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Uranus Orbiter, The other proposed ice-giant mission
Jyril
post Apr 22 2007, 05:08 PM
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And don't forget Neptune's second largest moon Proteus. It is larger than Mimas but still more or less irregular. Has it always been so?

We know so little about both of the planets so any mission to them would be scientifically very useful.


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David
post Apr 25 2007, 01:28 AM
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QUOTE (Rob Pinnegar @ Apr 11 2007, 03:28 PM) *
(3) Small inner satellites: how stable are their orbits? Do they hit each other from time to time as has been hypothesized?


Just the huge number of fast-moving inner satellites means that a close-in orbiter is always going to have something to look at.

QUOTE
But of course, it's going to be Neptune, not Uranus, because no politician is ever going to publicly support a mission that is going to get laughed at on the Tonight Show.


No American politican, I guess you mean. Happily, there are other countries which don't base policy on stupid puns (in somebody else's language).

QUOTE
If we want that to change, then we'd better think of a different name for Uranus. They shoulda named it Minerva or Apollo or something.


They did name it something different. "Georgium Sidus"...
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Spirit
post Sep 22 2007, 11:45 AM
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What about building two identical probes and launching one to Uranus and the second to Neptune? It will certainly lower the cost of the missions. And if we launch them at appropriate times, we can have the same team working on both projects - first on Uranus Orbiter and later on Neptune Orbiter.


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djellison
post Sep 22 2007, 12:12 PM
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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
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brellis
post Sep 22 2007, 04:37 PM
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A mothership with a bunch of micro-landers would be great. I wish Cassini had a few. Imagine dropping one into an Enceledan geyser!
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JRehling
post Sep 22 2007, 06:14 PM
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QUOTE (djellison @ Sep 22 2007, 05:12 AM) *
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.
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AscendingNode
post Sep 23 2007, 05:18 AM
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QUOTE (JRehling @ Sep 22 2007, 11:14 AM) *
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*
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JRehling
post Sep 23 2007, 05:56 AM
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QUOTE (AscendingNode @ Sep 22 2007, 10:18 PM) *
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.
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ugordan
post Sep 23 2007, 10:41 AM
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Regarding attitude-control propellant (i.e. monoprop for RCS thrusters), it is not the limiting factor, main engine biprop is. Cassini uses reaction wheels most of the time, only switching to thrusters if high slew rates are desired (mostly around closest approach observations) so it's pretty conservative about the RCS propellant. Even if that ran out Cassini would be able to do good science on reaction wheels alone. Apparently there's a way to offload built-up momentum solely via solar radiation pressure by taking advantage of the s/c assimetry. Using the very same thing that probably accounts for the majority of momentum buildup in the first place, rather neat.

It's the main engine fuel supply that, once exhausted, will lead to one of the endgame scenarios (regrettably, the crash-into-Saturn being most probable). Once you exhaust THAT, you can still control where you're pointed, but not where you're going.


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AscendingNode
post Sep 23 2007, 02:25 PM
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QUOTE (JRehling @ Sep 22 2007, 10:56 PM) *
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.

QUOTE (JRehling @ Sep 22 2007, 10:56 PM) *
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.
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ugordan
post Sep 23 2007, 03:00 PM
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QUOTE (AscendingNode @ Sep 23 2007, 04:25 PM) *
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.

You're neglecting the nontrivial fact of exactly how many Titan flybys do you need to even escape the Saturnian system? Escaping the solar system? Can you lay down some concrete numbers supporting that?

As much as mission ending scenarios like that are nice and dandy, romantic and all that, I join the others in thinking wasting the last drops of fuel on planning an escape trajectory when you could be doing what you came here for (squeezing as much Saturnian system science as you could out of an aging spacecraft) is pointless.

The minute Cassini ignited its main engine on July 1st, 2004 to brake into orbit, it effectively determined its final resting place. I'm all for making as much out of that fact as we can. We have only just began scratching the surface of discoveries over there anyway.
Trying to get Cassini over to Uranus would IMHO be akin to trying to slingshot MRO to Jupiter via repeated Phobos flybys after its mission is over.


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djellison
post Sep 23 2007, 05:21 PM
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Whilst orbital mechanics dictate that sending Cassini elsewhere is possible...it's not practically feasable, nor is it the best use of Cassini as a priceless scientific resource. It doesn't matter how long Cassini lasts - there will always be things for it to study in the environment for which its instruments were designed.

Doug
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tasp
post Sep 23 2007, 05:48 PM
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I admit a burning desire for a Uranian orbiter (heck, even a targteted Pioneer 11 flyby), but Cassini is not the craft for the job.


High value observations at Uranus:

*confirmation of current (or recent) geophysical processes on Ariel

*(I wouldn't rule out Titania in that regard either)

* darkening agent (ala Iapetus' CR) discoloring Umbriel

*(Umbriel is otherwise DOA, BTW)

* 'big mountain' on Oberon

* ring morphology changes since Voyager II (maybe ongoing durin mission!)

* Uranian magnetic pole shift

*deep infrared observations of Uranian atmosphere

* BACK SIDE OF MIRANDA !!!!!!
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djellison
post Sep 23 2007, 06:11 PM
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I think Neptune and Uranus orbiters are going to require a jump in technology before they become viable as anything other than a big flagship (and I don't think anyone would argue that we don't have enough on our plate in terms of targets for flagship missions already).

Aerocapture, NEP (or a large SEP for inner solar system accell), deep space flight tests Ka band - perhaps with a deployable HGA etc etc. All these things would make orbiters beyond Saturn a lot more achievable I think. Of course, we all want massive spacecraft to go everywhere, tomorrow...but we have to be realistic.

Doug
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JRehling
post Sep 23 2007, 09:07 PM
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QUOTE (AscendingNode @ Sep 23 2007, 07:25 AM) *
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.
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