As soon as MESSENGER gets to Mercury, the most poorly explored planets in the solar system will be Uranus and Neptune. Could this lead to a revival of interest in the ice giants and their retinue, in the same way that the existence of New Horizons is perhaps partly due to the Pluto stamp*?
*via Pluto Fast Flyby and later Pluto Kuiper Express
There is the opportunity, brought up in another thread, for a Jupiter > Saturn > Neptune tour with launch opportunities 2016-1018. It could then go on to visit one or more KBOs. A presentation on this opportunity as a New Frontiers candidate was made to a group reviewing the program in November, I think.
An ideal mission would drop a probe into the Saturn atmosphere as well as Neptune. It would also do a close fly-by of Triton. Don't know if the orbital mechanics will allow this and still do the probe relay.
"Economics engineering' seemed to be the key to Mariner 10 visiting Mercury (for the then bargain price of 98 mil). And was also a key factor in New Horizons and Messenger.
It might be a concatenation of ion drive, Sterling Cycle generators, follow on design revisons of existing hardware, some clever gravity assists, and the continued progression of Moore's Law that will bring a capable and affordable probe to either/both planets.
Does anyone have a link to any of the JSN proposals? I searched several different ways, but always kept getting Voyager material.
I suppose even ion drive doesn't make an orbiter all that much better a proposition. That is, assuming you use conventional means to launch the thing to Jupiter, only depending on the ion drive (and maybe some aerobraking) to slow it down at the end. Even so, it probably still busts the budget for a New Frontiers mission.
Trouble is, I agree it's hard to justify another Neptune flyby.
--Greg
Do you think this: http://planetary.org/blog/article/00001285/ could help or even be a reason to go ?
Re: JSNK mission. Front this more as a Saturn probe mission with Neptune and KBO flybys as extras. IIRC, the Decadal survey has a Saturn atmosphere probe higher up on the priority list. Make it international with ESA supplying the probe and better yet most of the spacecraft instruments similar to Dawn. Don't see ion propulsion being required. Use Earth flybys to get to Jupiter so a low end EELV like Atlas 401 can be used. Use NH experience with hibernation during cruise to reduce operations costs until the Jupiter flyby. We have enough images from other Earth flybys, so don't even turn on any instruments (except for bring up tests) until Jupiter if that will save a million dollars or more.
How much of the cost of one of these missions is the launch vehicle? Is it possible that the Falcon 9 Heavy could be enough to put a Neptune (or Uranus) orbiter under the New Frontiers ceiling?
--Greg
[...]
I would consider aerocapture an enabling technology for both Uranus and Neptune orbiters. Get that sorted, and you don't have to trade fuel vs flight time quite so much.
Doug
I think they should build two spacecraft, and send them both to JS and then one to Uranus and one to Neptune.
Building the spacecraft simultaneously will keep the building and development cost low.
Then the time it takes to get to Uranus and Neptune, if breaking is much a problem, then the speed to get there should be lower, at least much slower than New Horizons gets to Pluto.
You know if anyone's done a serious study on a NERVA-style mission to deliver an orbiter to either Uranus or Neptune, Gordan? Interested in what the passage time would be.
Neptune has been ranked as a higher priority as ranked by the scientific community for a follow on mission because of the interest in Triton.
I looked at Titon's orbit relative to Neptune in the mid-2020's on the Solar System Simulator. As viewed from the sun (which I presume is the direction of arrival for a flyby mission), Triton's orbit remains well away from Neptune for this time frame. However, Triton's distance from Neptune is less than the distance of Io from Jupiter. A mission that targets a Triton close encounter still comes reasonably close to Neptune, and probably close enough to act as a relay for a Neptune probe. (However, the ammonia in Neptune's atmosphere will weaken the probe's signal; I don't know how this would effect this scenario.) In fact, a more distant flyby of Neptune than was done by Voyager might be required to give adequate viewing time for the probe relay.
I get 16 years for the Hohmann to Uranus, but just 30 2/3 to Neptune -- not 40. Calculating as follows:
1) Since the Hohmann is at 1 AU at perihelion and at the target planet at aphelion, the SMA of the Hohmann is (1+a)/2, where a is the SMA of the planet. 10.1147 AU for Uranus and 15.5518 for Neptune.
2) Kepler's third law says period is SMA to the 3/2 power, but we only want HALF the period (since we're done when we reach the target). That gives me 16.08422 years to Uranus and 30.6650 to Neptune.
As a cross check, using these numbers I calculate an 84-year period for Uranus and 165 years for Neptune, so I do think I have it right this time. :-)
Even so, 30 years is a long time to wait. So's 16. Add to that the fact that currently we can't even swing the delta-V for a Hohmann to JUPITER for a large probe -- much less one to Uranus or Neptune. That's why Cassini and Galileo had to make all those swings by Venus and Earth first.
Aerobraking, ion propulsion, and cheaper boosters all seem needed to get a reasonable-sized package to either destination in a reasonable time. On the other hand, all three look like they might be just over the horizon.
--Greg
But it would have missed Ida and Gaspra, and the cool photos of Earth and Venus.
On the other hand, the main antenna might have worked, and the probe would have followed the SL9 impacts from orbit...
Yes, but during the post-Challenger delay and replanning, they also discovered that the rocket motors were defective. Galileo would have gone the way of CONTOUR had it launched in 1986 or 87. In other words, things could have been a whole lot better, but they could have been a thousand times worse.
IIRC, the Galileo Jupiter trajectory provided by the Centaur stage would have included a nice flyby of Amphitrite, an otherwise obscure, largish main belt asteroid. Had that occured, it would have been the largest asteroid flown by till Dawn reaches Vesta in several years.
Uh, I hope they didn't plan to open the main antenna before firing the Centaur upper stage. :-)
Anyway, I've heard the switch away from the Centaur blamed for the antenna problems. (e.g. Carting the probe around for so long led the lubrication to dry out so the pins didn't release.) But I've heard other explanations too.
Biggest problem with the upper stage (based on reading "Taming Liquid Hydrogen: The Centaur Upper Stage Rocket 1958–2002") was that the shuttle couldn't actually lift the Centaur-G-Prime because the shuttle never achieved its originally planned lift capability. They were talking about throttling up to 109% instead of 104% for the Galileo and Ulysses launches. The astronaut crews were already calling Centaur "the Death Star" before Challenger exploded.
After Challenger, safety changes made the shuttle heavier, and any changes to the Centaur would have made it heavier too. If it was marginal before that, it was hopeless afterwards.
