Jupiter Probe Juno Prepares For Earth Flyby on Wednesday

NASA's Juno spacecraft launched aboard an Atlas V rocket from Cape Canaveral Air Force Station, Fla., Aug. 5, 2011, beginning a five-year journey to Jupiter. But it wasn't charted on a direct path. Before it reaches its destination, Juno will greet the Earth one last time. The spacecraft will perform a flyby Oct. 9, passing within 347 miles of Earth.

The flyby will function as a gravity assist for Juno, with Earth's gravity accelerating the solar-powered spacecraft's velocity by 16,330 miles per hour. NASA launched Juno to an area just past Mars, then two main engine burns executed a year ago maneuvered it back around toward Earth. The purpose of using a gravity assist to get Juno on its way to Jupiter is one of cost.

"A direct mission to Jupiter would have required about 50 percent more fuel than we loaded," said Tim Gasparrini, Juno program manager for Lockheed Martin Space Systems. "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."

For Lewis and others on the team, a big part of positioning Juno for a successful gravity assist is to ensure the spacecraft steers clear of other objects in its vicinity.

"The day of the Earth flyby, the team will be on hand to monitor things," said Lewis. "We have a couple of possible collision avoidance maneuvers to select from, looking at all the satellites around the Earth. We are passing inside the orbits of geostationary spacecraft."

Catching a velocity boost isn't the only value in the effort. The operation also will permit officials to test Juno's instruments and observe the spacecraft's flight handling.

"We'll exercise the science instruments, since Juno's instruments will be operating in a magnetospheric environment for the first time," said Lewis. "The Earth's magnetic field will allow a number of the instruments to be tested. We're also using the flyby of the moon as an opportunity to gauge how the spacecraft operates. Since Juno is a spinning spacecraft, we need to sense the right time to take data as the Moon, or Jupiter, passes through the instruments' fields of view."

On Aug. 12, Juno achieved a milestone by reaching the halfway point on its trek to Jupiter as it had traveled 9.46 astronomical units, equivalent to 879,733,760 miles, at that point. Demonstrating fortuitous timing, the spacecraft is scheduled to reach Jupiter July 4, 2016.

Juno's primary mission is to study Jupiter's atmosphere as a means of better understanding how the planet, and by extension, the solar system originated and evolved. Juno will employ its suite of scientific instruments to peer beneath the planet's dense cloud cover to study the existence of a solid planetary core, map Jupiter's magnetic field, measure water content in the atmosphere and study the planet's auroras. -- Lockheed Martin