Jovian Magnetosphere Science from EJSM

Across the universe, plasmas are the dominant form of baryonic matter (that is the particles that make up the universe we can see and touch).  Many missions have been flown to investigate the (magnetic) fields and (charged or ionized) particles within them, both around our own planet and around the sun and other planets.  This blog entry continues the series that looks at contributions that the Europa Jupiter System Mission can make beyond the studies of the primary targets, Europa and Ganymede.  Although this entry will touch upon those worlds since understanding the magnetic and radiation fields at those two moons are key to exploring them.  The interaction of these moons with Jupiter's magnetosphere is a key method to explore their interior oceans, and the radiation delivered to Europa's surface may be a key source of energy to create the organic compounds that might enable life within its ocean.

Slides are from a presentation given at the last OPAG meeting in February (http://www.lpi.usra.edu/opag/feb2010/presentations/KruppEJSMSynergisticScience.pdf).

Before I get too far into the topic, I'll be the first to admit that I lack the training in physics to properly understand this field, so I apologize if I don't go into depth (and for any mistakes I make).  This is also not a topic that receives a lot of public attention.  There's no pretty pictures (beyond auroras) and little that we can compare to our everyday lives.  However, Jupiter offers an extreme laboratory to study magnetospheres and their plasmas.  To quote from Wikipedia (http://en.wikipedia.org/wiki/Jupiter#Magnetosphere and http://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter):

"Jupiter's broad magnetic field is 14 times as strong as the Earth's... making it the strongest in the Solar System (except for sunspots). This field is believed to be generated by eddy currents — swirling movements of conducting materials—within the metallic hydrogen core. The field traps a sheet of ionized particles from the solar wind, generating a highly energetic magnetic field outside the planet — the magnetosphere. Electrons from this plasma sheet ionize the torus-shaped cloud of sulfur dioxide generated by the tectonic activity on the moon Io. Hydrogen particles from Jupiter's atmosphere are also trapped in the magnetosphere."

  "The action of the magnetosphere traps and accelerates particles, producing intense belts of radiation similar to Earth's Van Allen belts, but thousands of times stronger. The interaction of energetic particles with the surfaces of Jupiter's largest moons markedly affects their chemical and physical properties. Those same particles also affect and are affected by the motions of the particles within Jupiter's tenuous planetary ring system. Radiation belts present a significant hazard for spacecraft and potentially to humans."

Both NASA's Jupiter Europa Orbiter and ESA's Jupiter Ganymede Orbiter will carry advanced sets of instruments for studying magnetic fields and their plasmas such as megnetometeris, plasma/energetic particle packages, UV spectrometers, plasma wave instruments, and ion/neutral mass spectrometers.  The JGO spacecraft may also carry an energetic neutral atom (ENA) camera (http://en.wikipedia.org/wiki/Energetic_neutral_atom) to image particles within the magnetosphere similar to the instrument carried by the Cassini spacecraft at Saturn.

The true power of the mission, though, comes from having two spacecraft in orbit around Jupiter at the same time that can measure conditions at different places within the magnetosphere.  That capability may be substantially enhanced with the addition of a dedicated Jupiter Magnetospheric Orbiter supplied by the Japanese JAXA space agency.

This is the third in a series of posts that look at the science of the EJSM mission.  You may also want to check out EJSM Jupiter Science and EJSM Satellite Flyby Science.

EJSM Satellite Flyby Science

Mercury as imaged from the MESSENGER spacecraft in January 2008.  2.8 km/pixel resolution at the equator.

This week's blog entry continues looking at the science the Europa Jupiter Science Mission (EJSM), which could consist of at least NASA's Jupiter Europa Orbiter and possibly ESA's Jupiter Ganymede Orbiter. (See this blog entry for the introduction to this series.)  Both missions still face a selection hurdle.  JEO must be prioritized by the Decadal Survey and JGO must win out in a selection against two astronomy missions.

Prior to entering orbit around their namesake moons, both craft would perform a number of flybys of the Galilean moons.  Messenger's flybys of Mercury and Cassini's flybys of Saturn's moons have shown what modern spacecraft with modern instruments can do from quick encounters.

