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NASA Space

Cassini's Elaborate Orbital Mechanics 116

Posted by kdawson
from the tour-engine dept.
jamie found an article at the NY Times about the extreme orbital mechanics gyrations required to extend the Cassini mission at Saturn by seven more years. Here's a graphic of the mission extension, which NASA took two years to arrive at. "The plans are for Cassini to keep working for seven more years, but it currently has only 22 percent of the maneuvering propellant it had when it started. Figuring out how to more than double the duration of the mission with less than a quarter of the fuel is hard. Cassini's orbital mechanics present an astonishingly complex exercise in Keplerian physics and geometry. The enormous array of science objectives and targets — moons, rings, Saturn itself — makes it one of the most complex missions ever flown. ... 'Without Titan,' Mr. Seal [Cassini's mission planning supervisor] said, 'we would go into one orbit around Saturn and be stuck there.' Thus Titan, in the argot of orbital mechanics, is Cassini's 'tour engine.' [T]he final 'reference trajectory' ... now includes 56 passes over Titan, 155 orbits of Saturn in different inclinations, 12 flybys of Enceladus, 5 flybys of other large moons — and final destruction."
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Cassini's Elaborate Orbital Mechanics

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  • What are the chances we can get some images from it entering Saturn's atmosphere?
    • by Coren22 (1625475) on Tuesday April 20, 2010 @03:22PM (#31914332) Journal

      My thought as well. How long after it enters the atmosphere, but before it breaks up will there be. Is it possible that it could end up "floating" on the atmosphere and not actually break up, that would be some great pictures. Though I am sure that is very unlikely. The most likely occurrence I see is that as the craft enters enough of the atmosphere to be called an atmosphere all the sensors would be ripped off from the deceleration and we would cease to receive signal from it.

  • by ItsJustAPseudonym (1259172) on Tuesday April 20, 2010 @02:15PM (#31913552)
    ...to see the software and user interface that NASA uses to plan the orbits? I wonder how much of it is automated and how much is interactive. We could envision a totally automated system in which they input a desired list of waypoints, some of which might have required time-windows, and the software cranks out the flight plan.

    Does anybody here have experience with this?
    • by EdIII (1114411)

      We could envision a totally automated system in which they input a desired list of waypoints, some of which might have required time-windows, and the software cranks out the flight plan.

      I thought that was Wesley Crusher's job...

    • by PotatoFarmer (1250696) on Tuesday April 20, 2010 @02:23PM (#31913644)
      Take a look at the graphic linked in the summary. I'm pretty sure their "software and user interface" is John Madden locked in a room with a whiteboard and dry erase marker.
      • Re: (Score:3, Funny)

        by Nick Number (447026)

        They were having him plot out the shuttle missions too, but it was considered to be in poor taste when he would punctuate each re-entry point with, "Boom!"

    • by Anonymous Coward on Tuesday April 20, 2010 @02:43PM (#31913892)

      I have worked on this problem with a teacher at A&M that is working on this exact problem. Even in simple cases of move from here to here in more then 1 burn can not be numerically solved with current technology (damn computers too slow). So I worked on apply genetic algorithms and Lambert’s equation to solve for minimum delta V. These calculating become much more complex when you can enter a 3rd body (a moon) into this type of calculations.

      I have also talked with people at Johnson Space Center about this and they use programs like Matlab to determine the orbit maneuvers and another program I can't recall offhand for visualizing it.

      • Re: (Score:2, Informative)

        by riboch (1551783)

        It is probably MATLAB linked with Astrogator/STK.

        MATLAB has the standard integrators, plus it has a decent optimisation toolbox and STK allows for neat visualisations.

      • by kybur (1002682)

        Even in simple cases of move from here to here in more then 1 burn can not be numerically solved with current technology (damn computers too slow)..

        I'm not sure why the parent has a 5 Informative tag... The writer probably meant that the case couldn't be solved *analytically*.

