The Outer Planets Assessment Group (OPAG), a community forum within NASA designed to help focus outer solar system exploration and to ensure that NASA mission decisions for the outer solar system have community input, released the results of a peer review of the Outer Solar System Flagship concept studies conducted and released last year. The peer review resulted in the downselection from four mission down to three. It had previously been known that the Enceladus mission concept was not selected for further study, but the document provides the rationale behind that rejection. Apparently, it was felt that the concepts developed did not adequately address the primary science goal of an Enceladus mission: to determine the biological potential of the south polar region. The other three missions, the Europa Explorer, the Jupiter System Observer, and the Titan Explorer, were given more favorable reviews with the Europa mission receive the best review.
For those who haven't seen them, there are pretty extensive PDFs covering each flagship mission concept on OPAG website. Io science is best dealt with by the Jupiter System Observer with four targeted flybys in the baseline trajectory. The baseline JSO mission also provides for repeated observations during the Jovian orbital tour as well as during the Ganymede orbit segment, where a top resolution of 775 m/pixel would be possible with the high-resolution camera. The Europa Explorer also includes a monitoring program (only before the Europa orbit segment of the mission), but EE as baselined never comes closer than 267,000 km to Io, providing images with a resolution of at best 2.67 km/pixel.
The other big news in the Downselection document is that NASA will be cooperating with ESA for the next phase mission concept studies. For this purpose, the Jupiter Science Observer and Europa Explorer missions have been combined into a single mission with two spacecraft (NASA providing Europa Explorer and ESA providing Jupiter System Observer). The Titan mission concept has been split between NASA and ESA with NASA proving the Titan Orbiter and ESA providing some type of in situ probe (either a lander or a balloon). Not sure what to make of this last bit of news. This might be good news since this could mean that both Europa Explorer AND Jupiter System Observer could fly as two component of a single, international mission, rather than only one being selected. With Europa Explorer getting favorable reviews and the favorite (at least IMO) to be selected, this could be good news.
Link: Downselection Results for 2007 Outer Planet Flagship Studies [www.lpi.usra.edu]