Three missions are in contention:
Jupiter Ganymede Orbiter (JGO) - Would orbit Ganymede following an intensive campaign of Callisto flybys. Would compliment a NASA Jupiter Europa Orbiter (JEO) if that mission is prioritized by the Decadal Survey and funded by Congress, but could also fly as a standalone mission. Apparently no technology readiness issues. Estimated cost: 710M Euros.
International X-ray Observatory (IXO) - A next generation X-ray astronomy mission that requires contributions from both NASA and the Japanese space agency, JAXA. The two articles make it clear that the mission depends on novel X-ray optics, but less clear on whether these optics still represent a technology development risk. ESA estimated cost: 660M Euros.
Laser Interferometer Space Antenna (LISA) - Three spacecraft would fly in formation to become a highly sensitive gravity wave observatory. The technical challenges behind this mission are substantial, and ESA plans to fly a technology demonstration mission in 2013. NASA contributions apparently would be required to fly the mission. ESA estimated cost: 750M Euros.
The ESA costs do not include the funding of instruments that are usually paid for separately by ESA member nations sponsoring individual instruments.
The articles stress that a key challenge for the ESA selection will be lack of commitment from NASA and JAXA for their contributions to IXO and LISA. The recently completed U.S. astronomy and astrophysics Decadal Survey did not make either mission a top priority, but left open the possibility of a NASA collaboration. If ESA selects either mission, NASA would have to find funds in its budget to support the mission and Congress would have to approve the funding. (I don't understand the Japanese budget approval process and don't know how easy or difficult securing its commitment to IXO might be.)
Editorial Thoughts: Perhaps the hardest part about international collaborations for space missions is aligning the decision making and funding processes of multiple space agencies. From these articles, it appears that ESA would be taking something of a leap of faith to select either IXO or LISA this summer in terms of international partner commitments and technology readiness. From that perspective, JGO might be the programmatically least risky choice. Should NASA not proceed with its JEO, then ESA would have the exploration of the Jovian icy ocean worlds to itself for the coming two decades. (I suspect that if JGO is selected and JEO is not flown, that JGO's mission goals would be expanded to include flybys of Europa. I've been told by a JPL mission planner that enhancing a Ganymede orbiter to perform Europa flybys should be relatively straightforward to do.) ESA will have the benefit of knowing NASA's planetary priorities (expected announcement in March) before making its large mission selection.
However, if ESA selects either IXO or LISA, there likley would be considerable pressure on NASA to find funds to support the mission -- both would be great missions. NASA is also considering partnering with ESA on other astronomy/astrophysics missions, and a larger program of cooperation might be put on the table.
Issues of international collaboration also may influence NASA's planetary priorities. NASA and ESA have a cooperative Mars program that currently includes the 2016 Mars Trace Gas Orbiter (MTGO) and the 2018 ExoMars and MAX-C rovers. It's unclear whether MTGO is a committed mission or is subject to Decadal Survey prioritization, but the 2018 rovers mission clearly will be among the missions that the Decadal Survey will prioritize. ESA does not have the funding to fly the ExoMars mission on its own -- it depends on a NASA launch and a NASA entry, descent, and landing vehicle. That puts the Decadal Survey in the position of either funding its contribution (I have not seen an estimate, but it would be hundreds of millions of dollars) or leaving the ExoMars rover -- an excellent mission -- stranded. The decision of whether to fly the NASA MAX-C rover as the first element of a Mars sample return campaign may be made separately. The Survey could recommend no 2018 Mars mission and leave ExoMars stranded, flying the 2018 mission with the ExoMars rover only, ExoMars and a NASA geophysical station, ExoMars with NASA's MAX-C rover, or ExoMars with NASA's MAX-C rover and a simple NASA geophysical station. (Hey, if all this was easy, there would be no need for this blog and I would have to find another advocation.)
Mars program planning aside, the two articles mentioned above make me more hopeful about the chances of JGO being ESA's selection is summer.