Inner circles at NASA continue to probe a set of outer planet flagship missions.

New word from James Green, Director of the Planetary Sciences Division at NASA Headquarters, is that phase-2 studies will focus on a Europa Explorer, a Jupiter System Orbiter, and a Titan Explorer.

Moreover, these studies will be carried out in cooperation with the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA).

There’s no question that coming to grips — or is it gripes? — regarding where to send an expensive outer planet flagship mission is a challenge. It looks like NASA can cough up roughly $2 billion for such a mission, a figure that likely puts a squeeze on how ambitious a new outer planet mission might be.

Collaborating with other nations on an outer planet flagship mission also brings up timetable issues, multi-nation processes and sticky International Traffic in Arms Regulations (ITAR) complications on the U.S. side.

Fran Bagenal, chair of NASA’s Outer Planets Assessment Group (OPAG) steering committee, tells me that international collaboration has always been understood to be part of the process. But how it is to work remains unclear…an issue that’s been noted within the OPAG membership.

NASA’s Green has informed the space science community that the space agency is in the process of developing new ground rules, plus the necessary programmatic changes that will be needed to include international collaborators in moving forward on a new outer planet mission.

The plan is to start working with ESA and JAXA, Green said, as soon as possible in 2008.