The maneuver will increase the speed of Juno, launched in August 2011, and send it on its way to a July 2012 rendezvous with the solar system's largest planet, they said.
The two years spent moving outward past the orbit of Mars before swinging past the Earth makes the trip to Jupiter possible, said University of Iowa research scientist Bill Kurth, lead investigator for one of Juno's nine science instruments.
"Juno will be really smoking as it passes Earth at a speed of about 25 miles per second relative to the sun," he said. "But it will need every bit of this speed to get to Jupiter for its July 4, 2016, capture into polar orbit about Jupiter."
"The first half of its journey has been simply to set up this gravity assist with Earth."
Juno will be the first spacecraft to orbit Jupiter over its poles, scientists said, in a highly eccentric orbit that will take the spacecraft from just above the cloud tops to a distance of about 1.75 million miles from Jupiter every 11 days.
The Iowa instrument, known as Waves, will examine a variety of phenomena within Jupiter's polar magnetosphere -- including the solar system's most powerful auroras -- by measuring radio and plasma waves.
"Jupiter has the largest and most energetic magnetosphere, and to finally get an opportunity to study the nature of its auroras and the role radio and plasma waves play in their generation makes Juno a really exciting mission for me," Kurth said.