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Uranus Orbiter, The other proposed ice-giant mission
tasp
post Sep 25 2007, 02:19 AM
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Yeah, the Longuski paper describes a mission phase for reducing the inclination of the initial orbit to the equatorial plane.

Actually, this is an advantage.

We get a couple of years of excellent field and particles study at Uranus (fascinating magnetically anomalied orb that it is), and if we are encountering Titania (or Oberon) every orbit, we might have some pretty precise control of our other ring plane crossing point too. Maybe some close flybys of Miranda (not approached closely in 'Galileo' phaseof the mission, btw) and the inner 'rocks' too.

Wouldn't hurt to check out the extent of the ring plane debris prior to orbiting in it, too. I'm curious if there are any co-orbital satellites lurking there.

The paper contemplates 2 years in the ring plane, and maybe a year at mission end orbiting Ariel. I wouldn't think 2 or 3 years cranking down the incliniation would be unendurable. Cassini has been doing interesting things in all kinds of inclinations at Saturn.


A follow on New Horizon styled orbiter doesn't seem too useful too me at Uranus (in an orbit at Neptune with a period around ~ 1 year is another story).

The flybys happen pretty quickly, and NH is optimized for high data rate collection in brief spurts at long intervals, Uranus is pretty taxing all the time. ( a Neptune mission, OTOH, with perhaps a 20 year 'loiter' time there, and close Neptune approaches once a year seems pretty NH friendly to me. Neptune has a big Hill sphere, might as well exploit it.)
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nprev
post Sep 25 2007, 03:40 AM
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QUOTE (brellis @ Sep 24 2007, 06:30 PM) *
It isn't "mass-producing". It's reproducing maybe a couple of dozen spacecraft that have common traits and interchangeable parts....
Missions to the outer planets are worth the wait, and the machinery should be designed accordingly.


Cannot disagree philosophically, but the problem is much more externally than internally driven.

We all know Moore's Law here, and that complicates long-term logistical support of software tremendously (older stuff gets REALLY expensive, really fast, to maintain; how to attract & retain coders for activities 30 years in the future?) Furthermore, achieving space qualification for IT hardware is not an insignificant effort. As of 2000, 80386 processors had finally achieved this for C-17 aircraft, which is a much lower level than that required for spacecraft. Add the fact that space exploration budgets do not generally enjoy stable long-term committments from decision makers for a variety of reasons...and thus very long-duration missions are correspondingly very difficult to sell.

A propulsion breakthough would obviate all this. Short of that, the overall risk profile for Cassini-equivalent outer-planet missions is questionable at best.


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JRehling
post Sep 25 2007, 05:03 AM
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QUOTE (infocat13 @ Sep 24 2007, 03:45 PM) *
(1) can a modular mass produce space craft design accept somewhat different scientific instrument suite for each outer planet?
(2) would a mass produced set off atmospheric probes work at each outer planet? each would need a somewhat different reentry cone?
(3) Could the Deep space network handle a outer planet mission launched every year or every two years?


Jupiter and Saturn have already been revisted since Voyager, and followups to those systems (eg, a Europa Orbiter, Juno) are going to be more specialized.

Saturn, Uranus, and Neptune coincidentally have about the same cloudtop gravity, so entry probes hitting all three places would accelerate similarly. However, Saturn has a greater escape velocity, so the Saturn probe would arrive at a higher velocity. I guess though that with a very superficial difference in design, one approach could work for all three.

Jupiter presents very different parameters, and has already received an entry probe. Although a design that worked for Jupiter would work in the other places.

The extreme differential in flight times would mitigate any worries about DSN time. Moreover, with probes in elliptical orbits, the encounter-rich time in each orbit could be staggered from one mission to the next.
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dvandorn
post Sep 25 2007, 05:42 AM
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QUOTE (nprev @ Sep 24 2007, 10:40 PM) *
...As of 2000, 80386 processors had finally achieved this for C-17 aircraft, which is a much lower level than that required for spacecraft.

I believe the most advanced processors being used in manned space flight today (on the laptops and integrated computer systems on the ISS, for example) are 80486's. We're not even into the Pentium era yet.

I could be wrong, of course... I haven't dug deeply into the latest news about ISS computers. I surely hope the Shuttles have upgraded their base computers from the PDP-8's they were originally fitted out with, though.

-the other Doug


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dvandorn
post Sep 25 2007, 05:51 AM
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Anyone have any charts handy telling us when we have Jupiter gravity assists to each of the outer planets? Jupiter orbits once every dozen years (roughly) -- that means we ought to have a gravity assist trajectory to each of Saturn, Uranus and Neptune every dozen years or so, right?

The order in which we send probes to the outer planets would seem to be dictated more by the availability of gravity assists than by our 'druthers, I think.

