If you've ever whirled（旋转，回旋） a ball attached to a string around your head and then let it go, you know the great speed that can be achieved through a slingshot（弹弓） maneuver¹. Similarly, NASA's Juno spacecraft will be passing within some 350 miles of Earth's surface at 3:21p.m. EDT Wednesday, Oct. 9, before it slingshots off into space on a historic exploration of Jupiter.

 

It's all part of a scientific investigation² that began with an August 2011 launch. The mission will begin in earnest when Juno arrives at Jupiter in July 2016. Bill Kurth, University of Iowa research scientist and lead investigator³ for one of Juno's nine scientific instruments, the Waves instrument, says that the two years spent moving outward past the orbit of Mars before swinging past Earth makes the trip to Jupiter possible.

 

"Juno will be really smoking as it passes Earth at a speed of about 25 miles per second relative to the sun. But it will need every bit of this speed to get to Jupiter for its July 4, 2016 capture into polar orbit about Jupiter," says Kurth, who has been involved with the mission since the beginning. "The first half of its journey has been simply to set up this gravity assist with Earth."

 

"One of Juno's activities during the Earth flyby will be to make a movie of the Earth-moon system that will be the first to show Earth spinning on its axis⁴ from a distance," says Scott Bolton, principal investigator for the Juno mission from Southwest Research Institute in San Antonio.

 

Kurth and colleagues UI Professor Don Gurnett and research scientist George Hospodarsky note that the real science will begin when Juno begins orbiting Jupiter some 33 times over the course of a year. Juno will be the first spacecraft to orbit Jupiter over its poles. The orbit will be highly eccentric, taking Juno from just above the cloud tops to a distance of about 1.75 million miles from Jupiter, every 11 days.

 

The UI-designed-and-built Waves instrument will examine a variety of phenomena⁵ within Jupiter's polar magnetosphere（磁气圈） by measuring radio and plasma⁶ waves. It's one of nine experiments to be undertaken of the gas giant.

 

In particular, Juno will explore the solar system's most powerful auroras -- Jupiter's northern and southern lights -- by flying directly through the electrical current systems that generate them.

 

"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," says Kurth.

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