Thursday, 1 November 2012

NASA's Juno Mission at EPSC 2012

The European planetary science convention (EPSC) took place in September 2012 in Madrid, and for the second year running I was asked to convene the session on the giant planets.  With European activities on JUICE, ESA's proposed mission to Jupiter in the 2030s, in full swing, I hoped to persuade the Juno team to 'cross the pond' to update us on the status of he Juno mission.  Scott Bolton, the mission PI, provided an overview, while Mike Janssen explained the microwave remote sounding of Jupiter's deep interior and Glenn Orton explained the need for coordinated ground based observations in support of these activities.  From the hastily-written notes as the rain fell in Madrid, here's a record of the Juno sessions at EPSC.

Bolton - The Juno Mission

With three solar arrays 8.5 m long, Juno is a big spacecraft, cartwheeling along towards the gas giant.  Juno's science goals at Jupiter will fall into four categories, studying the origin, interior, atmosphere and magnetosphere of the giant planet. The mechanisms by which Jupiter formed are a key question for the origins of our solar system and our habitable planet, and Juno will provide answers to two mysteries - how much oxygen is present, and does Jupiter have a rocky core at its centre? 

The motivation for this is the impasse reached in our understanding of Jupiter's formation since the Galileo probe 17 years ago.  We know that Jupiter is enriched in materials compared to the solar composition, and that all the elements are enriched by roughly the same amount.  Juno will help answer how those materials were incorporated into Jupiter by measuring the abundance of water (molecules were probably delivered trapped in water ice cages), something that escaped Galileo in 1995 when it descended into the Sahara desert of Jupiter (a dry hot spot). 

Probing the core of Jupiter, if one exists at all, will require both precise gravitational measurements as Juno orbits the gas giant, in addition to magnetic field measurements to understand how the interior rotates, where convection occurs, where the magnetic field is generated and why it appears to be asymmetric.   It will help answer how Jupiter's interior rotates, whether as a single solid body, or as a series of concentric cylinders with deep winds.   The polar orbits of Juno will be ideal for such measurements, as well as revealing the unique dynamics and chemistry of the polar atmosphere and auroras for the first time.  The trajectory will take the spacecraft through the regions where particles are accelerated along magnetic field lines.

To achieve its science aims, Juno must get closer to Jupiter than any other spacecraft, within the intense radiation belts.  Those belts, rather like the earth's van Allen belts, contain high energy particles that pour through sensitive detectors and electronics, meaning that Juno will be short-lived, and the prospects for an extended mission slim. There will be 32 primary science orbits roughly 11 days long, starting with the spacecraft close to the equator (perijove at 5000 km above the cloud tops) and distant over the poles, but eventually moving so that Juno enters more and more of the radiation belts as the planet's gravity tweaks the orbit.

Juno features a suite of instruments required to address its four science themes, including a gravity science instrument from JPL, a magnetometer from Goddard space flight center, a microwave radiometer from JPL, energetic particle detectors from APL in Baltimore, a UV spectrometer from SWRI, and a near infrared spectrometer from Italy.  Finally, there's a colour camera on board (JunoCAM, provided by Malin) intended for public outreach activities.  Bolton described Juno as a antenna farm, with more antennae than any other mission.

Juno was launched on schedule (!!) on August 5th 2011, and will return for a swing by of the earth in September 2013.  It will arrive at Jupiter on July 4th 2016 for a year-long mission, at approximately the same time as Cassini will be completing the proximal orbit phase at Saturn (i.e., close inside of the rings).  Juno's main engine was ignited in September for the first time, a correction manoeuvre in deep space, and came as a huge relief to the team.  That engine is needed to insert successfully into Jupiter orbit, without it they'd go sailing right past!

After orbital insertion, Juno will be in a polar orbit, with Jupiter rotating by 192 degrees longitude between each perijove.  For the first 15 orbits, we'll have ground tracks, north to south, spaced every 24 degrees longitude.  After 15 orbits, Juno will shift slightly and fill in the gaps, so the final product will be scans spaced by 12 degrees of longitude around the planet at very close perijove.  When complete, Juno will hopefully have revealed the existence of a core, the deep water abundance, and investigated the structure of the interior and polar regions for the first time.  Finally, Juno carries with it three Lego figurines, of the gods Jupiter and Juno, and Galileo himself, as a fitting tribute to our exploration of the giant planet.

Janssen - Microwave Observations below the Clouds

Microwave radiometry will be used to peer beneath the clouds of Jupiter to measure its internal water abundance and structure.  The MWR on Juno features six antennae from 1.3 to 50 cm in wavelength, with longer wavelengths probing deeper into the planet.  The antennae are huge, with one taking up a whole side of the spacecraft, and the other five mounted together on another side.  The antennae will provide footprints 12 degrees in size from 1.37-11.55 cm, and 20 degrees in size from 20-50 cm.  The longest wavelengths are most likely to probe the water cloud.  This type of science is impossible to do from the earth, as the synchrotron emission from Jupiter's powerful radiation belts hides the atmosphere from view.

The MWR uses a clever technique to calibrate, and get accuracy on the measurements below 0.1%.  Firstly, the spinning of Juno means that the antennae alternately see the synchrotron emission from the radiation belts and the radiance from the planet, meaning that the difference can be used to calibrate.  But the dependence of the radiance on emission angle is independent of the radiometric calibration, allowing the team to achieve high accuracy relative measurements.  This accuracy will allow Juno to peer down below Jupiter's churning clouds for the first time.

Orton - Ground-Based Support during Juno

EPSC had a session devoted to amateur contributions to astronomy, and this year Glenn Orton presented plans for earth based support of Juno in the years preceding the mission.  Juno's suite of eight instruments misses spectral ranges that could provide crucial contextual information. For example, although the near infrared instrument will cover the 2-5 micron range, and the microwave instrument covers beyond 1.3 cm, there's nothing in between (the mid infrared). The visible camera eill have no science-grade calibration.  Furthermore, Juno will be so close to Jupiter that data will come in narrow north-south strips, missing the global spatial context that telescopes on earth could provide. Finally, the Juno mission is short, and given that we know Jupiter's atmosphere varies over long timescales, we may miss the temporal context of our observations.

The Juno team will address these issues via a coordinated campaign of ground-based imaging, from both professional and amateur observatories.  Whole-disk images, at the same time as the microwave observations during orbits 3 through 8 (November 2016 to January 2017), will provide this context for the connection between the lower troposphere and the upper clouds.  Furthermore, ground based images may allow them to predict the location of features, allowing orbits to be tweaked to redirect remote sensing efforts. 

Real time support will be challenges, as Jupiter will be only 35 degrees from the sun in November 2016, so the team emphasises the need for contextual studies during the preceding apparition, before solar conjunction.  Spatially resolved visible and near infrared spectroscopy are desirable, filling in the gaps and calibration of JunoCAM and JIRAM. But the team are aware that all this will be done on a best-effort basis, with no formal contracts for image reduction and calibration, relying on the passion and expertise of amateur observers worldwide to support the Juno mission.

Personally, I don't think they'll have a problem! 

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