JEO and JGO would bring three key assets to a tour of the Galilean moons:

• They would have working antennas to return torrents of data compared to the trickle of data from the Galileo orbiter with its crippled antenna
• They would carry instruments new to studying icy moons including laser altimeters and ice penetrating radars
• They would have modern instruments that would be three decades more advanced than the instruments carried by Galileo

Perhaps the single greatest accomplishment these craft could perform would be to extend our high resolution coverage of these moons.  This map shows the resolution of the current coverage for Ganymede following the Voyager and Galileo missions:

The next map shows the resolution of coverage that would be possible from the current, nominal JEO mission, which may well be improved as mission design progresses:

The strips of high resolution coverage would be the ground tracks beneath JEO as it makes its closest approach.  The JEO instruments would be designed to capture data as the craft orbits Europa, and this ability to acquire data rapidly apparently would be employed to provide very high resolution images beneath the ground tracks during flybys.  The cameras would also be able to acquire images at greater ranges (and lower resolutions), much as the Cassini cameras have for Saturn's moons.  Certain instruments, such as the laser altimeter and ice penetrating radar, however, would operate only during closest approach.

The following chart shows the coverage the nominal JEO mission would provide for each moon from the flybys.  IPR is the ice penetrating radar and LA is the laser altimeter.

Of the two craft, JEO would have the more capable instruments (part of what the ~$3B JEO price buys compared to the ~$1.2B JGO price).  The following two maps show image coverage of Callisto following JEO's nine and JGO's fifteen flybys of Callisto:

The flyby science campaign would provide incredible advances in our understanding of these moons.  However, it is important to place the return in perspective.  The orbital geometry of the missions mean that the high resolution coverage occurs over only a single hemisphere for each moon.  It is my understanding that with time, the orbital geometries could be changed to view the opposite hemisphere and the science analysis group as asked if this could be reasonably done.  I expect that it is a tradeoff between more science from flybys versus the risk of the craft not surviving for their prime missions in orbit about their respective moons.

Multiple flybys also aren't a substitute for studies from orbit to understand the processes that have formed moons.  An orbiter can image the entire surface in high resolution and selected portions in very high resolution.  Certain instruments crucial to understanding the structure of the surface and interior such as the laser altimeter, ice penetrating radar would gain relatively little from flybys (see how limited the ground tracks would be in the chart above), but could provide global coverage from orbit.  Should NASA and ESA decide not to fund Galilean satellite orbiters, a Cassini-like spacecraft could substantially enhance our knowledge of these moons, but future orbiters would still be needed in the future.  For comparison, imagine if instead of orbiters around Mars, we had only a dozen flybys of that world by Cassini.  We would have global maps of what was there, but likely only a limited understanding of the processes that created the surface.

It's also reasonable to speculate how the missions might change if only one of the craft eventually flies.  JGO would not be designed to handle the radiation fields inside the oribit of Ganymede, so it would be difficult for it to add significantly to our knowledge of Europa and Io.  The JEO mission, however, could be lengthened to provide additional flybys of Callisto and Ganymede to extend high resolution coverage.  In theory, additional Io flybys could also be done, but the tradeoff there would be the additional radiation exposure that each of those flybys would cause.

References:

All images except the opening Mercury image are from these two presentations made at the last OPAG meeting:

NASA-ESA Outer Planet Flagship SDT
Ron Greeley, Arizona State University
Olivier Grasset, Université de Nantes


EJSM Satellite Science
David Senske, Jet Propulsion Laboratory

Note: Jason Perry has excellent summaries of what JEO could do during its flybys of Io at 

Io Science with EJSM

Io Science with EJSM Part Deux

Io Science with the Jupiter Ganymede Orbiter

Animation of the Jupiter Europa Orbiter's Four Io Flybys

JEO & JGO Jupiter Science - Overview

The two spacecraft planned for the Europa Jupiter System Mission -- NASA's Jupiter Europa Orbiter (JEO) and ESA's Jupiter Ganymede Orbiter (JGO) -- will focus on in-depth studies of those two moons with intensive studies conducted from orbit around their respective targets.  Both craft will spend 2-3 years in orbit around Jupiter before settling into their final orbits around their destination moons.  During that time, they can replicate science that Cassini has been performing for the Saturn system.  Jupiter has already had one orbiting spacecraft, Galileo, but it had mid-1970's technology instruments and a crippled antenna that reduced the data return to a bare trickle.  These two planned missions will have 2010's technology instruments and high data rate X-band and Ka-band communications.

