Thursday, July 14, 2011

"A Pragmatic Path to Investigating Europa’s Habitability"

Surface of Europa's Icy Shell (NASA)

There's a new plan in the works to enable missions to Europa that bears a resemblance to a proposal first put forward in a European study and to a proposal put forward in a Decadal Survey White Paper.

Europa has proven to be such a hostile world to explore that it has wrecked a flotilla of proposed missions before any were approved for development.  My library contains papers and reports on a dedicated mission to explore this moon and its ocean that date back at least fifteen years.  Any mission is challenging -- deliver a spacecraft with a battery of high data rate instruments deep into Jupiter's gravity well and survive harsh radiation for weeks or months.  Estimates of mission costs have run from ~$1B (a challenge goal, not met) to a behemoth that would have dwarfed any mission ever launched to the planets.  The most recent was the Jupiter Europa Orbiter (JEO) that would have flown a highly capable spacecraft to orbit that moon for the better part of a year.  Unfortunately, the estimated $4.7B price tag was politically unfeasible.
In an abstract ("A Pragmatic Path to Investigating Europa’s Habitability") for the upcoming (October) EPSC-DPS Conference, we are getting our first look at what a revised Europa program may look at.  While the abstract is brief -- likely reflecting the limited time that has been available for analysis -- it's clear that a radical restructuring of the mission is being proposed.  This work in progress comes from a NASA-chartered Europa Science Definition Team (ESDT).  At the conference, they will "will report on the status of this evolving concept, and will solicit community feedback."

The mission they are considering would split the goals for Europa studies between two spacecraft.  One would orbit Europa with a "very small" geophysics payload to perform those measurements that can "best carried out from Europa orbit."  A second spacecraft would orbit Jupiter and carry out imaging and other remote sensing during multiple flybys.  The abstract authors note that this minimizes the capabilities required by the Europa orbiter, lowering costs related to radiation hardening.  The multi-flyby spacecraft would need far less radiation hardening, and would have substantial time between flybys to relay the large volumes of data that would be collected by the imaging and spectral instruments.  The two spacecraft would be "phased in time", suggesting that they would be launched separately, perhaps years apart.

Editorial Thoughts: This new mission concept resembles the two spacecraft concept proposed by a European study. In that proposal, a small spacecraft would orbit Europa and use a second Jupiter-orbiting spacecraft to relay the Europa data and conduct independent Jovian studies.  This approach both minimized the size of the spacecraft that needed to be hardened to withstand the radiation at Europa, and reduced the power and communications capabilities required by that spacecraft.  While discussions about Europa missions often focus on the radiation (literally lethal to electronics), providing the power and communications systems to relay gigabytes of data in near real time during the short orbital life is equally a cost driver. Putting the high data rate instruments on a flyby spacecraft reduces the power and communications requirements because the data from each flyby can be sent in the days to weeks between encounters.

A Decadal Survey White Paper authored by David E. Smith of the Goddard Spaceflight Center also proposed splitting the JEO goals among multiple smaller orbiters.  His proposal did not include a multi-flyby spacecraft for remote sensing.

While the abstract doesn't discuss which instruments would be on which spacecraft, I'll hazard some guesses based on the requirements set forth by the JEO science team:

Europa Orbiter

  • Laser altimeter to measure tides in the icy-surface
  • Ice penetrating radar to measure ice depth and look for liquid pockets within the ice 
  • Radio science to measure mass distributions within Europa
  • Magnetometer to study magnetic fields induced by the ocean
Multi-flyby Spacecraft

  • Multi-color cameras
  • Visible-Infrared imaging spectrometers to measure surface composition

The idea of a multi-flyby spacecraft suggests some interesting possibilities.  If ESA selects the Ganymede orbiter as its next large science mission, it could be tasked with the remote sensing flybys.  The proposed mission already calls for a number of flybys of Callisto before orbiting Ganymede.  A mission architect who has looked at Jovian moon missions has told me that it would not be difficult to add the additional radiation hardening needed to enable a number of Europa flybys.

If ESA doesn't select its Ganymede mission proposal, then NASA may revive its own similar proposal.  Or the multi-flyby spacecraft could be kept as a Jupiter orbiter and used first for an Europa campaign and then for Ganymede and Callisto flyby campaigns.  (Io flybys would require additional radiation hardening that might drive costs too high, although flybys at the end of mission might be conceivable.)  

It will be interesting to see which mission the ESDT recommends for flight first.  The Europa orbiter would return unique measurements that don't duplicate the limited data previously returned by the Galileo mission.  On the other hand, the multi-flyby spacecraft would enrich our understanding of Europa and allow better planning for an orbital mission.  It's also possible that the orbital mission might carry a camera for higher resolution, stereo imaging a few high priority sites identified by the multi-flyby spacecraft.

NASA will not be in a position to decide on its own mission(s) for at least a year or two at best.  In the meantime, ESA will decide on its Ganymede orbiter, and NASA can make its decision in light of ESA's plans.  There is no reason why the Europa orbiter could not be an international mission with contributions from two or more space agencies.  Budgets are tight everywhere, and pooling resources makes sense.


A Pragmatic Path to Investigating Europa’s Habitability (EPSC-DPS abstract)

System concepts and enabling technologies for an ESA low-cost mission to Jupiter/Europa (ESA study published 2005)

A budget phasing approach to Europa Jupiter System Mission Science (Decadal Survey White Paper)  

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