest approach, Juno will fly within 350 miles of the Earth's surface, passing over the ocean off the coast of South Africa.

The rendezvous was designed to bump Juno's speed from 78,000 mph relative to the sun to 87,000 mph - enough power to cruise beyond the asteroid belt toward its destination.

During the gravity assist, the spacecraft's JunoCam, a wide-angle colour camera, will snap pictures of the Earth and moon.

Weather permitting, skywatchers in India and South Africa with binoculars or a small telescope may see Juno streak across the sky.

Ham radio operators around the globe were encouraged to say "Hi" in Morse code - a message that may be detected by Juno's radio.

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Nasa's Juno Zips Past Earth On Way To Jupiter