Anyway, the real problem wasn't failing to use the Centaur upper stage; it was using the Shuttle in the first place. Galileo should have been a Titan-Centaur launch, and I don't think there's much dispute over that now. Depending on the shuttle for launching unmanned probes turned out to be a huge mistake. It may have put us as much as 15 years behind where we'd otherwise have been.
But this is old news, long hashed over here. However, if you haven't read it, do have a look at "Taming Hydrogen." It's a great read.
--Greg
Actually, I think you're right. The only reason they folded the antenna up in the first place was to hide it behind a sun screen, and the only reason they had to do that was to protect it during the Venus flyby. Obviously they wouldn't have done a Venus flyby if they could have launched with Centaur.
Sorry about that. :-)
--Greg
Some years ago at a JPL open house I spoke with one of the engineers on Cassini. I asked about the reasoning behind using the Titan 4 vs. using the shuttle. It turns out it would have been cheaper to launch on the shuttle, but only by the way NASA does accounting. The price charged to the Cassini project would have been the processing fees and upper stage, and not the actual shuttle flight itself.
And it wasn't payload capacity or safety that were the main drivers either. It was schedule. The Cassini team figured they would rather rely on the Titan to get a launch accomplished during a planetary launch window. The shuttle, with it's frequent delays and down times was just too unreliable.
Not exactly on topic for this thread, but since everyone was discussing launch issues with the shuttle......
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Uh...say what?! We haven't launched a Galileo follow-up, which would be easy compared to Uranus/Neptune recon mission(s) (to say nothing of orbiters, the technical hurdles of which have been extensively discussed). We are extremely lucky that Voyager 2 did succeed, else I suspect that none of us now living would have ever seen these planets up close in any respect.
Think I know what you mean, but frankly it seems that you're overestimating the impetus for doing such missions. Remember that there was actually a serious proposal floated to turn off Voyager 2 while it was enroute to Uranus as a cost-cutting measure; thank God it didn't happen.
[...]
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Voyager 2 was only able to visit Uranus and Neptune because of the rare alignment of the planets which made the "Grant Tour" possible. No mission that could visit both ice giants is currently possible.
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One thing should be remembered when discussing about Uranus and Neptune systems: all the moons, including Triton have their axes almost coplanar to ecliptic meaning that parts of the moons are hidden from view for decades or more. The southern polar regions of Uranian satellites are now disappearing from view. If we sent a probe to Uranus today, it wouldn't see the same regions that Voyager 2 saw. On the other hand, having a probe there right now would have been perfect: currently the surfaces of the moons are fully visible because of the equinox. It'll take 40 years until the next time this is possible.
Come to that, Voyager itself was a scoped-down version of the Grand Tour, which IIRC would have also included Pluto had it been launched earlier.
There are many variables in UMSF, but none are so capricious (or, arguably, as influential) as budget environments. Still think that we were EXTREMELY lucky that the Voyagers flew at all.
You know, Nick, unlike a lot of the people here, you and I can recall when NASA and JPL were pushing hard to get the Grand Tour mission approved. I remember when it was first proposed in the late '60s, I remember when it was canceled, and I remember how elated I felt when its poor second cousin, a simple Jupiter/Saturn flyby, was "tweaked" into something approximating the original GT mission.
Also, IIRC, Voyager 2 could have been targeted for a Pluto flyby, thus completing the original GT mission plan -- but it would have lost its close flyby of Triton. The Pluto option was still possible after the Uranus encounter (though close to the limits of the remaining delta-V in the vehicle), but the craft was deteriorating (scan platform issues, among other things) to the extent that the decision -- and I think the right one -- was made to maximize science during the Neptune encounter.
When it came to deciding between doing as complete a recon of the Neptune system as possible with the healthiest spacecraft you could manage, or giving up Triton on the hope you'd still be operating well (or at all) when you got to Pluto, I think they did make the right decision.
-the other Doug
Nice account of the history of the Grand Tour project here:
http://history.nasa.gov/SP-4219/Chapter11.html
"Who killed Grand Tour? The demise of Grand Tour was less a simple case of its expensive price tag than its competition with other high-cost new starts (the shuttle and the space telescope) and Viking in a shrinking Federal and NASA budget. The smaller the budget became, and the more that costly programs competed for those shrinking funds, the more expensive each program appeared."
--Greg
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Well, don't we all, though? Sad fact of the matter is that planetary exploration budgets are very limited, launch opportunities to the outer Solar System are severely constrained, and harmonizing these two major domains of influence is anything but easy. We live in the real world, not the ideal one.
(Dammit; I am a geezer; now I'm lecturing!!! Gonna go drink beer till I reboot....)
[...]
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"...I'll always think of them as Mariners 11 and 12......"
My briefcase carries a VERY battered decal of "Mariner Jupiter Saturn". I'll have to take a photo of it and post it.
Dou you think that Alan will hate me if I privately refer to NH as "Mariner 13"...?
While imaging systems have certainly improved, Voyager's vidicon imaging was quite good. Much of the "inferior" quality comes from the use of old 1970s and 80s style processing and copying of the images. Also, many of the images are blown up to rather insane levels. The problem for Voyager with the Uranian moons was that when the planet is near solstice, approaching it from a flyby trajectory is like hitting a dartboard - you pass through the plane in which the moons orbit all at once, so you can only have one close encounter (and one more semi-decent one, such as Voyager-2 at Ariel), and all the encounters happen at about the same time. Since Voyager could only hold ~30 frames on its tape recorder and was limited by distance in what it could send in real time, the number of images that could be taken of the Urianian moons was limited. It was correct to say that Miranda would not have been picked for a dedicated Uranus mission - the Voyager team was quite frustrated by this, but it was the only moon that could receive a close flyby and still allow a trajectory that would send the spacecraft on to Neptune. Due to its small size, they were expecting it to be another Mimas, but by luck it turned out to be one of the most interesting worlds Voyager encountered. In fact, during the approach phase, when Voyager was bearing down on the Uranian system but not yet at closest approach, the lions share of the images were spent on Titania, which would likely have received the close encounter had they had the choice.
Voyager-2 at Triton was another spectacular encounter. It seem to me that the coverage seems in many classes cleaner than the coverage of the Galileans. While the increased speed, lower data rate, and lower light levels were an issue, when one looks at Voyager's early encounters, there is a lot of over and under exposure, partial (and total) misses of the target (a lot of close images of the Galileans were off the limb or on the dark side of the terminator), as well as smear from moving the spacecraft in the middle of exposures. While conditions at Neptune were more severe, by this time the Voyager controllers were veteran experts at operating the spacecraft and knew all its idiosyncrasies, rendering it almost like a new mission.
