Juno - halfway there and home again

Artist's impression of the Juno spacecraft near the planet Jupiter. Image credit: NASA/JPL

On August 12 2013, NASA's Juno Spacecraft reached the halfway point on it's journey to Jupiter. Since launching back in 2011, it has travelled over ten times the distance from the Earth to the Sun, and has performed a series of planetary flybys and deep space manoeuvres. Tomorrow on October 9, Juno will come within 350 miles of Earth's surface. This is known as a gravity assist, or a gravitational slingshot. After Juno says its farewells to planet Earth for the final time, it will race towards the Jovian system before its slated arrival time of 22:29 EST on July 4, 2016, give or take a few minutes!

But what has Juno done since launch? Well, the Jovian explorer has been sent out past the orbit of Mars, and performed crucial Deep Space Maneuvers to set itself up for tomorrow's flyby of Earth.

Juno Mission Project Manager Rick Nybakken explains further;

"On Oct. 9, Juno will come within 347 miles (559 kilometers) of Earth. The Earth flyby will give Juno a kick in the pants, boosting its velocity by 16,330 mph (about 7.3 kilometers per second). From there, it's next stop Jupiter... Almost like a second launch for free!"

Juno's path to Jupiter. Image credit: NASA

Confused? Well, think of it this way: if a golfer putts a ball towards the edge of the hole, and the ball does not fall into the cup, instead hitting the very edge of the hole and "lipping out", the ball will shoot off in another direction at a faster speed than before. You got the hang of this? Right, let's move on.

However, it is also worth noting that as well as this extremely predictable increase in speed, the spacecraft also experiences a tiny tiny change in velocity due to something else. But just why do spacecraft that flyby Earth receive this tiny change in acceleration? Well, to put it simply, no one is really quite sure! This is known as the Flyby Anomaly, and it's something that science doesn't really have an answer for at the moment.

What Causes the Flyby Anomaly? 

Scientists have theorised (and later dismissed) this unexpected source of energy as being caused by atmosphere, tides, magnetism or radiation. The possible remaining solutions for this problem include that there might be a halo of dark matter around the Earth, as well the theory that it is caused by the rotation of the Earth itself.