Jupiter-bound Juno races toward Earth flyby

CBS News

Returning to its starting point after a two-year billion-mile loop out beyond the orbit of Mars, NASA's Juno probe raced toward Earth Wednesday for a velocity-boosting flyby, using the planet's gravity to fling the spacecraft on to distant Jupiter.

The centerpiece of a $1.1 billion science mission, the solar-powered Juno spacecraft was launched atop a United Launch Alliance Atlas 5 rocket from the Cape Canaveral Air Force Station, Fla., on Aug. 5, 2011.

Even though Atlas 5 boosted the spacecraft to an Earth-escape velocity of 25,000 mph -- 7 miles per second -- Juno did not have the energy to head directly to its target.

A computer graphic showing NASA's Juno spacecraft in orbit around Jupiter. Arrival is expected on July 4, 2016. (Credit: NASA)

Instead, the spacecraft was launched on a trajectory that carried it beyond the orbit of Mars and into the asteroid belt before the sun's gravity finally began pulling the craft back into the inner solar system.

In September 2012, carefully planned rocket firings altered the probe's course slightly and set up Wednesday's gravity-assist flyby of Earth. After covering 994 million miles since launch, close approach was expected at 3:21 p.m. EDT (GMT-4) when Juno was expected to pass about 350 miles above South Africa.

"Even a large rocket couldn't provide enough propulsion to get us all the way to Jupiter, so we are flying by the Earth for a gravity-assist that will provide about 70 percent of the initial boost provided by the Atlas V 551 rocket," Scott Bolton, Juno principal investigator, said in a statement. "The gravity assist essentially provides as much propulsion as second rocket launch."

The flyby was set up to increases Juno's velocity, relative to the sun, from 78,000 mph to around 87,000 mph, according to the Southwest Research Institute.

Tim Gasparrini, Juno program manager for Lockheed Martin Space Systems, said a direct trajectory to Jupiter would have required about 50 percent more fuel than Juno could carry.

"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," he said in a company statement.

During the approach to Earth and its departure from the Earth-moon system, Juno's instruments will be calibrated and put to work.

"While we are primarily using Earth as a means to get us to Jupiter, the flight team is also going to check and calibrate Juno's science instruments," Bolton said.

"As another bonus, Juno is approaching the Earth from deep space, from the sunlit side. Juno will take never-before-seen images of the Earth-moon system, giving us a chance to see what we look like from Mars or Jupiter. We plan to release a movie of this unique perspective of the Earth-Moon system shortly after the flyby."

The Juno mission is managed by the Jet Propulsion Laboratory in Pasadena, Calif., which is operated for NASA by the California Institute of Technology.

While more than 90 percent of NASA's civil service workforce have been furloughed in the ongoing government shutdown, JPL is not directly affected and key NASA personnel remain on the job to protect operational spacecraft.

If all goes well, Juno will reach its target on July 4, 2016, braking into an elliptical orbit around Jupiter's poles five years and 1.7 billion miles after launch.

Using a suite of sophisticated instruments, the four-ton spacecraft will study Jupiter's turbulent atmosphere in unprecedented detail to learn more about how the huge world evolved, whether it has a solid core and what its constituents imply about the nature of the solar nebula that coalesced 4.5 billion years ago to form the solar system.

The titanium-armored spacecraft also will map out Jupiter's magnetic field and radiation belts, absorbing the equivalent of 100 million dental X-rays over the course of 33 orbits -- one year.

"What we're really going after are some of the most fundamental questions of our solar system -- how Jupiter formed, how it evolved, what really happened early in the solar system that eventually led to all of us and the terrestrial planets," Bolton told reporters shortly after launch. "These are really basic questions, who are we, where did we come from, how did we get here?

"We're kind of going after this recipe of how planets are made. We're getting the ingredients of Jupiter, we're going to understand what the structure is like inside, how was it built, and that will give us guidance as to what happened in that early time that eventually led to us."