[...]
"I'd argue that Galileo was Mariner 13 and Cassini is Mariner 14."
You left two out: Magellan was Mariner 14 and Galileo was Mariner 15.
There are also two "Honerable Mention Mariners": the two Viking Orbiters.
I have a rather technical but straightforward technical point that this is based on.
ALL, repeat, ALL, the Mariner and Mariner like spacecraft up to Galileo used a polygonal ring shaped set of electronics and equipment bays as the "core" of the spacecraft. EVERYTHING else, damn near, was attached to the ring. On Mariner 4 (as the classiest example of the layout) solar panels were mounted on 4 of the 8 sides of the octagon (number of bays varied from design to design), antenna and magnetometer boom on the sunside, scan platform on the anti-solar-side. Mariner 9 mounted a bit set of fuel tanks and a rocket engine on the sunside, scan platform on the cold side, solar panels as usual. Viking orbiters added the attached bioshield and lander on the cold side, scan platform on the side of the enlarged polygon. Voyagers had no solar panels, RTG booms and scan platform on the sides instead, mounted the big antenna on the solar side. Magellan stuck a monstrously large radar electronics box between the (Voyager derived) antenna and the (I think Voyager derived) electronics bay ring. Galileo was the last to carry the electronics bay ring, despun scan platform on the cold side, the @#$#@ antenna on the sun side. It's spinning attitude control didn't change the fact that it was essentially a Mariner.
Cassini is the first "Mariner" that uses the modern "brick" shaped rectangular box for the main spacecraft body.
After the proposal of a New Horizons-like mission to Uranus and KBOs (NH 2), a new mission to Neptune is now proposed (NH 3 ?) :
http://www.lpi.usra.edu/meetings/lpsc2008/pdf/1117.pdf
Will surely have the same fate as NH 2, even if the mission is interesting.
Marc.
Oh Great Maker, let it be so....
(for the record, I'm a staunch atheist, but if prayer has one part in a trillion chance of success...
)
To me, any new mission to Neptune that isn't an orbiter will be a tough concept to sell. Spending considerable time and money on an essentially Yet Another Flyby mission. An orbital mission would be significantly costlier, but the increased science return would most likely far outweigh the cost increase.
[...]
I think that Uranus has a bigger problem. The bland appearance of Voyager photos doesn't help. Also, it lacks a big moon, like Triton. Both planets could be combined with Kuiper Belt flybys, but with Neptune, you get a large (if melted down) KBO right off the bat. Plus, Triton is active, meaning that looking for changes since Voyager is a selling point for a flyby sooner rather than an orbiter later. I am not saying that Neptune out-merits Uranus, I am saying that I think a Neptune flyby mission has more of a chance of happening, perhaps tied to a New Horizons followup.
Unfortunately, any mission with Uranus as primary objective will be cheap joke fodder on the late night TV shows and elsewhere in the US. It would be difficult to garner support for such a mission if it is being so ridiculed.
Neptune also has the advantage of being the most distant official major planet (This was also true in the Voyager days, since Pluto was inside Neptune's orbit at the time). To the bean counters and to the novice "only the second visit to the most distant planet" has a ring to it that Uranus would have difficulty competing with.
[...]
Heh, heh...I see that some of our more distinguished members are tuning into marketing, however reluctantly and/or facetiously...
I see it as another 'gapfiller' initiative, much like Messenger. Uranus & Neptune are midway between the terrestrial planets and Jupiter/Saturn in terms of mass. Oddly enough, they have surface gravities not much greater then that of Earth, making surface exploration (if there is one on either of them) a tantalizing far-future possibility...we need to learn more, to say nothing of the satellites of each, Triton being one of only 5 known volcanically active bodies in the Solar System.
(It's all about framing the issue, guys.)
I was counting Titan; the atmosphere is extremely powerful circumstancial evidence.
I still wish I could see a breakdown of costs. I keep wanting to believe that two identical orbiters (one for Uranus and one for Neptune) wouldn't cost twice as much as a single one, and that if one could time them right, the Neptune one would arrive just as the Uranus one reached the end of its life, so you could just keep more or less the same team. Even if the Neptune probe took 20 years to get there, it would just be the second act.
--Greg
Again, this is why I wish that there was a 'library' of outer-planet launch opportunities. Seems like trajectory calcs only happen when there is a viable mission proposal like NH in the pipeline; might have the cause & effect relationship backwards here.
If we knew that there were favorable launch opportunities for Uranus & Neptune (even with inner-system gravitational assists) in, say, the late 2020s, then draft mission proposals could start development now. With that much lead time, it's even conceivable that Frontier-class missions would be feasible given assumed technology advances.
[removed in-line quote]
There may or may not be such a library, but there are usually a couple or more papers each year at the AIAA/AAS Astrodynamics conference that discuss trajectories for future missions to places all over the solar system. Probably >95% of these come to naught but some folks are having fun cranking out the plots.
Not sure what is gained by a stretched development cycle. There is the risk that product of earlier development efforts be obsolete or difficult to support when launch date comes around.
There really is no need for a launch opportunity library for the outer solar system if you want to go from A to C via B. As long as B and C are on the "same side" of the solar system there'll probably be an extended gravity assist trajectory with a varying efficiency over a couple of years. Jupiter would typically be your B body and from then on tweaking the launch date is a piece of cake if you have constraints on launch energy, launch date and/or arrival velocity. These sorts of calculations can be done on demand in a matter of minutes I figure.
More complex slingshot trajectories (involving say Jupiter AND Saturn) to get to Uranus or Neptune will occur rare enough that it's probably no use predicting them that far into the future. Furthermore, they usually impose bigger trajectory constraints which then constrain the slingshot gains. It might prove more efficient to use just an aggressive Jupiter flyby to catapult yourself outward than trying to fly by both J and S for what can turn out to be a weaker boost in the end.
I believe trajectory search and optimization software nowadays uses Keplerian orbits and http://en.wikipedia.org/wiki/Patched_Conics to find plausible/optimum trajectories (and things like impact parameter or miss distance during flybys) and then when a "conceptual" trajectory is selected, it's precisely worked out by numerical integration and taking into accounts other factors such as solar light pressure, etc. That last part might be iterative and computationally more expensive, but it's all within reach of modern computers.
[...]
This has got me thinking about if you could pull off a Uranus/Neptune Orbiter for less than $850 million...
