Onwards to Uranus and Neptune! Options

[SFJCody]

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

[tedstryk]

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

Here is what makes it difficult. Due to the expense and the long time commitment, it takes a lot more of a push to explore Uranus and Neptune than it does Mars, Venus, or even Jupiter. Also, the fact that Voyager-2 has been there. I am not saying Voyager is the be all and end all of exploration. What I am saying is that Pluto had the benefit of the "There is only one of the nine planets we haven't explored, and this flyby mission would get us there." I should add that this has nothing to do with the "is Pluto a planet" debate - the fact of the matter is that it was officially recognized as one at the time. In fact the questions about its status gave the mission media attention as well - as if New Horizons could somehow carry a Multispectral Mapping Planetometer (yes, I made that up, so don't ask me how it would work ) to answer this
"question." This gave the mission publicity and enough clout to lead congress to override its omission from the budget.

Selling another flyby of Uranus and Neptune requires much more explanation. And anything beyond a flyby would be a huge flagship. With active Triton and the exciting Voyager images of Neptune's clouds, I think it fares a better chance at another visit. Unless there is some probe going to explore the heliopause (in other words, something coincidental) and Uranus is a target of opportunity, I don't think we will be visiting it for a long time to come. Perhaps it could be sold as a chance to see the northern hemisphere of Uranus and its moons. I would really like to see a mission like this happen, but I am not holding my breath.

[vjkane]

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.

[tasp]

"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.

[Greg Hullender]

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

[climber]

Do you think this: http://planetary.org/blog/article/00001285/ could help or even be a reason to go ?

[mchan]

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.

[J.J.]

"There is only one of the nine planets we haven't explored, and this flyby mission would get us there."

I think that was definitely a factor--the whole "let's get this notch in our belt!" syndrome. Even I subscribed to it.

I'd *love* to see another Uranus or Neptune flyby with a NH-style suite, with instruments optimized for studying them (like Voyager 2's weren't), but I agree it would be a tough sell. Let's not forget that even NH came within an ace of getting cancelled...

P.S.--Unfortunately, I also think Neptune would win out over Uranus in the bid-war. Though I find Neptune fascinating, I think Uranus's unique axial tilt and larger retinue of major moons just edge it out over Triton's volcanoes and its primary's more active atmosphere.

[vjkane]

Re: JSNK mission. Front this more as a Saturn probe mission with Neptune and KBO flybys as extras.

Without more information on the trajectory through the Saturn system, it's not known if the Saturn portion allows a probe relay or probe entry with reasonable parameters. One hopes so.

The current Saturn probe proposal involves an equatorial and a polar probe, which requires the relay craft to traverse the Saturn system at high latitudes. The probably would not be possible for a gravity assist to Neptune.

If Neptune is a requirement of the mission, then the Saturn trajectory is fixed. One hopes that it allows a reasonably close pass to Enceladus (see if the geysers are sill operating) and a probe relay.

Then at Neptune, there may be a trade off between a close Triton flyby and probe relay.

In my opinion, getting the elemental compositions of a gas giant and an ice giant would be the primary goals. Then I'd do the best possible at Triton with the understanding that coming close will probably put the spacecraft well out of the elliptic, which may may KBOs harder (I'm not sure what percentage of inner KBOs fall well out of the elliptic).

[Greg Hullender]

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

[JRehling]

[...]

[djellison]

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

[CAP-Team]

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.

[ugordan]

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.

It will not keep the costs low. It will only enable you to get a second, identical spacecraft for less than 2x the cost of one. We're talking flagships here. You don't get two expensive launch vehicles at less than 2x of one to launch them, either.

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.

And you come back to what JRehling is saying that by the time the orbiter gets there, all the scientists will be retired. A minimum energy Hohmann transfer to Uranus/Neptune takes decades and is simply not worth it. What do you do for power? Even RTGs degrade over such long periods and you really want to get a capable and power-hungry instrument suite to orbit (since you're going through all this trouble already). The sad state of affairs is chemical propulsion is just not feasibly up to the task. We need new concepts, there are some feasible ones out there, but they need development. For serious delta-V in outer solar system you simply have to go nuclear (either reactor-based or RTG-electric).

[nprev]

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.