-the other Doug


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mchan
post Sep 25 2007, 07:30 AM
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To a first order, there can be Jupiter gravity assist trajectories every dozen or so years to Saturn, Uranus, and Neptune. Practically, a Jupiter gravity assist to planets further out is possible once a year for several years in a row.

The Jupiter flyby distance will vary from year to year depending on how much the trajectory needs to be "bent" and the energy of the orbit before the Jupiter flyby (which dictates the v-infinity of the hyperbolic trajectory around Jupiter). E.g., for "bend", compare that in the Saturn trajectories of Pioneer 11 vs the Voyagers and Cassini. E.g., for energy of orbit before Jupiter flyby, compare the "bend" in Jupiter flybys of Cassini (lower energy) vs. New Horizon (higher energy). Outer planet orbiters tend more to lower energy orbits to reduce orbit insertion delta-V, but must trade this off against longer flight times.

For an idealized calculation, Jupiter gravity assist opportunities are roughly spaced at the synodic period between Jupiter and the target planet.

For Saturn, 1 / (1/11 - 1/29) ~= 17.7 years
For Uranus, 1 / (1/11 - 1/84) ~= 12.6 years
For Neptune, 1 / (1/11 - 1/165) ~= 11.8 years

Time between Voyager (1979) and Cassini (2000) Jupiter flybys is 21 years which is within "several years" of the idealized calculated opportunities. Time between Pioneer 11 (1974) and Voyager (1979) was only 5 years, but look at how much Pioneer's trajectory got "bent".
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brellis
post Sep 25 2007, 08:42 AM
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QUOTE (nprev @ Sep 24 2007, 08:40 PM) *
Cannot disagree philosophically, but the problem is much more externally than internally driven.

We all know Moore's Law here, and that complicates long-term logistical support of software tremendously (older stuff gets REALLY expensive, really fast, to maintain; how to attract & retain coders for activities 30 years in the future?) Furthermore, achieving space qualification for IT hardware is not an insignificant effort. As of 2000, 80386 processors had finally achieved this for C-17 aircraft, which is a much lower level than that required for spacecraft. Add the fact that space exploration budgets do not generally enjoy stable long-term committments from decision makers for a variety of reasons...and thus very long-duration missions are correspondingly very difficult to sell.

A propulsion breakthough would obviate all this. Short of that, the overall risk profile for Cassini-equivalent outer-planet missions is questionable at best.


Back to my Synclavier analogy - relevance can resist Moore's law. That said, I'm entering this post on a Mac G5 that NASA scuttled after one year of service unsure.gif

Let's have everything: better propulsion, longterm hardware design vision, flexible software. Designing boxes to last a lifetime would help reduce our landfill deposits on earth while making outer planet exploration more feasible. smile.gif
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JRehling
post Sep 25 2007, 02:46 PM
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Another consideration, a happy one, is that for the biggest target in the uranian system, namely Uranus, there is a considerable and growing ability to monitor it telescopically from Earth at almost all times. Percentagewise, its distance from Earth is almost invariant. Excepting brief blackouts at solar conjunction, it is technically feasible to snap a multispectral image of Uranus practically *hourly* for years at a time.

Some images released in the press just a month back:

http://www.berkeley.edu/news/media/release...gcrossing.shtml

And from 2004:

http://www.newscientist.com/data/images/ns...n6657-1_370.jpg

The rings can also receive some useful studies from Earth. Obviously, the satellites cannot be resolved in any interesting way, so the scientific value of a UO mission depends most crucially on them. Since Miranda was wonderfully imaged (though only partially) by Voyager 2, the focus narrows even more.

In my mind, one of the key elements of interest in the uranian system is for us to have the contrast between many worlds of similar size that have nonetheless evolved differently. In the size range of 300 km to 1600 km, the Saturn and Uranus systems combine to give us 13 examples, all with approximately similar bulk composition and similarly low temperatures. Whereas the obvious view of exploration is that the unique places (like Io, Mars) deserve the most attention, you actually learn key things about the dynamics of planetary evolution by finding places that seem to have had similar initial conditions, but diverged. A good look at the uranian satellites would give us a pretty dense sample(s) of similar-sized worlds. At the top end of that group, there is a quartet: Rhea, Iapetus, Titania, and Oberon of almost identical size, and yet none of them look alike. There's really no opportunity in the solar system, except in the Kuiper Belt or among much smaller and less evolved worlds, to see a contrasting set of four worlds the same size and same bulk composition. (Since Mercury isn't made of the same stuff as Ganymede, Callisto, and Titan.)

None of this changes the fact that we'll have to wait a long time to get that next look at them. The main question is which multiple of 42 years we'll have to wait. (Alternate, worse possibility -- a flyby at an anti-Voyager solstice.)
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Spirit
post Sep 25 2007, 04:23 PM
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Maybe we'll have another opportunity to flyby Uranus earlier. I am pointing to the Jupiter Flyby with Probes and Saturn Flyby with Probes missions. If they get launched at the right time and have a technical capability to operate at Uranus orbit, we might be lucky. Yeah, we have a bunch of problems like power supply, telecommunications and money, but who knows.