Aerocapture is obviously the way to go, and the Titan Explorer orbiter seems a good place to start. It masses ~1800 kg wet, while the cruise stage ~1500 kg, mainly fuel. If we drop the balloon and lander, we cut out roughly 2/3 the mass of the cruise stage, giving a payload mass of ~2300 kg. That's about half the mass of TE. The option then is either to use a low-end EELV (Atlas 401, Delta IV sans SRBs, or Falcon 9) for a single launch (more likely), or launch two spacecraft (one Neptune, one Uranus) on an Atlas 551 (much more cost effective, but less likely).
The point is, I don't think it's a given that a ice giant orbiter has to a ridiculously expensive Battlestar Galactica style mission like Cassini. It's just like Mars Sample Return; if you're willing to use new technology (aerocapture and ASTG in this case) and make compromises, you can turn a perpetually paper mission into reality...
Simon
I think the problem with launching a Neptune and Uranus probe in the same rocket is that the next obvious launch window for that is about 80 years away.
Of course, gravitational assists are complicated beasts; there could well be some complicated sequence that managed to split them up at the right point and get them both to the right places, but it'd be a wonder to behold.
--Greg
[...]
I thought about that, but I think the trouble is that one of the two is going to get a pretty lousy gravitational assist, and given the distances involved, that seemed like a loser -- and maybe not even enough delta-V to get both to their targets.
And, as you say, every year it gets worse.
--Greg
How does that follow? You'd clearly have less mass to launch if you didn't drop anything at the earlier planets.
--Greg
Well then drop the second vehicle and fly the one with a faster trajectory. Point is, I still think it's possible to build a very capable New Frontiers-class Neptune Orbiter...
Simon
I hear you. I'd love to see a serious proposal. Say, starting with $90 M for a Falcon 9 Heavy to put 12 tons in GTO. What can you do with 12 tons in GTO with about $800M to spend on it? And, assuming no other never-before-flown technology.
--Greg
[...]
"Sedna would be a somewhat more appealing target."
After the Synoptic Survey Telescope's nearly whole sky survey's been going for a year and they've had time to chew on data to spot slow moving 24'th magnitude objects over the whole sky. we're going to have a LOT more Sedna's and other very interesting objects, including some that will blow our mind. There remains the possibility of a considerably larger object way-the-<bleep>-out-there that's one of the possibilities being invoked for explaining the dynamic structure of the KB.
We can keep updating our personal lists of our "KB and Beyond objects we'd like to explore", but till around 2015, when results will be coming in from the SST, etc., all these discussions will be based on a tiny sample of what's in the KB
[...]
Yeah, the LSST will undoubtedly provide a LOT of surprises; gonna be interesting, to say nothing of paradigm-shifting (such investigations nearly always are, of course)...
Getting back to Uranus & Neptune, I'm beginning to think that NH evolutes for flybys might be the way to go given the current state of the art for propulsion & on-board power. As has been pointed out on this thread, any given portion of any of the Uranian moons is illuminated for a few decades or so over an 84 year period, so a flyby really is as good as an orbiter there. Hopefully some innovative proposals might be forthcoming from the community...?
"So we should find any Sednas (now the 5th largest KBO) out to about 200 AU."
I thought Sedna is in the Oort Cloud rather than the Kuiper Belt. Is that incorrect?
Sedna is between what were supposed to be KB orbits and Oort orbits.
That "supposed" theory has been rattled more than a bit by our expanding understanding of KB dynamics, but it's still got a perihelion way beyond the classical KB.
"You can't get there from here" ... "Tiz a Puzzlement".
Mike Brown has sometimes called Sedna an "Inner Oort Cloud object," speculating that it's one of a number of bodies between the Kuiper Belt and the Oort Cloud. I just can't WAIT for the LSST to show us what's really out there!
--Greg
I just had a probably very silly idea, and would like to bounce it off of our orbital dynamicists: What about using a cometary heliocentric orbit to reach Uranus or Neptune?
Suspect that the required delta-V might be prohibitively large, but here goes anyhow. What I have in mind is basically using the Sun for a gravity assist by doing a close flyby & throwing an orbiter into a cometary trajectory with its apogee tangental to the orbit of either planet; if the planet just happens to be there at the time, then presumably minimal deceleration would be required to enter orbit.
Very simplistic, and probably not practical (thinking you might need as much as a 50 km/sec maneuver to swing by the Sun close enough)..but thought I'd throw it out there anyhow.
You could try it this way:
http://www.physics.uci.edu/faculty/STAEF2002Desorp.html
"Gravity assist" by definition involves a second large body in addition to the Sun. You don't get anything by flying by the Sun in terms of digging out of the Sun's gravity well. In fact, you suffer a net loss. Basically, if the Sun is the only body whose pull is significant (until the craft gets to Uranus/Neptune), then the orbit is going to be a conic section. And the minimum energy involves never being closer to the Sun than necessary (ie, Earth, for launch).
The down side of a minimum-energy orbit is that it will take a long time to get there. But it would only be worse if you tried dipping inside the Earth's orbit. Anything you gained by falling sunwards you'll lose, exactly, on the way out.
Also, as Star Trek IV showed us, is that by going close to the Sun for a gravity assist, you will go forward several hundred years in time. We could load the craft with nearly extinct animals so that it could repopulate the Earth on the way back out to a KBO. Oh, but then the craft will be several hundred years behind us and we won't get the benefit of its data. Hmm, back to the drawing board...
The Sun Diver idea is not a 'gravity assist'. It is using solar power (heat) to change the trajectory at perihelion by shedding mass with the right direction and speed. Have another look. It works. So does the idea of a solar sail edge on to the sun on approach and fully presented for illumination after perihelion. Going close to the sun pays off both ways.
My understanding has always been that gravity assists work by robbing a body of a bit of its rotational velocity by flying along with its rotating gravity field. The Sun rotates -- why ought this process not work with the Sun?
-the other Doug
Actually, this is not how gravity assists work - rather, they take advantage of an orbiting body's orbital motion relative to background space. Say a planet is orbiting with a speed V relative to the sun and a spacecraft approaches it (from the planet's point of view) from one side with a speed v. From the planet's perspective, the spacecraft ought to have the same relative magnitude of speed leaving it as approaching it, so the spacecraft leaves with speed v too. From, say, the Sun's point of view, though, the spacecraft approached the planet (which was moving at speed V) with some other speed z; by the time the spacecraft has left the vicinity of the planet, the planet has "dragged it along," and some significant fraction of the planet's orbital speed V is added to the original spacecraft velocity. (This can also be used to decelerate the spacecraft, too, by setting up the initial encounter differently.) The bottom line is that gravity assists depend on the mass and orbital velocity of the "assisting" body alone (robbing it of orbital, not rotational momentum), and have nothing to do with its rotation rate.
