After that time, NASA slipped the launch date to around 2020. The stated reason was to align NASA's schedule with ESA's schedule, which could not fit a launch before that into its funding profile. I suspect that the looming cost overruns of the Mars Science Laboratory might have made the delay on NASA's side inevitable. In any case, now that the MSL cost overrun is on the books, funding for preliminary studies of JEO are slim for the next couple of years. So, we have a 5 year push out in expected launch date over the last two years.
Could NASA pull in the JEO schedule if ESA does not select its potential contribution, the Jupiter Ganymede Orbiter (JGO) in 2011 to fly? Potentially. However, I suspect that it will take awhile for NASA to work through the problems created by the MSL slip and the earliest launch probably would be in the 2018 timeframe at best. Without access to NASA's funding expectations, this is speculation, however.
On another topic, what if ESA doesn't select JGO? How might the JEO mission change? The major loss from JGO would be the intense set of flybys of Callisto and the study of Ganymede from orbit about that moon. JEO has an excellent set of instruments for studying the moons of Jupiter during flybys. In fact, a series of flybys are planned of all four Galilean moons. JEO cannot carry enough fuel to orbit Ganymede as well as Europa, so the study of Ganymede from an orbiter would be lost. On the other hand, JEO could delay its orbit insertion at Europa to allow more time for more flybys of other moons. (More flybys than the planned handful at Io are probably out because of radiation issues.) JGO would spend approximately a year conducting 19 flybys of Callisto. JEO could spend an additional year carrying out flybys of either Callisto, Ganymede, or both. If such a plan was implemented, I would expect Ganymede to receive the lion's share of additional study. The scientific community has prioritized further study of Ganymede over Callisto.
Jason Perry has posted another excellent analysis on his website about what the selection of JEO with its planned Io flybys would mean for the proposed Io Volcano Observer (IVO). He correctly points out that IVO would conduct more flybys (at least 7, probably 14, and possibly even more) compared to JEO's 4. The flyby geometry selected for IVO would also be more optimised than JEO would be have. (JEO will use the Io flybys in part ot set up later encounters with other moons and hence is constrained in the selection of flyby geometries.) Jason points out that the proposed IVO instrumentation would be more optimized for Io studies than the proposed JEO instrumentation would be for Io. However, this may not remain the case. The proposed instrumentation for JEO is a strawman to show capabilities and allow conceptual design of the orbiter. The actual instruments will be selected from proposals submitted by scientists in several years time. It is very possible that the winning instruments will be tuned to do better at Io than the current strawman payload would.
Two missions to the Jovian system are on the official list for the next New Frontiers mission ($650M). One is an Io observer and the other a Ganymede observer. The selection of JEO and possibly JGO would seem to make the selection of the Ganymede observer unlikely in my opinion. JGO is the Ganymede observer done right. The Io observer faces hurdles in terms of being selected, too. As Jason points out, there are still valid reasons to fly IVO. The science it would return over what JEO will and may return has to be better (and at lower risk) than the science from all other proposed missions. That may not be possible. I would love to see IVO fly, but I have strong doubts that it will. (Note: IVO is a Discovery mission ($450M) and not a New Frontiers mission. However, I think this reasoning still applies.)
As my final thoughts, I'll speculate on what may be a crowded place the Jovian system could be in the 2020s. This year (2009) is the year of the moon with Chinese, Japanese, Indian missions there now. ESA just finished a lunar mission and NASA will launch its soon (with more to come). The 2020s could be the decade of Jupiter. NASA and ESA may have orbiters exploring the moons and observing Jupiter itself. The Japanese space agency, JAXA, is considering an orbiter to explore the Jovian magnetosphere. Russia is considering a lander for Europa. Other nations likely will have the capability to send missions to Jupiter. What might those craft do? Additional craft to explore the magnetosphere would be useful. A craft in polar orbit around Jupiter could study the polar regions of Jupiter, keep an eye on Io, and explore another corner of the magnetosphere.
Unless new news comes out about JEO, I'll take a break from this topic for a bit. In the next few weeks, the NASA FY10 budget will be released and will show the new administration's priorities for planetary exploration. There will be meetings of NASA's scientific advisory boards for Venus where a Flagship mission will be proposed, the Outer Planets where we may learn more about the issues that led the selection of Europa Jupiter over Titan Saturn, and Mars where replanning the exploration roadmap for the next decade will begin. I also want to complete the discussion of concepts for missions for the next New Frontiers mission selection.