        An analytic, or closed form solution is where you derive a model that explains exactly what will happen in any situation. As far as orbital dynamics is concerned, this is only possible with a two body problem (and perhaps a few special cases of three body problems). When you can't solve a problem analytically, you turn to numeric methods. No matter how fast your computer i

        • The "damn computers too slow" comment tells me that he really means the problem can't be solved numerically. A simple computer program can certainly tell you what will happen if you perform a certain burn. But it can't tell you what burn will achieve a certain outcome with minimal fuel expenditure. It can try to grind through all the possible variations, but if the variations are too numerous and the computations are too strenuous then it's not practical. Hence, the damn computers are too slow.

          • by gander666 (723553) on Tuesday April 20, 2010 @05:51PM (#31916140) Homepage

            Ding. We have a winner. Orbital calculations, and optimizing on burn, delta-v and minimizing fuel consumption is really hard. I took a celestial mechanics course as part of my graduate work in Physics, and while I was really good at analytical solutions where they could be achieved, the aerospace engineers who couldn't solve a closed form integral equation to save their lives, could give outstanding solutions for Hohmann transfer orbits, LEO mechanics solutions, and many harebrained options. I was amazed at their creativity, while I was grinding really hard for closed form analytical solutions.

            This is brilliant stuff, and their creativity of minimizing the burns yet extending the mission is way cool.

            • by trout007 (975317)
              Heck a simple pendulum doesn't have a closed form solution for anything but small disturbances.
          • by PachmanP (881352)

            The "damn computers too slow" comment tells me that he really means the problem can't be solved numerically. A simple computer program can certainly tell you what will happen if you perform a certain burn. But it can't tell you what burn will achieve a certain outcome with minimal fuel expenditure. It can try to grind through all the possible variations, but if the variations are too numerous and the computations are too strenuous then it's not practical. Hence, the damn computers are too slow.

            Fuel optimization is an NP complete problem or something those CS nerds talk about a lot if I remember correctly.

      • by vlm (69642)

        I have also talked with people at Johnson Space Center about this and they use programs like Matlab to determine the orbit maneuvers and another program I can't recall offhand for visualizing it.

        ORSA is popular with the ESA; perhaps folks at JSC use it. Its also used by the NASA and JPL crowd. Most likely, the name you cannot recall is ORSA.

        http://orsa.sourceforge.net/atwork.html [sourceforge.net]

      • by Sir_Dill (218371)
        Turn off the computer and use the force.....
    • by dotancohen (1015143) on Tuesday April 20, 2010 @02:58PM (#31914044) Homepage

      Not for Cassini, but Linux Journal did a report on the UI that the Mars rover drivers use:
      http://www.linuxjournal.com/article/7570 [linuxjournal.com]

    • Re: (Score:3, Interesting)

      by Facegarden (967477)

      ...to see the software and user interface that NASA uses to plan the orbits? I wonder how much of it is automated and how much is interactive. We could envision a totally automated system in which they input a desired list of waypoints, some of which might have required time-windows, and the software cranks out the flight plan.

      Does anybody here have experience with this?

      As awesome as something like that sounds like it could be, we probably don't have much automation for that yet - it's probably just a bunch of physicists doing math on a computer.

      But if we do have or ever make something like that, it would probably be pretty awesome. I'm imagining a big 3d solar system view with all kinds of neat loopy lines for trajectories and such.

      The math is so complex though - you have to take relativity into account because time literally slows down the closer to a massive body you g

      • I've been reading a Brief History of Time lately and this shit is insane.

        You think that is insane? Read Penrose's "Road to Reality" next!

      • by necro81 (917438) on Tuesday April 20, 2010 @03:39PM (#31914512) Journal

        The math is so complex though - you have to take relativity into account because time literally slows down the closer to a massive body you get. I've been reading a Brief History of Time lately and this shit is insane.

        By and large, unless you need super-precise calculations, you can rely solely on Newtonian physics to do orbital calculations. This makes the problem much easier to tackle computationally. The equations of motion cannot be solved analytically, but discrete simulations can be done to arbitrary accuracy extending out for years and years. Relativistic effects will appear as a small cumulative error, but it's small enough that it would probably require only a little more fuel to correct for.

        While Saturn is heavy compared to the Earth, the curvature it produces in spacetime is tiny in the grand scheme of things. Even for calculations where the Sun dominates, relativistic effects can safely be ignored in all but the most exacting situations.

