Why Go to Jupiter?

At 11:53 P.M. EDT on July 4th, as the last of the fireworks begin to fade, NASA will be eagerly awaiting a signal from the Juno spacecraft that it has entered orbit around Jupiter.  This will commence twenty months of exploration of Jupiter’s polar regions which is the epicenter of the giant planet’s massive magnetic and auroral activity.  It will also signify the beginning of the end of NASA’s second wave of space missions to the gas giants that began in 1989 with the launch of the Jovian Galileo probe.  In September 2017, Cassini will cease operations with a decent into Saturn.  Five months later, Juno will meet the same fate as it plunges into Jupiter.  NASA’s exploration of the outer planets will go dark until the 2020’s.

Juno, named after the Roman goddess wife of Jupiter, was launched in 2011 and embarked on a 1.8 billion mile odyssey to the giant planet that included a flyby past Earth.  Why flyby Earth?  The pull of Earth’s gravity whipped Juno into sufficient velocity to reach Jupiter.  This maneuver, while more time-consuming, saves fuel and cost.  Not an insignificant consideration as Juno was hatched during an era of flatline budgets for NASA.  In all, the Juno mission will cost $1.1 billion or roughly the same as a NFL stadium.  Below is a video of Juno’s trajectory to Jupiter.

Normally, we associate planetary missions with spectacular imagery.  Juno does have a camera on board but that will be used for outreach purposes.  The science of Juno involves magnetometers and particle detectors.  Jupiter has a massive magnetic field that produces aurora activity several times the size of Earth and radio emissions as well.  Juno intends to use its measurements to study the interior of Jupiter which in turn will reveal the processes that drive its magnetic activity and origins.

Jupiter’s aurora was discovered in 1979 by Voyager I.  On Earth, the aurora is created by ionized particles embedded in the solar wind spiraling down Earth’s magnetic field lines towards the poles (charged particles will follow the path of magnetic fields).  Here, in the upper atmosphere, the ionized particles slam into oxygen and nitrogen atoms exciting their electrons to a higher energy level.  As the electrons subside back to a lower energy level, the kinetic energy of these particles are converted to electromagnetic energy in the form of green and red light.

Juno’s elliptical orbits will avoid zones of high radiation surrounding Jupiter. Credit: NASA/JPL/Caltech/Institute for Aeronautics and Astronautics

On Jupiter, the process is a bit different.  The solar wind contributes to the aurora, but there is another major source of ions from the moon Io.  The most volcanic active body in the Solar System, Io spews out oxygen and sulfur ions that travel along Jupiter’s magnetic field to the poles.  The aurora has been viewed by the Hubble which recently released this image.

aurora
Credit: NASA/ESA

When electrons are accelerated, radio waves are transmitted.  This is the principal that radio towers work on.  Electrons are accelerated up and down the transmission tower producing the broadcast received by your radio and converted to sound waves by its speakers.  Around Jupiter, electrons are accelerated as they spiral down the magnetic field lines.  Io also acts to accelerate electrons as its presence distorts Jupiter’s magnetic field. A change in a magnetic field induces an electric field pushing the electrons.  This action creates radio transmissions from Jupiter that are received on Earth in the 8-38 MHz range, the same range shortwave radio is transmitted.

Ham radio operators have received these transmissions from Jupiter and NASA’s Radio Jove project allows schools to purchase receivers for a few hundred dollars to detect Jupiter’s radio waves.    Samples of these radio observations can be heard here.

One might ask, why should we care about Jupiter’s magnetic field and how does it relate to Earth?  The answer lies in the fact that while we can map Earth’s magnetic field as it extends into space, we are unable to map the dynamo process generating the field in Earth’s interior.  Jupiter, being a gaseous planet, will allow Juno to map the magnetic field down to the interior where the dynamo lies.  Jupiter formed before the solar wind blasted away the primordial material of the solar nebula.  The more we learn about Jupiter’s interior, the more we’ll know how the Solar System originated.  The video below describes how Juno will explore Jupiter’s magnetic field.

Juno’s instrument package includes a radio transmitter to detect variations in Juno’s velocity as it orbits Jupiter.  Doppler shifts in the radio waves will allow for measurements of variations in Jupiter’s gravity field providing hints to the make up of its interior.  The Jovian Auroral Distributions Experiment (JADE) and Jupiter Energetic Particle Detector Instrument (JEDI-video below) will measure the ions and electrons traveling along the magnetic field lines that eventually produce Jupiter’s aurora.

The Jovian Infrared Auroral Mapper (JIRAM) will provide images of Jupiter’s aurora.  Juno’s magnetometer will construct a 3-D map of Jupiter’s magnetic field, both the field lines and their magnitude.  The Microwave Radiometer’s (MWR) function is to detect thermal radio emissions from six layers beneath the clouds of Jupiter.  This will provide a 3-D map of the Jovian atmosphere.  The Ultraviolet Imaging Spectrometer’s (UVS) mission is to examine the aurora in ultraviolet allowing for measurements on both the day and night sides of Jupiter.  The aptly named Waves instrument will measure radio waves produced by the magnetic field.  Last, but not least, is the JunoCam which will take the first pictures of Jupiter’s poles and allow for the public to participate on deciding other targets to image.

Image of Antarctica taken by JunoCam during Earth flyby. Credit: NASA.

On February 21, 2018, after completing 37 elliptical orbits of Jupiter, Juno will crash into Jupiter ending its adventure.  The next mission to the outer Solar System is not scheduled until the 2020’s with NASA’s planned Europa mission.  This gap was caused by funding curtailments created by the Great Recession.  This is similar to the gap between the Pioneer and Voyager missions launched in the 1970’s and the Galileo mission launched in 1989.  That first gap was caused by budget cuts during the Reagan administration in the 1981-82 recession. In fact, that gap almost became catastrophic as the administration proposed to terminate Voyager funding before the mission reached Uranus and Neptune.  Fortunately, Voyager was kept alive and is still returning data today.  So, what can we hope for in the meantime?

The Hubble Space Telescope will still take high quality images of the outer planets, and will be joined by the James Webb Space Telescope in 2018.  Of course, both have other mission objectives and are not dedicated to viewing the Solar System.  The next generation of ground telescopes featuring mirrors in the 30-40 meter range will be able to peer deeper with more detail into the Solar System, possibly mapping surface characteristics of Kuiper Belt objects.  New Horizons just received funding approval to visit the Kuiper Belt object 2014 MU69 beyond Pluto on New Year’s Day in 2019.  Despite the upcoming lull in deep space exploration, the future still looks interesting for planetary science.

*Image on top is workers testing the solar panel for Juno prior to launch.  Credit:  NASA.

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