The Jupiter-bound Juno spacecraft flew by Earth on Wednesday, zooming 350 miles over South Africa and getting a gravity boost to sling the robotic voyager on a course to reach the solar system's largest planet on July 4, 2016.

Artist's concept of Juno flying past Earth. Credit: NASA/JPL
 
Juno made its closest approach at 3:21 p.m. EDT (1921 GMT) about 347 miles over the Indian Ocean off the coast of South Africa, dashing through a region of space populated by communications and navigation satellites on the flyby's entrance and exit from the vicinity of Earth.

"The purpose of the flyby is to change speed so that our orbit will reach all the way out to Jupiter," said Scott Bolton, Juno principal investigator from the Southwest Research Institute in San Antonio. "We had enough energy to reach out to the asteroid belt when we left Earth and went into orbit around the sun."

The European Space Agency's ground stations in Argentina and Australia tracked Juno its encounter with Earth. ESA's Perth antenna acquired a signal from the solar-powered spacecraft after its point of closest approach to Earth, confirming it was on the expected path following the flyby.

Juno blasted off from Cape Canaveral, Fla., on Aug. 5, 2011, aboard a United Launch Alliance Atlas 5 rocket, but the powerful launcher could not put the 8,000-pound probe on a direct trajectory to Jupiter.

"A direct mission to Jupiter would have required about 50 percent more fuel than we loaded," said Tim Gasparrini, Juno program manager at Lockheed Martin Corp., the spacecraft's prime contractor. "Had we not chosen to do the flyby, the mission would have required a bigger launch vehicle, a larger spacecraft and would have been more expensive."

Officials predicted Earth's gravity would accelerate Juno by about 16,330 mph.

"Effectively, we're changing speed by going by the Earth, but we're also changing direction and making a turn, and we're using Earth to help us make that turn," Bolton said.

Diagram of Juno's trajectory during the Earth flyby, showing its change in direction and increase in speed. Credit: NASA
 
Interplanetary probes have commonly used flybys of Earth, Venus, Mars and Jupiter to bend their trajectories toward their ultimate destinations.

NASA exempted Juno from the ongoing partial shutdown of the federal government, now in its second week due to political squabbling in Washington, because the mission was at a critical phase. Juno is also managed by the Jet Propulsion Laboratory, which is still operating because it is run under contract by the California Institute of Technology.

"The only thing that has been affected is the fact that NASA, in general, is shutdown," Bolton said. "The press releases and the press conferences that normally NASA would do are all gone."

Bolton said Juno was collecting images of Earth during the flyby. The spacecraft's main camera, named JunoCam, was programmed take photos throughout the maneuver. Bolton said he expected the first close-up imagery to be downlinked back to Earth by Wednesday evening, U.S. time.

"We have a camera as a public outreach tool," Bolton said. "We release the raw images and the public is able to go in and process them."

Juno also recorded a series of images with a navigation camera on approach showing Earth and the moon in their cosmic dance.

"We were able to get a distant shot of Earth and moon together about a day ago," Bolton said in an interview Wednesday. "We're still processing that data, but I can already see that it worked. We see the Earth spinning on its axis and the moon going around us."

Officials hope to release a movie of the Earth and moon by the end of this week.

"It will be very different from anything any human has ever seen before," Bolton said. "This is what we look like from afar."

Scientists also activated Juno's instrument suite for the flyby, using the opportunity to gather data, calibrate sensors and rehearse the spacecraft's hectic science mission at Jupiter.

"Most of our instruments are on, and we're either doing calibrations or doing full science with them," Bolton said.

This montage image of Jupiter and its volcanic moon Io comes from imagery collected by NASA's Pluto-bound New Horizons spacecraft when it flew by the giant planet in early 2007. New Horizons was the last spacecraft to visit Jupiter. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
 
Juno was programmed to measure charged particles and plasma in the Earth's magnetospheric bubble, observe the auroras, and look at the Earth and moon with infrared and ultraviolet instruments.

"We do some calibration and we also get to measure Earth's magnetospheric environment from multiple points because Juno is flying through with a snapshot view while other spacecraft are orbiting," Bolton said.

Juno scientists also solicited help from amateur radio operators to simultaneously send a "HI" message using Morse code while the probe is closest to Earth. If enough radio operators participated, Bolton said the craft's Waves experiment should detect the message.

About the size of a basketball court with its three solar array wings extended, Juno is scheduled to enter orbit around Jupiter on July 4, 2016, beginning a one-year science mission to examine the giant planet's crushing atmosphere, powerful magnetic field and deep interior.

So far, the $1.1 billion mission has gone according to plan, according to Bolton.

"The overall health is excellent," Bolton said. "We're at 100 percent. Everything has worked exactly as we hoped. There have been no issues at all. All the instruments are behaving, all the subsystems are working, and the navigation is as perfect as we hoped for."

Scientists say discoveries at Jupiter could yield insights into the planet's origin and the formation of the solar system. Clues hidden inside Jupiter, likely the oldest of the solar system's planets, could lead researchers to understand how the solar system formed and evolved 4.5 billion years ago.

Juno will orbit Jupiter 33 times, looping over the gas giant's poles and returning the closest images of the planet's ceaseless auroras.

Bolton said the Juno science team will spend the next two-and-a-half years meticulously planning the mission's activities once it arrives at Jupiter.

"We will process all the Earth data and make sure all the operations worked, and we will look at what lessons we learned so we can address any ideas on how to do things more efficiently at Jupiter," Bolton said.