        Put it this way: if relativistic effects mattered, then Kepler, Galileo, Newton, and others wouldn't have been able to work out the mathematics of non-relativistic orbital mechanics in the first place. Newtonian orbital mechanics is plenty accurate to predict the motions of the planets and other bodies to many decimal places over long stretches of time.

        About the only noticeable orbital relativistic effect that I know of in the solar system is a slight perturbation in the orbit of mercury that only became apparent after we'd been observing it for a few centuries. Relativity also comes into play in GPS, but that has a lot more to do with the precise timing of their radio signals than with their orbits.

        • The math is so complex though - you have to take relativity into account because time literally slows down the closer to a massive body you get. I've been reading a Brief History of Time lately and this shit is insane.

          By and large, unless you need super-precise calculations, you can rely solely on Newtonian physics to do orbital calculations. This makes the problem much easier to tackle computationally. The equations of motion cannot be solved analytically, but discrete simulations can be done to arbitrary accuracy extending out for years and years. Relativistic effects will appear as a small cumulative error, but it's small enough that it would probably require only a little more fuel to correct for.

          While Saturn is heavy compared to the Earth, the curvature it produces in spacetime is tiny in the grand scheme of things. Even for calculations where the Sun dominates, relativistic effects can safely be ignored in all but the most exacting situations.

          Put it this way: if relativistic effects mattered, then Kepler, Galileo, Newton, and others wouldn't have been able to work out the mathematics of non-relativistic orbital mechanics in the first place. Newtonian orbital mechanics is plenty accurate to predict the motions of the planets and other bodies to many decimal places over long stretches of time.

          About the only noticeable orbital relativistic effect that I know of in the solar system is a slight perturbation in the orbit of mercury that only became apparent after we'd been observing it for a few centuries. Relativity also comes into play in GPS, but that has a lot more to do with the precise timing of their radio signals than with their orbits.

          Hmm, interesting.

          Yeah, I knew about the GPS thing, and that Newtonian physics didn't quite get mercury's orbit right, I guess I imagined that if you need to use relativity for those things, plotting a trip around a bunch of bodies properly would need relativity too.

          You said they might just need to use a bit more fuel to correct, but the story also mentions that that they went to great lengths to preserve as much fuel as possible, so i still wonder if they needed to use relativity or not...

          I guess i don't kn

          • Re: (Score:3, Informative)

            by vlm (69642)

            You said they might just need to use a bit more fuel to correct, but the story also mentions that that they went to great lengths to preserve as much fuel as possible, so i still wonder if they needed to use relativity or not...

            Random variation in solar activity, random outgasing of surfaces, and random light pressure effects on decaying surface patterns/paints should totally swamp any relativity effects.

            There are also experimentally observed effects that have no current explanation. Perhaps they are just noise, perhaps not. The topics you need to search for are Flyby Anomaly and Pioneer Anomaly. That will give you enough background on the scale of unknown orbital forces to compare with the theoretical effects of relativity cal

            • You said they might just need to use a bit more fuel to correct, but the story also mentions that that they went to great lengths to preserve as much fuel as possible, so i still wonder if they needed to use relativity or not...

              Random variation in solar activity, random outgasing of surfaces, and random light pressure effects on decaying surface patterns/paints should totally swamp any relativity effects.

              There are also experimentally observed effects that have no current explanation. Perhaps they are just noise, perhaps not. The topics you need to search for are Flyby Anomaly and Pioneer Anomaly. That will give you enough background on the scale of unknown orbital forces to compare with the theoretical effects of relativity calculations.

              http://en.wikipedia.org/wiki/Flyby_anomaly [wikipedia.org]

              http://en.wikipedia.org/wiki/Pioneer_anomaly [wikipedia.org]

              Cool, thanks!
              -Taylor

    • Re: (Score:1, Informative)

      by Anonymous Coward

      For the Juno mission, they have simulation of the orbits in nice 3D. More detailed information of what can be simulated in this paper from 2008. http://trs-new.jpl.nasa.gov/dspace/handle/2014/41455. A video of the simulation is available from the Juno website: http://juno.wisc.edu/animation_simulation.html

    • Re: (Score:3, Informative)

      by Nyeerrmm (940927)

      From what I've seen (we had a seminar at my university about it recently) the JPL system for trajectory planning and spacecraft simulation all looks like Python.

