That would leave a multiple flyby mission as the likely proposal. If a multiple Io flyby mission is compelling, then why not a multiple Europa flyby? Such a mission would not provide all the information needed to select landing sites for future missions and would not come close to replicating the depth of science a Flagship Europa orbiter would provide. During each flyby, however, such a mission could image the surface in greater resolution and coverage than the crippled Galileo orbiter was able to enhance our understanding of the processes shaping the surface. It could carry a modern infrared spectrometer to analyze surface composition in greater spatial and spectral resolution to look for locations where subsurface ocean material may have been carried to the surface. A radar sounder could measure the depth of the icy shell along the ground track below each flyby path. A magnetometer would seem a likely instrument to measure the interaction of the ocean with Jupiter's magnetosphere and continue measurements begun by Galileo.
Two types of orbits around Jupiter might be considered. The first might be a highly inclined orbit such as the one proposed for an Io multi-flyby mission that would avoid Jupiters equatorial radiation belts except for the brief time of each Europa flyby. The second would be a Galileo-style equatorial orbit that just touches Europa's orbit at each perijove. If this orbit was chosen, the spacecraft could also do multiple flybys of Ganymede and Callisto as an extended mission.
The kind of mission I've described would not replace dedicated orbiter for either Europa or Ganymede. The option to continue study of the Galilean icy moons on a Discovery budget, however, might be compelling and could mean that we don't ignore these worlds in the coming decade. The following charts from the Jupiter Europa Orbiter planning documents give an indication of the types of coverage that might be possible with a multi-flyby mission. (The JEO mission is no longer feasible given its cost and NASA budgets.) The JEO encounters with Europa were designed to pump the orbit around Jupiter down to enter orbit around Europa. As a result, the encounters occur over the same equatorial real estate. A mission designed to maximize science from flybys presumably would vary the encounter geometry with coverage more like what JEO would have done at Ganymede and Callisto.
Example of coverage from multiple flybys for four Io, six Europa, six Ganymede, and nine Callisto flybys from the planning for the Jupiter Europa Orbiter (JEO) mission. The imager on a Discovery mission might be less capable than that planned for JEO and might image smaller portions of the moons at these resolutions. All images from http://www.lpi.usra.edu/opag/feb2010/presentations/Senskev8.pdf
Example image resolutions for Europa from JEO flybys.
Example image resolution for Ganymede JEO flybys.
Editorial Thoughts: If a multi-flyby Europa Discovery mission were proposed, I worry about whether the science would be compelling enough to compete against missions proposed for other destinations. A multi-flyby mission would advance our understanding of Europa, but might not answer the fundamental questions the science community has. If not, then an eventual orbiter would still be needed. The same arguments could be made about a multi-flyby Io mission or the proposed Journey to Enceladus and Titan Discovery mission. In the case of Io, the radiation levels are so high that a follow on orbiter mission is all but inconceivable. The question, then, is when to fly a multi-flyby mission. In the case of JET, it's two instruments provide measurements that fill gaps in the Cassini mission's measurements and neither instrument requires global coverage to add significantly to our knowledge. Even so, I worry about whether review teams will consider it compelling enough for a $6-700M mission (with launch costs).
One concern for a Europa multi-flyby mission would be acquiring coverage of both hemispheres. To minimize radiation exposure, the mission likely would have its perijove, and hence maximum radiation exposure, at the orbit of Europa. (If radiation was not an issue, the perijove could be inside the orbit of Europa and have encounters on both the inbound and outbound legs of its orbit to image Titan on both hemispheres.) This makes it difficult to change the encounter geometry over a reasonable mission lifetime. Gravity assists could be used to walk the perijove around, but that would entail additional costs for a mission operations staff and a longer flight. (Editorial note: I was surprised at how expensive mission operations for the multiple gravity assists for an Enceladus orbiter would be -- it would be a substantial portion of the Principal Investigator's budget for a Discovery mission.) Perhaps mission designers have a good solution to this problem.
If I can see issues with a possible mission, then a team of experienced scientists and mission planners will have seen them, too. If an Europa Discovery mission has been proposed, it likely has clever solutions to these issues.
If ESA's Jupiter Ganymede Orbiter is selected for flight, then I hope it's mission would be enhanced to include a number of Europa flybys. One mission designer I've talked to says that the additional cost likely would be low. In this case, the mission is justified by the in-depth, global measurements at Ganymede. Flybys of Callisto and Europa would make nice bonuses.