Little physics tidbit: Imagine a spacecraft orbiting the Sun in an elliptical orbit, and imagine that it (for the time being) will not pass closely enough to any planet for them to have a significant pull on it.
The sum of the gravitational potential energy of the craft and its kinetic energy is utterly constant. (This is true of circular, parabolic, and hyperbolic trajectories as well.) What it loses in kinetic energy from perihelion to aphelion (or at any other time) is exactly equal to what it gains in gravitational potential energy. And this will be true forever.
So imagine a spacecraft that has left Earth, and that proceeds in to some closer distance to the Sun, then passes on the other side back to (and then past) the orbit of the Earth. (Again, assume that Venus and Mercury don't get in the way.) Between launch (once it gets sufficiently outside the Earth's sphere of influence) and perihelion, what it gains in kinetic energy will be lost, exactly, in gravitational potential energy. Between perihelion and crossing, again, the Earth's orbit, what it lost in kinetic energy will be gained, exactly, in gravitational potential energy. And moreover, because gravitational potential energy is determined by the distance from the Sun, the kinetic energy of the craft when it crosses the Earth's orbit on the way out will be EXACTLY what it was when it left the Earth in the first place. You gain a net sum of nothing. You may as well have just launched in the outward direction in the first place (at some different launch window).
OK.
Just to be nitpicky, wouldn't some miniscule amount of energy be imparted on the spacecraft due to the sun's rotation during a "solar dive"?
I would assume this would be similar to tidal effect. In theory, there should be a (teensy) tidal bulge raised on the Sun due to the spacecraft's gravity, and the friction of this bulge on the Sun's surface relative to the rotating surface material should fling the spacecraft forward a teensy bit. [Assuming the spacecraft's approach velocity is smaller than the rotational velocity].
This is similar to the process that is making the Earth slowly fling the Moon to a higher orbit, while the Moon is slowing down Earth's rotation.
(Any math whizzes out there able to calculate the amount of (de/ac)celeration of a typical sized spacecraft from the Sun's rotation field due to tidal effects?)
-Mike
Tidal effects are harder to model than your typical sophomore calculus class lets you do. I know that in some instances, natural satellites spiral in due to tidal effects and in other cases they spiral out. I guess it's a question of whether the sub-satellite point is tracking on the surface faster or slower than the rotating surface or the speed of a wave through the surface. Something passing at a few tenths of an AU from the Sun would be moving slower, so the tidal bulge could track it fairly well, whereas something speeding over Jupiter's cloudtops would outpace the bulge.
But as for integrating over the density of various layers through the depth of the Sun... sounds like a major research problem.
Or are you expecting to get a boost from the relativistic frame-dragging effect? That'd be tiny, but compared to the tidal effect caused by the spacecraft on the Sun, . . .
--Greg
Well, clearly, JR answered the question in its fundamental form: there ain't no such thing as a free lunch. We can game planetary orbital motion & rotation only so far; sure looks like genuinely revolutionary propulsion systems are needed (very badly) for outer-system exploration.
I get it now; thanks very much for that clarification, Ralph!
Still sounds like Sun-diving with conventional propulsion systems isn't quite worth it. You'd have to expend a massive amount of fuel just to get into a close solar approach trajectory anyhow, then execute another major maneuver to take advantage of the C/A dynamic.
I don't know; could doing a few pump-down gravity assists with the inner planets make this more feasible? Seems to me that if you can save the maximum amount of thrust for solar C/A you still might be able to make some money here. Not sure if conservation of angular momentum for the entire Solar System wouldn't turn around & bite you in the butt, though.
Emily posted an interesting blog on Planetary.org regarding a proposed flyby of Neptune that could be done by 2027 if it launched by 2020. Called Argo, it would be the fourth New Frontiers mission if approved it in the next selection process, which is to take place in 2013.
http://www.planetary.org/blog/article/00001729/
The original proposal page:
http://www.lpi.usra.edu/opag/march_08_meeting/presentations/hammel.pdf
Thanks for pointing to that entry -- but Argo's being discussed actively in http://www.unmannedspaceflight.com/index.php?showtopic=5413.
--Emily
Emily,
You have mentioned that New Horizons may try to photograph Uranus and Neptune. Do you have any information about whether pictures have been taken?
http://www.unmannedspaceflight.com/index.php?showtopic=675&pid=133721&st=1185&#entry133721
Alan Stern has stated that they have indeed been taken.
Thanks, Ted.
I've been following the Review of U.S. Human Space Flight Plans Committee meetings the past few weeks and one of the presentations had a relevant chart regarding the lift capability of the Ares V in regards to unmanned planetary missions. Without going into the taboo discussion of human spaceflight or the likelihood of the Ares V being built in this configuration [or at all], I thought that it would be interesting to include the chart.
Here's the entire slide for context and volume considerations.
50 mT to Mars! _wow_. That would open up a whole new vista for the fun game of Fantasy UMSF.
Yes, except for two inconvenient facts:
1) should the "classic" Ares V ever materialize, it will be expensive as hell
2) Spacecraft are usually many times more expensive than the launch vehicle. It's not a problem of launching a largish spacecraft into space, it's a problem of funding it in the first place. See JWST.
I would put my hand in the fire that no unmanned spacecraft would ever be launched on a dedicated Ares V launch.
Sad but true. The Saturn V was never used for UMSF for the same reason, even though the original Voyager Mars concept (which evolved into Viking) did envision two Saturn launches.
I think a 5-fold to 10-fold reduction in launch costs would result in more twin-probe or multi-probe missions. It costs maybe half-again more to produce two new probes than a single probe (if you take the MER project as an example). A third probe adds quite a bit less to your costs, as does a fourth, etc.
So -- if you can design four probes that are nearly identical (with perhaps some variation in their scientific payloads) and launch them to the same destination for the same cost as what you'd pay today for a single flagship mission, you can get a potential for a lot more bang for your buck. I can envision flying a really solid NetLander mission to Mars this way, or sending a flotilla of four to six Jupiter-system probes, each with its own unique program to execute, and each perhaps half as capable as a flagship probe.
