Juno: Journey to Jupiter

Next Friday, the 5th of August, all things going as planned, I will be watching NASA’s latest scientific planetary mission launch into space, aboard at Atlas V rocket, from Kennedy Space Center Florida. The mission, named Juno, is a spacecraft bound for Jupiter, the largest planet in our solar system and the fifth planet from our Sun.

Juno is the second mission of NASA’s New Frontiers program, with the first being the New Horizons probe launched in 2006 and headed to Pluto. The program focuses on exploring the solar system with frequent (approximately one every 36 months) spacecraft missions that will conduct focused scientific investigations designed to enhance our understanding of the solar system.

Juno will take five years to reach Jupiter – arriving there in 2016. Even though it is “only” 2 planets over from Earth, Jupiter is far enough away (400 million miles from us) to be considered in the “outer” solar system. Once it reaches Jupiter, Juno will enter a polar  orbit and at its closest will be a mere 3,100 miles (5000 km) above the colorful clouds. Its orbital journey will be harrowing to say the least. In a highly elliptical orbit, Juno will race by the equator at a rate of 60 km/s (37 miles/s) threading a narrow gap between Jupiter and it’s hazardous “radiation belts” before swinging out further into space and returning. In total, Juno will complete 33 orbits – each taking about eleven days resulting in a yearlong mission.

Juno mission insignis/patch

The Juno mission insignia

Juno will be only the second spacecraft to ever orbit Jupiter (the last being Galileo in 1995) – which makes this mission particularly exciting. Although eight probes have passed by the gas giant planet, they have not been close enough to Jupiter for long enough to give us the answers to the big questions that still remain about the planet. Shrouded by clouds, astronomers have been able to study only the very outer layers. The images of giant storms across reddish bands stretching around Jupiter are familiar to most. Even with a small telescope it is possible to make out the parallel light bands (called zones) and darker stripes of cloud (called bands). The oft-observed “Great Red spot” is also popular with observers and is actually a “hurricane” bigger than our Earth that has been swirling across Jupiter’s surface for hundreds of years!

Close-up image of Jupiter and the Great Red Spot taken by Voyager I in 1979. Credit: NASA

“Our knowledge of Jupiter is truly skin deep,” said Juno’s principal investigator, Scott Bolton of the Southwest Research Institute (SwRI) in San Antonio, TX

What we can’t see is what is happening beneath Jupiter’s clouds. The Juno science team believes the answers to many perplexing questions lie hidden here. They want to know what exactly makes up the atmosphere of Jupiter, how Jupiter was formed and, ultimately, how  solar systems and planets form in general.

Juno’s name, which comes from Roman mythology, seems particularly apt. In the myth, the god Jupiter would hide behind clouds. But his wife, the goddess Juno could see through them.

Why Jupiter?

Despite the fact that Jupiter is by far the largest planet in our solar system (in fact it could easily hold every other body in our solar system excluding our Sun) we still know surprisingly little about its composition, atmosphere and the physical processes that occur there. Composed predominantly of hydrogen and helium like our Sun, scientists believe Jupiter was the first planet in our solar system to form from the “leftovers” of the Sun’s creation. So in studying Jupiter we can also test theories related to the formation and evolution of our entire planetary system, and answer the question: “how did our solar system come to be?”

As we have also started to discover many Jupiter-massed planets orbiting in distant solar systems, understanding Jupiter and how it was formed becomes more relevant.

NASA Juno spacecraft

Artists impression of the NASA Juno spacecraft -- credit:NASA

As Bolton puts it, “If we want to go back in time and understand where we came from and how the planets were made, Jupiter holds this secret, because it got most of the leftovers after the sun formed, and we want to know that ingredient list. What we’re really after is discovering the recipe for making planets.”

The science

The Juno mission will gather data that will help in completing several scientific goals.

One of the first goals is to measure how much water and oxygen are present inside Jupiter. Since oxygen is “locked-up” in the atmosphere as water vapour, we can use water vapour measurements to tell us how much oxygen is present

“Understanding the history of water across the early Solar System is a fundamental question, and Jupiter is going to give you the first clue,” says Bolton

You might wonder if the previous Galileo mission had already done this – well, yes and no. When the Galileo orbiter dropped a probe into the atmosphere of Jupiter in 1995, scientists were surprised by what they found. The probe reached about 97 miles below the outer cloud cover and showed that there was far less water present than expected. This indicated that scientific theories of Jupiter’s formation were incorrect.

However the Galileo probe is thought to have possibly plunged through a rare “dry spot” in Jupiter’s atmosphere, which may have affected results.

Juno will see beneath Jupiter's clouds to its internal structure. Credit: NASA/SwRI

This time Juno will try to measure water content by detecting microwaves emitted by Jupiter’s atmosphere. The amount of water present at different depths in the atmosphere alters the strength of the emission at different frequencies. Knowing the amount of water and oxygen can give us an idea of whether Jupiter formed close to its position today or alternatively whether it formed further away and “migrated in” toward the Sun. These questions form the basis of planetary formation theory.