      They have a backend framework of compiled C/C++ code with general components for building spacecraft, including different physics components (everything from standard gravity models to solar influences to atmospheric drag), and applying controls. Then they use Python to combine these into practical models for testing.

      Its probably not as impressive

    • by TheKidWho (705796)

      They most likely use matlab and run the equations and positions through it.

      I've written my own 3-body solvers and they can be very very very computationally expensive.

    • To the extent that the burn orders must be manually inputted, it's "interactive" but there's zero feedback involved since it takes about 1.5 hours each way to get a signal out there. I also don't think the hardware on Cassini is that sophisticated (processing-wise) since it was launched in 1997. Think about that - the top of the line desktop computer was a Pentium II, 300 mHz.
    • Having just finished up the first season, and looking at the plans, I would say that the lead engineer's brother's name is Lincoln Burrows. Maybe he looks like this?
      http://www.zhippo.com/cfscripts/imageWindow.cfm?image=http://www.zHippo.com/PleasurePointsHOSTED/images/gallery/space-sleeve-tattoo-L1.jpg&imageWidth=371&imageHeight=550&imageAlt=space%20sleeve%20tattoo&title=space%20sleeve%20tattoo [zhippo.com]

    • Actually, they're just handing the controls to the Pinball Wizard.

    • by jvillain (546827)

      You mean there isn't an iPhone app for that?

    • The participants to last year's ICFP Programming Contest [icfpcontest.org] received a short sharp introduction to orbital mechanics. It was only a 2D version, but still not an easy task. I ran out of brain power and time just short of the last set of problems which is where it got really interesting.
  • by sznupi (719324) on Tuesday April 20, 2010 @02:16PM (#31913558) Homepage

    Cassini spent a lot of time in the vicinity of Enceladus and its water geysers; another place where there could be life, with some traces of it hatched for the ride on our spaceship.

    Alas, there's way too litle fuel even for routes with lowest energy requirements [wikipedia.org] :(

    • by BitZtream (692029)

      While discovering life would be great, any one with any clue about the environment will tell you its a REALLY REALLY bad idea to transport an organism from its current environment to a new one, far far away, that it has never seen before.

      Bad things happen when species on Earth get transplanted by humans a 100 kilometers across land.

      I suspect the danger from even pulling the probe back to Earth orbit is not worth risk at this time due to the possibility of potential contamination should it accidental or mal

      • by sznupi (719324)

        I didn't say "aim it at Earth". Just somewhat...closer. If a very (think beyond Moon) high Earth orbit makes people nervous, it might just as well a high one around Mars or nearby solar orbit. So in a few decades we would have a realistic chance of visiting it, when we'll have a means to examine it in situ.

    • Alas, there's way too litle fuel even for routes with lowest energy requirements :(

      Wait, why did you send a link to an article that says there are an infinite number of routes that require no energy to transit then? I think you meant to say "there's too little fuel to do anything useful before the satellite dies."

      • by sznupi (719324)

        Those routes would require initial braking off from Saturn gravity though; something Cassini can't do anymore, with the miniscule amount of fuel it has (one could cosnider using Saturn moons I suppose...that still almost certainly wouldn't be enough)

        I don't think it would "die" anytime soon. It uses RTGs which do age of course...but still supply plenty of power (all instruments functioning; and you could gradually shut most of them down on decades-long return trip). Heck, we still have contact with Voyagers

    • Dump it into a parking orbit and go get it with something shipped there on a SpaceX launcher. It's not like time is something we don't have.
      • Re: (Score:3, Informative)

        by robot256 (1635039)
        Problem is there aren't any parking orbits within range of the spacecraft. Those darn moons keep dragging you out of them. That's why they spent two years figuring out how to stay in orbit after as many "drags" as possible--and use them to our advantage. But you still can't stay in orbit indefinitely without any fuel, not at the low altitude Cassini is at now.
  • Sounds like... (Score:1, Insightful)

    by Anonymous Coward

    they used genetic algorithms to solves this? Or something alike

    • by b0bby (201198)

      I think they've developed a 3D Spirograph...