You're still talking flagship mission funding, of course, and so only looking at seeing such multi-probe missions once a decade or so. Even so, in the meantime, you'd at least be able to spend a little more on Discovery-class mission spacecraft and a little less on their launch costs. In other words, reduction in launch costs is always a good thing, but I think it'll have more of an impact on flagship-class planning than on intermediate-class missions.
-the other Doug
A little levity this Sunday morning as we enjoy our coffee and doughnuts:
the mighty Saturn V did manage on 2 occasions to loft craft appropriate for discussion here. PFS-1 and PFS-2 were used to study fields and particles from lunar orbit back in the seventies. Each were under 40 kilograms and probably set the record for the largest launcher used on the smallest spacecraft.
The other thing to remember is that a putative Neptune orbiter would be approaching the planet with a great deal of excess velocity to shed (assuming that it's to make a reasonable transit time, say 15 years from launch to beginning of mission).
So, unless aerobraking or some other exotic deceleration method is used, you're looking at sending a good-sized engine + lots of fuel along in addition to the spacecraft, sort of like keeping the Apollo CSM + LM attached to the S-IVB all the way to lunar orbit (bad analogy, but you hopefully see what I mean.) Bottom line is that the total throw weight (and cost) would go way, way up, which would constrain the actual payload considerably.
That is his point, though, is it not? If huge launchers get 10x cheaper, you design the Neptune Orbiter you want and can also afford the huge detachable propulsion module for NOI for the same launcher price... maybe. And you get there in 10 years rather than 30. Or whatever.
(Isn't aerobraking useless for orbital insertion, anyways? It's for circularization, right?)
It is useful for both. The difficulty with aerocapture comes from the fact that, a) it will take a great heat shield to hit the atmosphere that fast and be slowed down enough to go into orbit, and b ) the parameters of the atmosphere of Neptune are poorly known. If it goes to a depth where the atmospheric thickness is too great, it could end up being an entry probe, might end up being permanently disabled/rendered inoperable, or might be put in such a short orbit that it decays before the periapsis can be raised to a safe distance. If it doesn't go far enough into the atmosphere, it could end up flying by the planet never to return or in some five year orbit that lasts longer than the probe's designed lifetime.
Here's a timely little tidbit about an inflatable (=potentially cheaper) heat shield in this space.com http://www.space.com/missionlaunches/090817-inflatable-heat-shield.html.
But low cost still won't mitigate the risk of unknown atmospheric characteristics (like at Neptune or Uranus).
Also, dual-probe missions have increased the science return from their given opportunities. Primarily, we've been able to target the two probes on dual-probe missions somewhat differently, using the same instrument suite to look at different locations. That was actually done on Mariners 6 and 7, Vikings 1 and 2, Pioneers 10 and 11, and Voyagers 1 and 2. There were also, of course, plans for the Mariner 3/4 and 8/9 missions to use the same instrument suites to look at different locations.
Dual- and multi-probe missions, though, also give you the opportunity to send probes with different instrument suites to the same location. I can imagine this kind of approach would work very, very well for outer planets missions, where your orbiters all have shots of their own at observing the primary planet and each of its moons. Once you finish your pre-planned primary mission, you use the results of one instrument suite to define how the rest of the instrument suites are used on each extended mission.
I'd think you'd have more flexibility, as well -- imagine if you had 4 mini-Cassini's at Saturn now, one with SAR and general imaging, one with high-resolution multi-spectral imaging, one fields-and-particles vehicle, and one with VIMS and additional spectroscopic analyzers to look at the chemistry of the planet, rings and moons.
Now, each of these guys would be on their own mission orbits, with the SAR probe staying out near Titan a lot. One suite would follow up the results from the other suites. Perhaps the vehicle with all the spectroscopic analyzers could be designed a little mechanically tougher than the others, so you could dive it through plumes and the vaporous edges of rings.
Figure that each of these spacecraft could share designs (and manufacturing, etc.) for a common physical bus, common attitude control and propulsion systems, common data handling and communications systems... You'd be developing maybe 20% more sensor systems for the various instrument suites, but your designs, fabrications, engineering, etc., can actually get into savings-of-volume. If you can spend maybe 50% more on your multiple spacecraft than you would on a single Flagship mission, but spend only 60% of what you'd currently spend on launch costs, then it tends to even out. You get more science, a more flexible mission, and the ability to follow up on discoveries by some instruments with detailed analyses by others, without hauling all of the rest of your instruments along on your detailed analyses.
Does that kind of mission architecture sound exciting to you? 'Cause it sure does to me!
-the other Doug
With missions like MSL and Europa Orbiter, failure is a really scary thing. Remember when the quasi-flagship Mars Observer failed and Galileo's antenna didn't open? Then again, with those failures (in Galileo's case I mean antenna failure, not mission failure), lightning might have struck twice...I am especially thinking of Galileo. If they didn't lubricate one antenna, having a second spacecraft to deal with wouldn't have changed much. I would be interested in knowing how much it would cost to have a backup vehicle that could, if necessary, be made flight-worthy in the event the initial mission failed. You would at least save the second rocket cost of the first mission succeeded.
Well yes, you are correct. What I am referring to is that despite a decade and the truck rides, no one checked on the condition of the lubricant before launch. My point is that had there been a pair, it is unlikely that this error could have been recognized and corrected for the second spacecraft since it would have likely been already in space when the problem was discovered.
In the case of Jupiter, the radiation environment is also a factor.
I think the only more or less realistic (partial) aerocapture scenario at the outer planets would be using Titan to assist a Saturn orbit insertion. I read that the Titan atmosphere is "quite good" for that - dense atmospere & low gravity, and known to quite a degree. It will still require orbit trims and possibly a retro-burn to "finish up" the orbit insertion, but still use much less fuel than without the slowdown at Titan (hopefully the fuel savings vastly outstrip the weight of the heatshield+related equipment). The day-or-so it will take the spacecraft from the Titan atmosphere encounter to periapse should give enough time to calculate the additional burn needed.
No such option exists at Uranus or Neptune AFAIK.
Even if you have a strong rocket, budget constraints will still force missions to go as low-weight and low-cost launch as possible.
The real problem is that, with outer planet missions, you play transit time against approach velocity. To get out to Uranus or Neptune in 10 to 15 years, you have to be traveling pretty fast relative to your destination by the time you arrive. You can design a trajectory that results in a much lower approach velocity, but such a trajectory will take 30 or more years to get out to the farther reaches of the System.