This is the first of several blogs that will look at the science these two missions can perform prior to their ultimate investigations in orbit around their respective moons.  The missions studies that previously have been carried out focused on the studies from orbit, with only minimal attention to studies of the moons from flybys and of Jupiter from orbit.  That is beginning to change as the missions undergo further definition.

I believe that looking at what these missions can do prior to their Europa and Ganymede orbit science campaigns is instructive for three reasons.  First, both missions can carry out synoptic studies of the weather of Jupiter.  The Juno mission will study the interior of Jupiter and the weather in slices that will be tiny both in spatial and temporal coverage.  These two follow on missions can study the entire planet's weather for years.  Second, Io and Callisto will be studied only from flybys and distant observations.  The MESSENGER flybys of Mercury has shown what kinds of science can be performed by a modern suite of instruments.  And third, the present mission concepts are not guaranteed to fly.  NASA's mission is subject to prioritization by the Decadal Survey.  ESA's mission is in competition with two attractive astronomy missions for funding.  Supply problems for NASA with plutonium-238 to power their craft may force a radical restructuring of the mission with the result that more focus goes to remote studies.

The February OPAG meeting had several presentations on planning for the current mission concepts, with greater focus on NASA's mission (OPAG is a NASA-chartered group).  The presentations state that the current mission designs are preliminary and have had limited attention.  They suggest that the oribital tours are likely to be considerably refined as mission definition continues.

Click on any image for a larger version.

Currently envisions mission timelines. (From http://www.lpi.usra.edu/opag/feb2010/presentations/GreeleyEtal.pdf)

Example JEO tour showing satellite encounters and distance and phase angles from Jupiter.  This tour design includes an Io science campaign (four science flybys) and a system science campaign that includes numerous remote observations of Jupiter and Io as well as flybys of the other Galilean moons.  JEO will also conduct magnetospheric studies (not shown). (From http://www.lpi.usra.edu/opag/feb2010/presentations/GreeleyEtal.pdf)

Example JGO tour with Ganymede and Callisto flybys and extensive monitoring of Jupiter and the magnetosphere. (From http://www.lpi.usra.edu/opag/feb2010/presentations/GreeleyEtal.pdf)

Example activities for one JEO orbit that includes a flyby of Ganymede and remote monitoring of Jupiter and Io.  The JEO and JGO spacecraft will be able to probe the atmosphere of Jupiter by tracking each other's radio waves as they pass behind Jupiter as seen by the other craft.  (From http://www.lpi.usra.edu/opag/feb2010/presentations/LockScienceScenarioModeling.pdf)

A science definition team (SDT) has been looking into issues relating to the current mission design.  Some items they suggest for further consideration include:

• Could the orbital tours be modified so that the apoapsis of some orbits occur on the day side of Jupiter?  This would allow exploration of additional portions of the magnetosphere and allow longer observations of the daylit side of Jupiter. 
• Can the orbital inclination be increased to allow observation of Jupiter's poles, the rings, and better coverage of the magnetosphere?

Editorial Thoughts: Either of these missions has the potential to do considerable Jupiter system science.  That science could be considerably enhanced if the Jupiter tour prior to beginning the orbit of the their target moons was extended.  This has been propsosed for JEO if the JGO mission were to launch later than expected.  The synergistic science opportunities with two orbiters is great enough that NASA is willing to consider lingering longer in orbit around Jupiter to wait for JGO rather than rushing to orbit Europa.  Should the JGO mission not be selected, extending the JEO Jupiter tour would make good science sense.  An extra year or two could enable a number of additional flybys of Ganymede and Callisto to partially make up the loss of the science from the JGO mission.  However, NASA has to balance this against the increased risk of a malfunction on the JEO craft if it delays its final science campaign at Europa.