When NASA's Juno spacecraft was launched to Jupiter in 2011 there wasn't a rocket available powerful enough to send the heavy spacecraft directly to the Jovian giant. Mission managers had to build in a flyby of Earth into the spacecraft's trajectory to provide the boost in speed that Juno needed to complete its 1.74 billion-mile journey. Therefore, on Wednesday, the Juno spacecraft will fly to within 350 miles of Earth's surface. This close encounter with Earth, called a "gravity assist," will boost the spacecraft's speed by about 16,330 miles per hour.
The slingshot effect of a gravity assist makes it possible for a spacecraft to reach the distant outer planets without using vast amounts of propellant. Astronomers had long known that the orbit of a comet was altered by an encounter with a planet. Consequently they theorized that the principle could be applied to spacecraft trajectories as well.
When a spacecraft flies by a planet, the planet pulls the spacecraft with its gravity. But the spacecraft has gravity, too, and it pulls on the planet a tiny amount. This causes the planet to lose a little energy from its solar orbit while the spacecraft gains the same amount. A small change in energy for a massive planet like Earth causes a minute reduction in the planet's speed, but the same energy applied to a tiny satellite causes a great change in speed.
The Earth flyby on Wednesday will help Juno, the first solar-powered spacecraft designed to operate at a great distance from the sun, reach Jupiter in July 2016. Juno's primary goal is to give planetary scientists a better understanding of the origin and evolution of Jupiter. The spacecraft is also expected to shed new light on how the giant planets formed and the role these titans played in putting together the rest of the solar system.science