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.

[ugordan]

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.