I thought about adding Ulysses - but in terms of the science people are talking about here, I thought it would ignore it.

Doug

Don't forget the two Voyagers.

While there's been a lot of missions, this is the king of planets with 5 planet-classed bodies, many asteroid-classed moons, and a magnetosphere. Both Jupiter and Saturn could be the subject of almost endless missions. Just like the moon or Mars.

He was counting post-Voyager.

Having skimmed through these reports (I need to read them more carefully later) I strongly prefer JSO over EE, mainly thanks to its much better instruments, far broader coverage of the Jovian system and longer lifetime once in Ganymede orbit, compared to EE's lifetime in Europa orbit.

Just one close flyby of Io (and in fact there will be four) would return more and higher quality data than all of the Galileo Io flybys combined. There would be high resolution long term monitoring and atmospheric movies of Jupiter at multiple wavelengths, something Galileo couldn't do well because of limited bandwidth and which Voyager, Cassini and NH couldn't do properly due to lack of time. And once JSO is in Ganymede orbit Io can be monitored at 1-2 km/pixel for a long time.

It is true that EE would return a lot more Europa data - still JSO's Europa data would be vastly superior to the Galileo data. However, there will be a lot less data for everything else. And let's not forget that Ganymede is interesting. Not as interesting as Europa but still interesting.

That said, my 'post-Cassini' view is that I find the Saturn system more interesting than the Jupiter system. Before Cassini I expected the Jupiter system to be more interesting but Cassini has revealed a huge amount of interesting things.

Saturn does have some quirky little moons that Cassini has been able to take a close look at.
Are Jupiter's small moons really devoid of the same interesting variety, or is it that we have
not looked at them close enough to reveal their quirks? Could there be hidden gems among
them, puzzles like Enceladus' plumes, Iapetus' equatorial ridge, or Hyperion's spongy look?
Or have we seen enough already to know that no such surprises await a closer look?

Jupiter doesn't have any/many mid-sized moons, which is where most of the surprises have been coming from at Saturn. From what we've seen of smaller worlds, there's less opportunity to surprise. We won't know til we look, but exploring many of those outer jovian satellites would be tough with just one craft. Lots of empty space and wacky orbits out there. You would either need to burn a lot of propellant or have lots of little mini-craft zipping around.

I should clarify that the main reasons I find the Saturn system so interesting is that Enceladus and especially Titan turned out to be more interesting than I expected before Cassini (even though I knew Titan would be interesting). There's more out there that's interesting but these are the biggest factors.

Saturnian and Jovians systems - They're both interesting, they're both scientifically compelling, they both offer excellent outreach potential. Essentially, the decision has to come down to personal taste and ultimately, sadly, money.

Doug

How much more expensive would a Saturnian mission be over one to Jupiter? How much longer? Could it just come down to factions within NASA and the scientific community duking it out for their priorities will Dr. Griffin making the final decision?

On the surface of things, you would think that a Saturn mission would naturally take a lot longer to fly since it is so much more distant than Jupiter. But that's not necessarily the case. With the modern approach to do multiple gravity assists the trip times get all jumbled.

Galileo took a Venus-Earth-Earth path, and was in flight for 6.5 years to get to Jupiter.

Cassini took a Venus-Venus-Earth-Jupiter path and was in route for 6.75 years to get to Saturn.

Three months longer. Big deal.

And then it depends on your target. what if you are really asking is how long to attain Europa or Ganymede orbit, or be floating in the clouds of Titan? Well, the Jovian targets invlove a lot of sattelite flybys before final insertion, which can add 2-4 years. But many of the Titan scenarios I've heard about use aerocapture. In that case Titan becomes a much faster flight than Europa.

So the relative distances to Saturn and Jupiter turn out to be not that relavant. Just another counter-intuitive example in the wide wide world of space travel.

True - but there's one problem: A Jupiter gravity assist (JGA) to reach Saturn isn't always available. It's available for only a few years every ~20 years.

Looking at the near future a JGA is available if you launch around 2015 or so (I'm writing this from memory so it's not 100% accurate). If you miss that opportunity you have to wait until the mid 2030s or reach Saturn without a JGA which takes a longer time.

Does JPL or another agency maintain a launch opportunity "library", by any chance? Seems like the operations research folks should be doing just that: looking for optimal launch opportunities for various targets, and publishing them in order to solicit & align mission proposals with the realities of orbital mechanics.

Reason I ask is that the Galileo & Cassini transit trajectories were pretty complex, and at least in the case of Galileo this was certainly not the preferred option for a variety of other reasons. Seems as if determining all feasible options in this regard would be quite beneficial for future mission planning.

Well, probably just re-invented the wheel here, but given the inherent variability in planetary exploration initiatives just thought I'd ask...

Granted, but my point was that there are many ways to get out to the outer planets.

Even if the 'standard' Jupiter gravity assist is not available, remember Pioneer 11? It used a JGA, but took a very extreme route, much more circuitous than the Voyager probes four years later.

And some proposals include a solar-electric propulsion stage, presumably also using Venus, Earth or Mar gravity assists along the way.

Unfortunately it seems that we don't see many mission proposals (involving orbiters, at least) that use anything remotely like a good-old fashioned direct route like we use to Mars or Venus.

Sigh..... six or seven years to Jupiter instead of two. Heck, I'd settle for even three or four... I'm not greedy. Just impatient.

That's actually how Voyager came about, someone calculated that it would be possible to launch during the 1970's and hit all the major outer planets with one mission then pushed as hard as possible to get that mission flown.

The NASA technical reports server has all sorts of documents related to trajectory planning, one of the more interesting online documents is one written at a time NASA was considering creating a Solar-Ion drive 'common bus' for future space exploration. It covers outer planet opportunities for that type of spacecraft from Jupiter to Pluto for the period 1975-1990. (Download 11.9mb PDF)

One of the offline documents discusses missions that would flyby Saturn for gravity assist purposes before heading on to Uranus, that covered 1979 -1987, the next set of such opportunities will not occur till 2025, so there may be the chance to start a lobbying effort that pays off for the next generation of Space enthusiasts.

Yeah, I knew that; wasn't the guy who found that Italian?

What I'm asking is whether there's a dedicated group looking for opportunistic trajectories to the outer planets via possibly complicated inner planet pump-ups, etc. Does not appear to be a trivial effort, but seems as if such a group could generate some real, positive planning data for future outer planet missions.