[Planet X]

One tradeoff with orbiter missions is how long we can tolerate the cruise phase to be. With a flyby, it's not such an issue, but with an orbiter, the faster you get there, the more fuel (or heatshield) that you need to slam on the brakes at the end of the cruise. A minimum-energy trajectory to Neptune is prohibitive unless we expect the people running the mission at the onset to be retired (or dead) by the time the science mission begins. The more we shave off of that cruise, the more mass problems intrude. That's one big factor in Uranus's favor -- it's 11 AU closer.

Personally, I see Uranus as the better target. Though Neptune may have more to offer, it's just too distant for an orbiter mission, barring some miracle advancement in technology. For instance, the Hohmann transfer time to Neptune is 40 years, while the time to Uranus is 16 years. I see no problem with designing a probe to last a couple decades (16 year trip time + 4 year length of primary mission) for a Uranus mission. Besides, there's always the possibility of a mission profile similar to that of Cassini, involving Venus, Earth, and Jupiter that could shorten the trip time somewhat. Later!

J P

[vjkane]

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.

[Greg Hullender]

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

[Del Palmer]

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.

Galileo could have reached Jupiter directly, had it been allowed to use its original liquid-hydrogen-fueled Centaur IUS instead of that pansy solid-fuel IUS...

[Jyril]

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...

[ugordan]

and the probe would have followed the SL9 impacts from orbit...

It would have been on the wrong side of the planet at the time.

[Jyril]

It would have been on the wrong side of the planet at the time.

How do you know that? Wouldn't the Jupiter tour have been different, if it had traveled directly to the planet? It would have been there already for years.

And even if wouldn't have seen the impacts, it would have had much better view of the impact effects than any observatory at Earth.

[ugordan]

How do you know that?

Murphy's law.

[tedstryk]

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.

[tasp]

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.

[Toma B]

On the other hand, the main antenna might have worked,...

It would have worked.
If I remember correctly it was plan to open HGA while still near shuttle so that if any problem occurred it could be fixed right away... <spacewalk>
Also that heat shield for HGA would not be there.....
I am not sure about this just remembering.....

[Greg Hullender]

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

[Toma B]

Uh, I hope they didn't plan to open the main antenna before firing the Centaur upper stage.
--Greg

Why not?
Actually that's what I thought they were planing.....I really don't know...

[Greg Hullender]

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

[Mariner9]

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......

[Gladstoner]

.

[nprev]

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.

[JRehling]

[...]

[Gladstoner]

.

[nprev]

And the bean counter who considered switching off Voyager 2 before Uranus..... I hope he is now wrapping burrito supremes at Taco Bell.

No argument there! Still, the fact of the matter is that Uranus & Neptune are really off the radar screen right now, and have been for a considerable period of time. I don't like it either, but it is what it is. NH took advantage of an excellent launch opportunity to get to Pluto within a reasonable time to complete the initial recon of the major objects in the Solar System (Note: NO planet/ain't a planet comments welcomed!!! I'm serious! Uh, would be remiss without mentioning Dawn as part of this effort as well), and I think that the vibe is to wait on Uranus & Neptune until a viable means to undertake a Cassini-style mission to each of them becomes available.

[Jyril]

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.

[Gladstoner]

.

[Jyril]

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.

[mps]

Instead, we currently have inferior vidicom images to play with, and probably will for the rest of our lives.

So, Gladstoner, would you cancel the next flagship mission, so that our grandchildren could see a little bit better quality images of Galilean moons or Titan?

[centsworth_II]

Considering that New Horizons is on its way to Pluto, I would surely think that recon
missions to Uranus and Neptune would have at least as high a priority.

New Horizons just barely made it as a priority:

"The Bush Administration canceled it twice, NASA claimed its budget couldn't cover it
and Congress earmarked funds to be cut in mid-development; yet the trail-blazing
New Horizons Pluto-Kuiper Belt mission has survived....

It is a successor to a long line of planned Pluto missions, none of which ever left the
drawing board. New Horizons' immediate predecessor, the Pluto Express, got farther
than most, but in the summer of 2000 NASA canceled mission."
http://www.planetary.org/programs/projects/pluto/

Not exactly an example of the easy time a Uranus or Neptune mission would
have getting approved and flown, even if those planets had not yet been visited.

[nprev]

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.

[dvandorn]

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

[nprev]

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

...approaching geezerhood does have its advantages! I agree, oDoug, but IIRC (fading memory, not sure it's accurate), a Pluto flyby was off the table for both Voyagers after Saturn. V1 would have had to give up its close Titan encounter, and V2 would have had to have made some kind of ungodly close approach to Neptune, perhaps even within its atmosphere...in any case, not feasible.

