Uranus Orbiter, The other proposed ice-giant mission Options

[Rob Pinnegar]

Since the Neptune Orbiter thread has started to veer into talking about a Uranus orbiter as well, it seemed like a good idea to start a topic for Uranus.

[djellison]

Sounds great! You paying?

Seriously - that'd be an ideal way to do it - but if they can't find money to do one of them, they're not going to be able to find the money to do both.

Doug

[JRehling]

Sounds great! You paying?

Seriously - that'd be an ideal way to do it - but if they can't find money to do one of them, they're not going to be able to find the money to do both.

Doug

Yes, I think just about any Outer SS science is looking marginal these days. Only the heavy-hitters (Europa, Titan, Enceladus, Jupiter and Saturn themselves) have a good shot at attention.

I will always be somewhat heartbroken that NH2 didn't come to pass, because the uranian flyby would have done an awful lot to address post-Voyager science there. With the extreme inclination and long seasons, the uranian satellites lose a lot of value as a potential target for the next *wince* 84 years. Given how long it was between Mariner 10 and Messenger, that sadly doesn't seem like such a long time.

I think the opportunistic use of a flyby en route to some deep-space (heliosphere) mission is the best bet that Uranus has, if we get a mission where the direction doesn't matter so much as the distance and we may as well point the ship at Uranus just to take advantage (and get a little gravity assist). But even such a mission would only carry nice camera in order to take imagery at Uranus, so it's not a total freebie.

Neptune/Triton carry a bit more clout, and lack the extreme seasonal constraints of Uranus, and I guess they'll get their closeup in, oh, about 30-40 years or so.

[AscendingNode]

I think the opportunistic use of a flyby en route to some deep-space (heliosphere) mission is the best bet that Uranus has, if we get a mission where the direction doesn't matter so much as the distance and we may as well point the ship at Uranus just to take advantage (and get a little gravity assist). But even such a mission would only carry nice camera in order to take imagery at Uranus, so it's not a total freebie.

Cassini could fly by Uranus as part of an end-of-life disposal. It could arrival anywhere from 2030 on (even at the equinox). There was another Longuski paper that talked about this at the AIAA/AAS Astrodynamics conference this summer. The big question, of course, is if Cassini could last that long. I hope they look into it to see if it could make it.

I've also calculated that you could launch something new horizon's sized to Uranus and not just do a flyby, but get it into orbit and do a tour. The flight time to Uranus would be 12 years, and the ops cost would probably be outrageous.... and you'd arrive at the solstice which wouldn't be great. So while it is possible with current launch and current chemical engines to do a Uranus mission, it is probably a lot of money to spend for a Uranus solstice mission.

And Neptune isn't even possible without some sort of new technology or 20+ year flight times. *sigh*

[JRehling]

Cassini could fly by Uranus as part of an end-of-life disposal. It could arrival anywhere from 2030 on (even at the equinox). There was another Longuski paper that talked about this at the AIAA/AAS Astrodynamics conference this summer. The big question, of course, is if Cassini could last that long. I hope they look into it to see if it could make it.

There's no way Cassini is going to visit any other planets, even if the theoretical possibility exists.

Its lifespan is primarily limited by the attitude-control propellant that is used up with every occasion in which the spacecraft is pointed in a particular direction (eg, for imaging during a satellite flyby). Now that it's in saturnian orbit, with Titan and Enceladus becoming more interesting the more we find out about them, the team is not going to stop using that attitude-control propellant on anything but Titan and Enceladus until the tank is empty. Saturn, the rings, and the other moons may get a few pictures here and there, but Titan and Enceladus will deservedly dominate the extended mission(s). Uranus can't hope to steal the show from those two heavy-hitters, especially with the exceptional risk involved in a 20 year cruise with the attitude-control propellant almost spent.

Put it this way: Imagine an upcoming mission could visit either Uranus or Titan and Enceladus. Which target would be more attractive? Now imagine that the craft is already AT Titan/Enceladus. The decision is extremely lopsided.

[AscendingNode]

There's no way Cassini is going to visit any other planets, even if the theoretical possibility exists.

Its lifespan is primarily limited by the attitude-control propellant that is used up with every occasion in which the spacecraft is pointed in a particular direction (eg, for imaging during a satellite flyby). Now that it's in saturnian orbit, with Titan and Enceladus becoming more interesting the more we find out about them, the team is not going to stop using that attitude-control propellant on anything but Titan and Enceladus until the tank is empty. Saturn, the rings, and the other moons may get a few pictures here and there, but Titan and Enceladus will deservedly dominate the extended mission(s). Uranus can't hope to steal the show from those two heavy-hitters, especially with the exceptional risk involved in a 20 year cruise with the attitude-control propellant almost spent.

It don't think it would be a terrible hardship to reserve enough propellant for a single Uranus encounter. In any disposal scenario, there is going to still be propellant in the tanks (both monoprop and biprop). And if that propellant is need to insure a safe disposal, there's going to be margin on top of what is needed. And I maintain that that margin may be enough for worthwhile pointing during a Uranus encounter.

Also the spacecraft id disposed of as soon as it leaves Saturn.... (the spacecraft is guaranteed to escape the solar system after a cleanup maneuver) so the risk analysis only goes that far. Uranus can be viewed as a "lucky" event to be tried for, but that doesn't have to be insured.

Anyway, this can be calculated and verified along with other lifetime constraints... and the merits and demerits of a solar system escape disposal can be discussed if Cassini is interested in doing this.

Put it this way: Imagine an upcoming mission could visit either Uranus or Titan and Enceladus. Which target would be more attractive? Now imagine that the craft is already AT Titan/Enceladus. The decision is extremely lopsided.

An upcoming mission to Titan/Enceladus would have to have better instruments than Cassini or do something different (like go into orbit) to be worthwhile.

It still is a valid argument that a mission to Saturn should stay at Saturn. But I just point out this alternative because it is interesting.... and because I don't think it should be ruled out without deeper study... I think the potential payoff is high enough to give it a shot.

[JRehling]

An upcoming mission to Titan/Enceladus would have to have better instruments than Cassini or do something different (like go into orbit) to be worthwhile.

Expanded SAR coverage of Titan is reason enough to keep directing flybys that way. The fraction that has been covered by SAR is going to be about 25% when the main mission ends, and has just about no prospect for reaching past 45% when the extended mission ends. With that fraction mapped, we will have a peek at representative terrain for all of the major terrain types, but Titan is a place where a unique and fascinating feature may just be hiding in the last 10% that we'll get around to SAR-ing.

Titan's seasons (and Saturn's) are also an abundant justification for a long follow-up there. We haven't even seen Titan's north pole yet in ISS/VIMS.

In terms of terrestrial seasons, Cassini arrived when Saturn and Titan were at February 1, and 4 years of extended mission would only take us to about May 10. It will take 7 years of extended mission before we get to see Titan experience a solstice. As the seasons change, who knows what we'll see. Maybe the lakes in the north will shrink... or grow. Maybe the south will flood. Maybe midlatitude rains will lead to storm wash hitting the equatorial sand seas. Cassini's got the instruments to tell us quite a bit, but we have to stick around for 30 years before we've seen every season on Titan (15 years if there's north-south symmetry).

Enceladus also has more to show us, like the temperature profiles across the tiger stripes when they are in the darkness of seasonal polar night. And more observations to determine if there is temporal variation in the geysers.