    • Why? Genetic algorithms are used to solve equations, using a system of guided trial and error, that are deemed unsolvable. In control theory, optimizing for "lowest fuel use", is usually an exercise in variational calculus.

      • No, this would be a great problem for a GA, in general. You can't do a variational calculus approach to tour selection because the parameter space is huge and not a differentiable function, in general. A GA would be fine if you had the time, I suspect. You just give it a goodness of fit metric (like meets so many priority goals of the science team) and let it loose.

        In reality, this is kind of what they did, but they did it iteratively with the scientists themselves. I'm not sure how much they numericall

        • I agree that if what you said is true:

          the parameter space is huge and not a differentiable function

          then it would push more into the realm of genetic algorithms. I don't believe that to be true, because with numerical modeling, almost anything is differentiable (x'=x[n]-x[n-1] or the like) Additionally, GA is to be avoided because (as I understand it) the answer are not guaranteed to be optimal in any form, which means you never know if there is a better solution, just that this one might be good enough.
          So it's possible, but w/o more info, I doubt it. Perhaps we co

          • Your definition of x' is only an approximation and not useful for many purposes. As such, it doesn't really usefully expand the set of differentiable functions. In this case, x may be a wildly non-linear function and a small change in input may yield radically different results. As such, it's not differentiable in those areas.

            Intuitively, you know your logic doesn't hold. If it did, no optimization problem would ever be unsolvable with the calculus of variations. That's clearly not the case, however.

            As

  • kdawson actually posted a story worthy of the front page of Slashdot. I acutally did a doubletake.

    • Re: (Score:2, Insightful)

      by Anonymous Coward

      And yet your comment is the usual back-biting. Ah well.

    • by sznupi (719324)

      Well, happens; perhaps kdawson managed to, say, avoid contact with MS software in the last week and hence wasn't traumatised by it.

  • by SnarfQuest (469614) on Tuesday April 20, 2010 @02:26PM (#31913686)

    Why don't they just schedule another shuttle mission to refuel this, like they do with the Hubble telescope?

  • by Mr 44 (180750)

    The article refers to a delta-V budget [wikipedia.org], which I hadn't heard of before, and my first thought was that they'd gotten their units wrong, but apparently "delta-V" in this context is actually measured in m/s, not m/s^2.

  • Why does it seem to me like people dealing with probes in space always pull Scotty's?

    'Oh capt'n, she can not take it, I'll need at least 2 days to repair it'

    Six hours later

    'Its ready capt'n'

    EVERY ONE of these missions either fails right off the start (understandable, this is rocket science after all) or ... gets extended. Why are we not planning better to start with rather than getting there and saying ...

    'oh gee, our probe can live longer, but we'll have to 'figure out how' since we didn't plan on actuall

    • Re: (Score:1, Insightful)

      by Anonymous Coward

      You plan for what you know your resources will do for sure. If there is anything left you make a plan to use what's left. Repeat until you can't any more.

    • Re: (Score:3, Interesting)

      by geekoid (135745)

      It's very simple:

      You have no fucking clue how this kind of science works, and you don't seem to remember the 'failures'

      But hey, you let your experience with fictional TV show dictate how you think of the world, because I don't think you could really do any better then that.

    • Re: (Score:1, Insightful)

      by Anonymous Coward

      That's because, unlike most, they DO expect the Spanish Inquisition. The only extend the mission if the inquisition never gets off the ground.

    • Re:I call bullshit (Score:5, Insightful)

      by Nadaka (224565) on Tuesday April 20, 2010 @02:49PM (#31913952)

      Because everything costs so much to launch that they design something to absolutely be capable of fulfilling its mission parameters. If they screw it up, it crashes and dies with billions lost. If they get it right, they might have some triplicate backup resources used to ensure function for 90 days that are left over. And because it would cost billions to get those resources up with a new mission, they might as well take advantage of what is left.