Must confess that in my heart of hearts I'll always think of them as Mariners 11 and 12...the proud cumulation of an historic series of spacecraft that gave us the initial recon of the Solar System. (When you get to be a geezer, you get sentimental easily.. )

[Greg Hullender]

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

[Gladstoner]

.

[nprev]

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....)

[JRehling]

[...]

[Gladstoner]

.

[edstrick]

"...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.

[nprev]

Dou you think that Alan will hate me if I privately refer to NH as "Mariner 13"...?

[tedstryk]

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.

[JRehling]

[...]

[dvandorn]

Dou you think that Alan will hate me if I privately refer to NH as "Mariner 13"...?

I'd argue that Galileo was Mariner 13 and Cassini is Mariner 14.

And here they thought they'd get around any possible numerological issues by renaming these programs before they got to a Mariner 13...!



-the other Doug

[edstrick]

"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.

[MarcF]

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.

[Big_Gazza]

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...
)

[ugordan]

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.

[Doc]

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 totally agree. But try and tell that to the bean counters. They'll never take their eyes off the $numbers.

[vjkane]

An orbital mission would be significantly costlier, but the increased science return would most likely far outweigh the cost increase.

From various outer planet mission studies, we can safely say that a Neptune or Uranus orbiter will likely cost in the $2-3B range (one Flagship mission to outer planets every 10-20 years). (The last outer planets new start was Cassini in 1990.) This proposed flyby is in the ~$850M range (one New Frontiers mission every 3-5 years). We'll pick either Jupiter or Titan for the next Flagship mission, and then presumably will pick the other one 10-20 years later. That pushes an ice giant mission out to the 2040-2050s or so by the time it arrives. So if we want to learn anything about ice giants from a spacecraft mission in the working lifetime of the present cadre of scientists (and for many of us, in our lifetime at all), it will have to be a mission like this. My only complaint about the proposal is that it doesn't have an atmospheric probe, which could be very minimal and provided by an international partner. Understanding the elemental composition of ice giants is key to understanding the formation of the solar system.

[nprev]

I have a sort of Moore's Law optimism for far-future missions that the ability to perform operations rapidly and store everything will increase so that the very crowded encounter sequence can be managed. It would definitely be a challenge with a slower spacecraft.

I do too, but am convinced that it will have to come in the form of both increased data-handling capability and major advances in propulsion technology. The whole IT revolution experience makes the idea of sustaining (or re-developing & procuring) the necessary systems to interpret data received decades after launch seems like a significant added consideration.

[JRehling]

[...]

[vjkane]

It's unsatisfactory to have a small set of programs for mission selection, and to thereby allow targets of moderate interest to attract zero attention for decades on end. Much better if a broad long-term plan included contingencies for Uranus/Neptune (including, if the priorities weight out that way, to ignore them) than to see them perpetually bridesmaids because of the dynamics of one-upping.

Extremely well stated, John. For the foreseeable future, Mars (e.g., MSL, sample return), Jupiter, and Titan are going to take all the flagship spots. Discovery missions are severely limited in their scope and range, especially with the new rules to constrain develop cost risks. That leaves New Frontiers as the only mechanism to explore the solar system from the surface of Venus, Mercury (can't top MESSENGER and Bepi-Colombo on a Discovery budget), to the solar system beyond Jupiter. (You can throw the surface of Mars in this list, too. A Mars network mission has been recommended for New Frontiers). If our knowledge of these places is going to increase in any systematic way, then we need a list of prioritized missions. Otherwise, it's a crap shoot of which proposal looks best each time.

Right now, the New Frontiers queue includes (this is from memory, so I may miss something):

Lunar sample return
Venus lander and/or balloon
Comet sample return

A Saturn dual-probe mission has been strongly recommended (and if it launches ~2016-2018, it can also fly by Neptune; perhaps it could even carry a third probe for Neptune)

At 2-3 New Frontiers missions a decade, this list of (in my opinion) superb missions will take 1-2 decades to complete.

A number of missions have been suggested to augment this list or even to throw each announcement of opportunity wide open.

I don't favor the latter approach. I think the science community needs to set priorities so that this funding mechanism can ensure a systematic approach to studying all the places that Discovery can't reach and Flagship missions won't reach. This also allows the engineering community to focus on finding solutions for a constrained set of options. I do favor reviewing the list every time a mission is selected from it. For example, the discovery by Stardust that at least one comet's dust is composed of highly reworked, solar origin material may greatly lower the attractiveness of a comet sample return. Similarly, another nation may decide to fly a mission equivalent to one of the candidates.