So, unless you want to launch a probe that will be managed by multiple generations of PIs, flight support personnel, etc., you have to deal with taking out a pretty hefty amount of velocity upon arrival. This will be the case right up until we can design a constant-acceleration propulsion system and we can accelerate for half of the outbound trip and decelerate the other half. When we eventually develop such a propulsion system, we'll be able to travel to Mars in weeks and the outer planets in months.
Until then, though -- you gotta slow down when you arrive, so you need to carry enough fuel and/or aerobraking equipment to do so. And, the film 2010's fictional flight planning aside, you gotta have enough fuel to raise your periapsis out of the atmosphere after a primary approach aerobrake, and the amount of fuel plus the mass of the aerobraking equipment required for the aerobrake vs. the amount of fuel you need to do a rocket-only insertion generally comes out in favor of the rocket-only option.
-the other Doug
Don't forget one other possiblility, aerocapture followed by a satellite swingby to raise the periapsis. In Neptune's case you might be able to use Triton. For Uranus, well... probably not.
There are various ways to address that problem. Observations of stellar occultations of Uranus and Neptune can help refine the models of their upper atmospheres. This is especially true of space based observations where you can observe a broader range of the spectrum, plus less noise in the signal.
Then there is the heatshield. The manned Gemini and Apollo capsules had off-center shields that allowed for some adjustment and piloting of the trajectory during entry. The same principle can be applied to aerocapture. With a sensitive enough inertial guidence system and enough built-in intelligence, the craft could fly its way through the upper atmosphere to the proper outcome for orbital speed and direction (within a reasonable initial approach, of course).
Finally, the craft doesn't have to immediately aim for Triton on the way out. The orbit would be a long loop outward that could have a period of maybe half a year. Such an orbit would be very sensitive at apoapsis to small adjustments leading to large changes in targeting later on, putting Triton well within range. On the way back to Neptune you make a close pass to the moon and ... there you go. It might be possible to "flatten out" the orbit, lowering apoapsis and raising the periapsis at the same time.
Plus, I don't think it really would take all that much fuel at apoapsis to raise the orbit out of the atmosphere, so a satellite flyby might not be all that necessary to save the spacecraft (highly desirable, though, to reduce the period).
There is a thread here somewhere (sorry don't have time to check right now) about aerobraking at Triton. As I recall, the scale height of that atmosphere is useful for the technique, and having an orbit about Neptune with your low point at the height of Triton's orbit might be handy.
tsp my be confused. Two earlier post were talking about Neptune aerocapture and swingby of Triton to raise periapsis--while avoiding lithobreaking.
http://www.unmannedspaceflight.com/index.php?showtopic=3768&st=15&start=15
Would it be possible to use an atmosphere probe to get enough info to mitigate the risks of aerocapture? It seems you'd want to have an atmosphere probe along for the ride anyway.
(And btw - isn't it lithobraking, rather than breaking? Though I guess both ultimately amount to the same thing... (and in Triton's case, wouldn't it be cryobraking? (And if you cried when your probe cryobraked and cryobroke, would anyone be so cruel as to call you a cryobaby?)))
I always assumed that "lithobreaking" was a deliberate joke. I prefer to call it "impact." :-)
--Greg
In conjunction with the Decadal Survey, the "Giant Planets Panel" had a study performed in regards to a Neptune Orbiter/Flyby/Probe.
http://www.spacepolicyonline.com/pages/images/stories/PSDS%20GP2%20Spilker%20NeptuneRMA.pdf
I'm just absorbing the fact that a 14-year mission launched in 2035 (the latest date mentioned) would enter its extended mission just after I turn 90.
--Greg
recent updates and white papers on proposed Uranus and Neptune missions
http://www.spacepolicyonline.com/pages/images/stories/PSDS%20GP1%20Hofstadter_Uranus%20Orbiter.pdf
http://www.spacepolicyonline.com/pages/images/stories/PSDS%20GP1%20Hansen_Argo_Neptune%20Mission%20Concept.pdf
this one is the Argos mission in its latest version, it might make it as a new frontiers mission.competition for the ASRG's for the Argos mission would come from a sample return flyby through the geysers of Enceladus mission being proposed!
I've been curious whether a Falcon 9 or Falcon 9 Heavy could improve the Argos mission, but this was the first time I actually saw an estimate for the cost of the launcher spelled out in the proposal. I notice that they don't mention either as a possiblity, but I figure a Falcon 9 could save $165M and a Falcon 9 Heavy could save $110M and increase the payload into the bargain.
To arrive at this, I started with their report. On page 58, they talk about mission concepts using the "smallest" Atlas V, a "mid-sized" one and the "largest" with a Star-48 upper stage. For each, they give a mass that could be delivered to a particular C3 (hyperbolic excess velocity squared). Figuring the smallest to be the 401 the medium to be the 541, and the largest to be the 551, and using the Atlas V numbers from Wikipedia, I figure one can convert from mass-to-GTO into mass-to-C3 if you multiply by 5 and divide by the desired C3.
Taking the Falcon 9 numbers from Wikipedia, the Falcon 9 could deliver slightly less payload than the Atlas V 401 (making it viable for the C3=25 scenario) while the Falcon 9 Heavy could deliver over 600 kg for the C3=162 scenario -- a big increase over the 478 kg scenario in the paper.
On page 63, they give a $200M estimate for the 551 with Star-48, $190M for 541, with $10M being the cost of the Star-48. Falcon 9 is only $35M and Falcon 9 Heavy is quoted at just $90M.
Of course, the Falcon 9 Heavy hasn't launched yet, so I could see not wanting to risk anything on it. Also, it seems they're currently not required to include the launch vehicle cost to get under the New Frontiers spending cap. Still, I'm surprised they included so many other options but left this one out.
Maybe once a Falcon 9 actually launches we'll see some proposals start to include it.
--Greg
I don't think anyone would ever submit a mission proposal with total cost estimates based on the manufacturer-projected price of an as-yet unproven booster, Greg. That would really be doubling down on assumed risk; NASA probably wouldn't even bother to finish reading the proposal once they saw that.
I wouldn't expect it to be the main proposal, but I'm still a bit surprised not to see it mentioned at all -- if only to show that the stated cost is conservative.
To turn it around, I'll be curious to see at what point we DO start seeing Falcons in the proposals.
--Greg
What is really interesting is accessibility of Eris in Argo mission.
But no mention of how long it would take to get there? Just 'date of KBO arrival depends on which KBO is chosen.' Mmm . .
I think around another ~20 years (29 years after start). I suppose, that it's technically possible after some closer KBO flyby like mission of opportunity (or more precisely mission of survive).