    • Re: (Score:1, Informative)

      by Anonymous Coward

      It's not padding. There's no way to know exactly how long your probe will function. Scientists take a guess (I mean use statistics) at what they expect the life of the probe to be and plan a primary mission itinerary. Anything else is a bonus. Considering there's no way to service these things after launch as well as the environmental stresses they must endure, I think scientists are doing a pretty good job.

    • by prograde (1425683)

      Well, it took them two years to plan this extended phase of the mission, so that would mean that they would have had to wait two extra years before launching (for planning better). Plus waiting for the next launch window.

      Also, some of these missions last long enough that the science advances back on Earth before the probe dies. They can now answer questions that no one had considered asking before they launched. If you ask me, that's pretty freaking cool.

    • Re:I call bullshit (Score:4, Informative)

      by blair1q (305137) on Tuesday April 20, 2010 @03:14PM (#31914250) Journal

      No, they got their project picked out of the hundreds submitted for funding because they had a particular plan that met particular objectives.

      They engineered the spacecraft with normal margins over and above the basic requirements to allow for the sort of uncertainties that enter into designing every part of a device this complicated and limited in mass and power. One of the simplest of these is to put in twice as much fuel as your nominal model tells you to, giving yourself 100% margin to deal with exigent circumstances.

      This sort of margin is not just good for business, it's generally required by the funding authority (NASA and the government), because they got tired of being bitten on the ass by penurious aerospace craft design in the '00s. That's the 19'00s, not the 20'00s.

      Then, as the mission goes along, their managers and technical staff made careful decisions that didn't waste their fuel margins. The result is that they have a free spacecraft on-orbit with which to do new science. Which again they have to propose to the funding authority, since ground systems and personnel still cost money. The funding authority sees the variable costs as incredibly cheap compared with developing and emplacing a whole new device, so they give it a green light.

      A few months later, we get to see a pretty picture that blows the minds of the smaller-minded among us, and makes the bigger-brained among us yawn at the idea that anyone is impressed with 64 loops around a couple of rocks that aren't going anywhere fast...

      • Re:I call bullshit (Score:5, Insightful)

        by BJ_Covert_Action (1499847) on Tuesday April 20, 2010 @04:05PM (#31914820) Homepage Journal

        and makes the bigger-brained among us yawn at the idea that anyone is impressed with 64 loops around a couple of rocks that aren't going anywhere fast...

        That's a bit of an arrogant statement. While you hit the nail right on the head with the margins in spacecraft design, I have to say that seeing the orbital plots of a mission as complex as Cassini, is, in no way, a yawning experience. While you may think that it's nothing more than a few geometric loops around some rocks that aren't going anywhere, those of us who have worked on orbital mechanics problems understand, in no small part, just how complex the math must have been to work out those solutions. Furthermore, we have a fascination with the idea of getting to fiddle with problems like that ourselves one day. In fact, this particular plot has inspired me to pull out my 3-body problem source code when I get home and revisit some of the algorithms used to optimize fuel consumption for 3 body orbital problems.

        What I am getting at is that those 64 loops around a couple of rocks, which you so trivialize, represent, to those of us involved in this field, a visual depiction of a very high level of hard, computer intensive work that we are familiar with. It's kind of like hearing a symphony play a composer's final musical construct. Those who are composers can appreciate the symphony as it depicts, audibly, just how intensive, deep, and subtle the composer must have been in his work. Likewise, those of us that are familiar with these types of problems can look at the visualization and appreciate the incredible mathematical nuance and finagling that must have gone into doing these calculations.

        Then again, very few people tend to have any appreciation for the engineering intensive processes that go into even common, everyday appliances like computers and cars, so I suppose I shouldn't be surprised when a self-proclaimed 'bigger-brained' person cannot appreciate the depth and culmination of work that a plot like the one linked to in the summary represents.

        • by syousef (465911)

          and makes the bigger-brained among us yawn at the idea that anyone is impressed with 64 loops around a couple of rocks that aren't going anywhere fast...

          That's a bit of an arrogant statement. While you hit the nail right on the head with the margins in spacecraft design, I have to say that seeing the orbital plots of a mission as complex as Cassini, is, in no way, a yawning experience.