By the way, I've heard that the next New Frontiers mission will not be allowed to use nuclear power (to save costs or a dwindling supply of nuclear material or both?). All the missions on the current list can be flown with solar power, although that would constrain any Saturn mission to just Saturn.

By the way, my favorite sequence of the next selections would be the Venus lander (which also likely would include atmospheric composition) followed by a combo Saturn-Neptune mission. But like I said, there's not a dud in the bunch.

[tedstryk]

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.

[mchan]

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.

[tedstryk]

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.

[JRehling]

[...]

[nprev]

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.)

[kwp]

Triton being one of only 5 known volcanically active bodies in the Solar System.

Five?

-Kevin

[tedstryk]

Five?

-Kevin

Earth, Io, Enceladus, and Triton. Titan and Venus likely fit in that group, and some data suggests the same is true for Dione and even Mars.

[nprev]

I was counting Titan; the atmosphere is extremely powerful circumstancial evidence.

[Greg Hullender]

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

[nprev]

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.

[mchan]

[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.

[ugordan]

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.

[kwp]

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.

Since this is a three body problem (spacecraft, Jupiter, Sun), I believe there is no closed, analytical solution to the problem of finding the best trajectory. I have thus assumed that it is done via Monte Carlo simulations. Alternatively, while there is no analytical solution, there might be good approximations. I'm hoping one of our resident experts will weight in and tell me how slingshot trajectories are calculated, and how computationally intensive the process is.

-Kevin

[ugordan]

I believe trajectory search and optimization software nowadays uses Keplerian orbits and 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.

[JRehling]

[...]

[simonbp]

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

[cawest]

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.

This had me thinking... why can we not have three droppers, one at each? This would make an orbit of Neptune cheaper. How do you ask.. if you drop of a probe into Jupiter, Saturn, and before you make Neptune orbit you have less mass to slow down. what do you all think?

[ugordan]

you have less mass to slow down.

What about mass you actually need to launch? Does it go down as well?

[tasp]

This has got me thinking about if you could pull off a Uranus/Neptune Orbiter for less than $850 million...

Simon

Not to put you off of working through cost savings for interesting mission concepts, but my ideas for a 'cheap' Neptune orbiter were pretty thoroughly discredited and chewed up here a while back . . .

[Greg Hullender]

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

[JRehling]

[...]

[Greg Hullender]

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

[cawest]

What about mass you actually need to launch? Does it go down as well?

it would be less fuel mass to launch than a reguler orbiter.

[Greg Hullender]

How does that follow? You'd clearly have less mass to launch if you didn't drop anything at the earlier planets.

--Greg

[simonbp]

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

[Greg Hullender]

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

[Big_Gazza]

$90 M for a Falcon 9 Heavy to put 12 tons in GTO.

Sorry, but i'll believe it only when I see it fly. And even if it ever does, it'll cost way more than 90M.

[Greg Hullender]

Sorry, but i'll believe it only when I see it fly. And even if it ever does, it'll cost way more than 90M.

Good points, but my question is "taking Falcon 9 Heavy on faith -- but nothing else, does that change the picture? That is, is the launch vehicle cost the only thing that keeps us from having a New Horizons-class Uranus or Neptune orbiter?"

--Greg

[Mark6]

No argument there! Still, the fact of the matter is that Uranus & Neptune are really off the radar screen right now, and have been for a considerable period of time. I don't like it either, but it is what it is. NH took advantage of an excellent launch opportunity to get to Pluto within a reasonable time to complete the initial recon of the major objects in the Solar System (Note: NO planet/ain't a planet comments welcomed!!! I'm serious! Uh, would be remiss without mentioning Dawn as part of this effort as well

By that logic a flyby of Eris & Dysnomia should take priority over Uranus or Neptune. After Dawn and New Horizon reach their targets, Eris is the only "major object" left unexamined.

[JRehling]

[...]

[edstrick]

"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

[Mark6]

Eris would be a good target, but loses out big-time in terms of the cruise time needed to get there. What's more, I bet you a soda-pop that before the craft got there, Eris will be down the list of largest remaining unexplored objects.

I would not take this bet. My previous post was only semi-serious.

[JRehling]

[...]

[nprev]

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...?

[laurele]

"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?

[edstrick]

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".

[Greg Hullender]

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

[nprev]

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.