I can't find a figure for Argos' expected final speed, but if we take Voyager 2's average speed from Earth to Neptune (19 km/sec) and divide that into the difference between Neptune (30.1 AU) and Eris (96.7 AU) I come up with 16.7 years. That's very crude, of course (Neptune and Eris aren't aligned THAT well and the speed PAST Neptune must be a good bit slower than the speed before it) but I'd say that's got to be a lower bound. Even 20 years seems optimistic.
Eris is just too far.
--Greg
http://voyager.jpl.nasa.gov/science/neptune.html is my reference for 19 km/sec.
The F9 is going to need an EDS - what are they thinking of? One of the Star solids?
Even then I cant see the F9 being up to an outer planets mission unless the trajectory is a LOOONG one
P
F9 doesn't inherently need an "EDS" any more than Atlas V needs one. The upper stage has performance for an escape trajectory. The problem as you say is it lacks performance required for outer planet missions. In terms of C3 energy capability, F9 is somewhere between a Delta II and a vanilla Atlas V 401. For comparison, New Horizons used 5 solids on the boosters and an additional solid kick stage and JUNO will use 4 solids IIRC.
Theoretically, a F9 Heavy could do it, but with 27 engines and being very much a paper rocket, it's only a theoretical what-if.
I can see F9 winning Discovery class missions in the future, New Frontiers missions will still be ruled by Atlas.
Any new development on the Argo mission proposal? Just wondering.
Hadn't seen any mention of this proposal yet.
http://www.mssl.ucl.ac.uk/planetary/missions/uranus/
Here is a short write-up about the proposal as well: http://skymania.com/wp/2010/12/mission-planned-to-probe-uranus.html
I hope it never flies under that name, because I can't read it and keep a straight face. It reminds me of my days as a planetarium intern, where I would have real trouble during live shows explaining to people how to "find Uranus" without cracking up.
Better than the National Geographic headline: http://blogs.nationalgeographic.com/blogs/news/breakingorbit/2011/01/europe-asks-to-probe-uranus.html
But yeah, I entirely agree the name of the proposal probably shouldn't stick but likely wasn't unintentional.
I've learned to embrace Uranus jokes (there, I did it again). About 75% of space-related conversations between me and my husband (of which there aren't very many, admittedly) involve Uranus jokes. Don't view it as a handicap, view it as a way to warm up the audience!
My problem was that I was running slides on timed sequences from a reel to reel tape (we had a more modern-for-the-time system, but it died and forced us back to the old one), so by the time I finished laughing the slides were visualizations of the Kuiper Belt. And yes, my wife and I make plenty of jokes...the last name Stryk (pronounced "Strike" for those who don't know) doesn't help.
Thanks for that, ZLD.
Couple of good Hubble shots I don't recall seeing before too.
Article didn't say if the craft was a flyby or orbiter, but at my age, this is probably my last chance for a Uranus mission.
If the craft is an orbiter, I hope they can utilize the Longuski/Heaton plan to visit the satellites. There is a thread here somewhere on that . . .
LOL.
It was the obscurely titled 'Uranus Orbiter' thread.
I bumped it to the top of the pile.
The Longuski/Heaton trajectory is modeled after the Galileo Jupiter orbital tour. Turns out Uranus system is scaled appropriately from Jupiter system to make an analogous tour possible.
Amazing idea they had. Also, they note that at mission end, a (IIRC) 1000 km/hour decel would park the craft in orbit around Ariel.
This is sounding pretty sophisticated for a $400 million mission . . . .
Just curious (I don't have access to AIAA during break), how does the orbital insertion work for Uranus in such a trajectory? I hadn't even really thought about how that would work with Uranus being basically on its side.
Same as at any other planet, as long as you arrive during Uranus' equinox, the initial orbit around the planet won't be too inclined relative to its moon system.
This will be an interesting mission competition. In addition to Uranus Pathfinder (something to look forward to after age 50 ) there is also the http://futureplanets.blogspot.com/2010/11/titan-on-budget.html concept. If the latter can figure out a decent data relay (the http://futureplanets.blogspot.com/2010/12/jet-journey-to-enceladus-and-titan.html proposal?), then my money is on that mission.
I'm sure there are more planetary proposals that were submitted along with many astronomy and astrophysics proposals.
As I understand it, the proposals are competing for 2-3 slots for Phase A funding from which the final mission will be selected. Flight time early 2020s.
Uranus Pathfinder has not been selected for the initial Assessment Phase
http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=48467
the good new is that Marco Polo-R is one of the four finalist mission (and EChO looks great as well!)
Since no one has mentioned this yet, I guess I will. The NRC Planetary Decadal Survey for 2013-2022 mentions a Uranus Orbiter and Probe. A link to the report is available through Emily's Blog.
http://www.planetary.org/blog/article/00002945/
More info including Steve Squyres' Presentation, slides and a copy of the original mission concept study are available at the PDS site.
http://sites.nationalacademies.org/SSB/CurrentProjects/ssb_052412
Keep in mind that this is a report from National Academy of Science, not NASA. And that NASA's budget is highly volatile.
http://www.unmannedspaceflight.com/index.php?showtopic=6922
vjkane has excellent blog on future missions, see http://www.unmannedspaceflight.com/index.php?showtopic=5565
I didn't know it until yesterday, but the ESA medium class mission proposal included a Neptune and Kuiper Belt flyby and General Relativity mission.
see this prez http://moriond.in2p3.fr/J11/transparents/christophe.ppt
Document about Uranus Pathfinder: http://www.google.cz/url?sa=t&source=web&cd=3&ved=0CDsQFjAC&url=http%3A%2F%2Fwww.mssl.ucl.ac.uk%2Fplanetary%2Fmissions%2Furanus%2Fdownloads%2Fup_expastron_submitted.pdf&ei=fnDdTfKxAdHusgbDq6jEBQ&usg=AFQjCNEE-gj956WeKBnbjKT_mpSVg3FrRg
This mission wasn't selected, but still it's interesting pdf.
Interesting mention that the Decadal Survey recommended a Uranus mission:
http://www.planetary.org/blogs/guest-blogs/20120904-hammel-uranus-dark-spot.html
... not that Decadal Survey, but the Solar Physics decadal survey. However it's not exactly binding and would mostly fund a magnetometer. But cool, nonetheless.
just out on the arXiv: http://arxiv.org/abs/1301.5781
I am posting it here because the determination of the D/H ratio was one of the objectives of the decadal survey Uranus mission
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