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.

In 2005, ESA's Rosetta spacecraft made its first of three Earth flybys planned for the mission. The first flyby resulted in a predicted increase in velocity, of 4km/s. However, the Rosetta mission team also found a really small change in velocity of 1.82 mm/s (that's 2 million times smaller than the main boost!) as a result of the anomaly¹. This is why mission planners can safely ignore it, but scientists who have been trying to figure out the exact cause of this puzzle will be eagerly awaiting Juno's flyby tomorrow to see how much of an increase in velocity it will receive from the Earth and watching closely for any unexpected variations due to the flyby anomaly. It seems Juno might be able to solve some tough questions before it even gets to Jupiter!

Mission Science

When Juno arrives at Jupiter in 2016, it will orbit the gaseous planet for around a year, completing 33 orbits around the planet's poles, using onboard scientific equipment to probe beneath the planet's obscuring yet beautiful clouds.

The main goals of the Juno mission are:

• To find out how much water is in Jupiter's atmosphere, which helps determine which planet formation theory is correct (or if new theories are needed).
• To look deep into Jupiter's atmosphere to measure composition, temperature, cloud motions and other properties.
• To map Jupiter's magnetic and gravity fields, revealing the planet's deep structure.
• To explore and study Jupiter's magnetosphere near the planet's poles, especially the auroras – Jupiter's northern and southern lights – providing new insights about how the planet's enormous magnetic force field affects its atmosphere.

Much like the Northern Lights here on Earth, Jupiter has aurorae. Juno's magnetometer and other instruments will help scientists to study these phenomena. Image credit: NASA/Chandra/Hubble

One of the main mission objectives is to discover how a giant planet like Jupiter came into being and how it evolved. This cloudy world is a primary example of a giant planet, and can also give us clues as to how other giant gas planets (called "Hot-Jupiters") which we have discovered orbiting other stars, may have formed.

Juno will accomplish this by studying the planet's cloudy atmosphere and its overall composition. One such scientific instrument aboard Juno is its Magnetometer. This sensor, located on the spacecraft's magnetometer boom, will accurately map the magnetic field of Jupiter to try and understand how it is generated deep within the planet's core. Juno will do this by observing and imaging Jupiter's core where the field originates from.

End of Mission

After completing primary mission objectives and 33 orbits around Jupiter in 2017, Juno will perform a series of de-orbit burns to take the spacecraft out of Jovian orbit and into a destructive re-entry in Jupiter's upper atmosphere.

But first, on October 9th 2013, Juno will return to within 350 miles of Earth to say one last goodbye to planet Earth, before heading to Jupiter to try and unlock some of the secrets of our solar system's biggest planet.

¹ A big thanks to the Rosetta mission team for letting me have the numbers!

This guest post was written by Cian O'Regan, who also writes the Irish Space Blog.

EDIT (July 2014): This article is now also available in video-form through our YouTube channel, thanks to a group of media students at Sheffield Hallam University!