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JRehling
post Sep 25 2007, 06:06 PM
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QUOTE (Spirit @ Sep 25 2007, 09:23 AM) *
Maybe we'll have another opportunity to flyby Uranus earlier. I am pointing to the Jupiter Flyby with Probes and Saturn Flyby with Probes missions. If they get launched at the right time and have a technical capability to operate at Uranus orbit, we might be lucky. Yeah, we have a bunch of problems like power supply, telecommunications and money, but who knows.


Jupiter Flyby with Probes has most of the life taken out of it by Juno. Although an orbiter is technically a very different mission profile than a flyby with entry probes, Juno addresses many of the science goals that the multi-probe mission would have pursued. The AO for the New Frontier mission to Jupiter sort of expected entry probes but had a loophole that allowed a good remote-sensing mission to replace the actual entry probes, and Juno successfully convinced the powers that be that it will get the job done from orbit.

A Saturn entry probe is not what I'd call high on the list of missions to fly, but it is desirable sooner or later. When it does fly, it *could* work out that the bus/flyby craft could take a trajectory to Uranus (or Neptune, or some KBO), but I think the trajectory may be constrained by the need to get gravity data from the Saturn flyby.

On the other hand, Cassini might be able to fulfill that goal with a close dip over the cloudtops one or more orbits before its death plunge.

The Saturn flyby craft would *still* presumably have trajectory constraints as a data relay, etc. To my mind, top-down constraints would encourage planners to FIND a way to see two planets with the same mission, where feasible, and I don't think missions get enough credit for fulfilling off-prime science goals when selection time occurs. The Uranus flyby would have to end up coming almost for free, because if it increases the requirements (besides, obviously, a ground crew and DSN time) for a Saturn mission, it's going to be very unlikely.

To my mind, a $50 million (say) cost increase to a Saturn mission so it could fly by Uranus is like a $50 million mission to Uranus. But with bottom-up planning, that only looks like a complication of the "main" mission.
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algorimancer
post Sep 25 2007, 07:45 PM
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Perhaps slightly off-topic, but there have been serious discussions recently about putting an interferometric telescope array in orbit which could resolve (2+ pixels) earth-sized planets about other stars. A quick back of the envelope calculation assuming an earth-sized planet 6 light years away corresponds to a resolution of 1 kilometer at the distance of neptune. Increase this by a factor of 10-100 and we're getting distinctly competitive with what spacecraft can do, without needing to venture much beyond earth orbit. Just playing devil's advocate here smile.gif Still, the polar regions would still be troublesome, and this would never compete with a rover on the ground or a dedicated orbiter. smile.gif
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tedstryk
post Sep 25 2007, 09:08 PM
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QUOTE (algorimancer @ Sep 25 2007, 07:45 PM) *
Perhaps slightly off-topic, but there have been serious discussions recently about putting an interferometric telescope array in orbit which could resolve (2+ pixels) earth-sized planets about other stars. A quick back of the envelope calculation assuming an earth-sized planet 6 light years away corresponds to a resolution of 1 kilometer at the distance of neptune. Increase this by a factor of 10-100 and we're getting distinctly competitive with what spacecraft can do, without needing to venture much beyond earth orbit. Just playing devil's advocate here smile.gif Still, the polar regions would still be troublesome, and this would never compete with a rover on the ground or a dedicated orbiter. smile.gif


Ignoring the technical difficulties (such as that orbital motion and parallax would become HUGE problems at such high resolutions), there is also the problem of the fact that we see the outer planets at full or nearly full phases - high and moderate phase observations are impossible, so topographic mapping would be difficult. Also, building and operating an array with such imaging and tracking capabilities in orbit would cost a lot more than some orbiters and landers.


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algorimancer
post Sep 26 2007, 12:37 PM
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QUOTE (tedstryk @ Sep 25 2007, 04:08 PM) *
...technical difficulties..., ...we see the outer planets at full or nearly full phases - high and moderate phase observations are impossible, so topographic mapping would be difficult. Also...would cost a lot more than some orbiters and landers.

I believe the notion was to place it in one of the earth-sun Lagrange points, rather than earth orbit, minimizing the problems of orbital motion. As to cost, I would guess several billion dollars. Definitely more expensive than sending a dedicated orbiter or lander, but far cheaper than sending orbiters and landers to every possible target in the solar system. I agree that the limited view & phase angles would be problematic for lot's of things, plus particles & fields would be out of luck. I'm just throwing the idea out there as a broad but imperfect solution which would in many ways be equivalent to sending flyby missions all over the solar system, not to mention the opportunity to do comparative planetology, since if you can resolve earth-sized bodies around other stars consider what you could do with the gas giants there.
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