Another key question Juno will try to answer is “what is the structure inside Jupiter?” Juno’s Microwave radiometer (MWR) will probe beneath the visible cloud tops to provide data on the structure, movement and chemical composition to a depth of up to 342 miles (550km). This will be especially important in understanding those “zones and belts” – for example, are the zones and belts just surface features or does the structure go down deeper? Are they showing us how Jupiter is structured internally perhaps? Also, just how deep are the roots to the Great red Spot (GRS) and how has it maintained itself for so long?

When we consider Jupiter’s structure, we also want to know about its gravitational and magnetic fields. The gravitational field is measured by way of the “Doppler shift” effect so as the spacecraft flies – we watch how Jupiter pushes and pulls at the spacecraft’s velocity. This can tell us how the mass is distributed within Jupiter and also how the planet is rotating – for instance, whether it is rotating as a solid body or a series of concentric shells.

Part of the way down into Jupiter’s depths, the hydrogen that we are familiar with on Earth comes under so much pressure that it actually becomes “metallic” and starts behaving like a fluid (a little like mercury in some thermometers). This metallic substance starts to conduct electricity and the result is the production of a magnetic field. Investigators are very interested in understanding this field and that is why Juno includes a magnetometer (MAG), which will measure the strength and direction of Jupiter’s magnetic field lines.

Theories for Jupiter's internal structure vary. Credit: NASA/SwRI

Jupiter has the 2nd strongest magnetic field in the solar system (after the Sun). Juno will study Jupiter’s magnetosphere and because it is in a polar orbit it has the distinct advantage of being able to observe the polar magnetosphere where the phenomenon known as the “great aurora” occurs. Jupiter has the brightest aurora in the solar system: almost 100 times brighter than those on Earth. Juno can detect high-energy particles coming down into Jupiter’s magnetosphere and forming aurora. Scientists can then compare these with the aurora that occur on Earth, and also those on Saturn (which are being studied now with the Cassini space probe) giving us a much better understanding of the auroral processes.

Juno will also tackle the question as to whether  the center of  Jupiter is solid.  And if so, how large is it?  Currently, scientists do not know if there is an ice and rock core beneath the metallic hydrogen region under incredibly high (40Mbar) pressure. The atmospheric pressure at sea level on Earth is, by comparison, a meager 1 bar – so at 40MBar or 40 million times this pressure, it is almost impossible to imagine what form this solid “rock” substance may take.

Finding evidence of a core made of heavy elements would suggest that that these elements were made in rocks early in solar system before Jupiter formed. This would constrain how and when the planet formed and give us more of a picture of what the early solar system was like. While some believe the existence of a solid core of roughly ten Earth masses would have been necessary to allow the runaway accretion of the hydrogen and helium gases that make up most of the planet, other models can explain the formation of the gas giant without the presence of a solid core.

Will Juno be able to definitively tell us the story of Jupiter’s formation and evolution? Possibly not, but it will help to constrain different theoretical models being proposed.

The Spacecraft

The Juno spacecraft in itself is extremely interesting.

One of Juno's massive solar panels. Credit: NASA JPL

The $1.1 billion dollar craft consists of a “vault” measuring 11.5 feet high and 11.5 feet wide and it weighs about 8000 pounds. Outstretched from this vault are three enormous solar panels, which will help to power the spacecraft. Juno is the first solar powered spacecraft designed to operate as far away as Jupiter.  Since Jupiter receives about 25 times less sunlight that Earth, the panels need to be extremely efficient and extremely large. Each solar panel wing measures about 9ft by 29ft so about the size of a tractor-trailer. With solar panels extended the craft spans more than 66 feet (20m). These panels will be unfurled shortly after the deployment of Juno in space.  At Jupiter’s distance the panels generate the power equivalent of four 100 watt light bulb; half goes to powering the instruments and the other half to engineering systems like heaters to warm equipment in harsh cold conditions of space.

The inner vault is made of titanium, houses delicate instruments and acts as armour for Juno to protect it from hazardous “radiation belts” which encircle Jupiter’s equator. In these regions the magnetic field traps and whips up electrons to almost the speed of light. The belts are similar to the “Van Allen” belts that exist around Earth but are many millions of times more intense. Without the armoured tank design of Juno, sensitive instruments and detectors would not survive the spacecrafts very first pass by Jupiter.

Juno's orbit (in green) has it dipping beneath dangerous radiation bands that surround Jupiter. Credit: NASA

Juno is a “rotating spacecraft”. Essentially this means that the spacecraft is spinning as it moves through space. Why? Because this spinning action makes the direction of pointing (where Juno is looking) extremely stable and easy to control (it has a gyroscopic action). During science operations Juno will spin at a rate of two revolutions per minute (RPM).

All instruments within Juno are fixed, so as Juno cartwheels past Jupiter the spacecraft will sweep the fields of view of each instrument. In fact, at 2RPM the instruments’ fields of view sweep across Jupiter about 400 times in the 2 hours it takes Juno to fly from one pole to the other.

The scientific payload consists of 9 instruments fed by 29 sensors. One of these instruments, “JunoCam” is simply designed to take images especially of Jupiter – especially the never before seen poles: to be used for public outreach and education.

“JunoCam will show us what you would see if you were an astronaut orbiting Jupiter,” said Bolton. “I am looking forward to that.”

Me too, Dr Bolton. Me too.

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