          Anyone that sees that and claims to have a bigger brain is either lying about the bigger brain, or the bigger brain was installed to store more manure to better fertilize the next crop. What percentage of the population has even done orbital mechanics calculations/simulation? A larger group would be capable of it but there are plenty of people who have trouble calculating change and would have NO hope. Yet some of them have other skills of value. Unlike this bloke who yawns at what he has no idea about.

          • by blair1q (305137)

            I have.

            Orbital mechanics is fiddly, I'll grant you that, but it's constant fiddly. Everything in the system moves predictably.

            In other words, it's NP-easy. Toss the players into the computer and let it grind. Adjust dv here and there, and you can spline together 64 blue loops.

            The trickiest thing here isn't the 64 loops. It's finding the 64 loops that get you the 200 scientific datasets you most want. Which isn't done in this case by a computer or a brain. It's done by a campaign of political refinemen

      • Regardless of the size of your brain, I think you may have a small mind ;)

  • science!

  • Did they use some fantastical optimisation algorithm to find the optimal path? I would be curious to know how they did this.
    • by tbischel (862773)
      They probably are using a tool like this [stk.com] to analyze possible trajectories... these tools do use numerical optimization libraries to pick paths between targets and destinations. Just guessing based on what was said in the summary, they are using the Saturn-Titan Lagrange points to alter their trajectories without using much fuel. You can do some pretty crazy maneuvers around these points.
      • by caerwyn (38056)

        STK was probably used for visualization of trajectories; I'm pretty sure it wasn't used to actually create them, though. My guess is a lot of computer time with custom optimization code.

        Also, I imagine it has more to do with simply getting close enough to Titan to slingshot off it than it has to do with the lagrange points; those are more useful if you *don't* want to move than if you *do*.

        • by tbischel (862773)
          well I certainly am not a trajectory analyst or anything, but the guy next to me uses STK to do his analysis (the astrogator plugin [agi.com]). You could be right that they are using Titan to do a swingby, but I've seen some pretty crazy trajectories as a result of unstable orbits around Lagrange points. That is the basis of the space highway [universetoday.com].
    • Mostly from what I saw of this process, they build trajectories using a toolkit of tricks to make certain specific objectives. (Objectives are things like, "fly by this moon" or "spend N hours above this latitude on the planet.") I don't know how much they can optimize within the vast parameter space of the orbital trajectories, mostly it seemed like a lot of man-hours and skill.

  • by smoothnorman (1670542) on Tuesday April 20, 2010 @02:49PM (#31913950)
    "If all goes as planned, on Sept. 15, 2017, Cassini will die a warrior’s death, diving inside the rings for 22 spectacular orbits on the fringes of Saturn’s atmosphere before plunging into the planet." ...they hope to orbit inside the rings! that's just as cool as it gets outside of riding a comet out of the solar system.
    • Re: (Score:3, Funny)

      by corbettw (214229)

      that's just as cool as it gets outside of riding a comet out of the solar system.

      Know what would make that even cooler? If Cassini were wearing a cowboy hat and yelling "Yee-haw!" as it dove into the planet. Best. Re-entry. Ever.

    • Yes, it is cool. And you don't have to wait 7 years. Cassini already completed a trans-ring intersection on it's breaking maneuver when it arrived at Saturn. You can probably find videos on the Net. There are some awesome high-res shots available. Go and find them.

      The fear then was that a mis-calculation might be the cause of the worst man-made environmental impact in the solar system -- crashing into the rings and disrupting their delicate balance, causing them to collapse into the planet. Of course with

  • Optimal control theory gives you a function (trajectory over time, or acceleration vector over time) which minimizes a given functional (a function of functions) in this case I imagine that the total amount of fuelt spent (with the constraint that it reaches the lifetime desired). Summary: find the trajectory x(t) such that Fuel(x(t)) is minimized. This algorithm is well developed, you can even use it in quantum mechanics (give the desired hamiltonian(t) such that the evolution of the state is such and such
  • by rotenberry (3487) on Tuesday April 20, 2010 @03:07PM (#31914156)

    I had the good fortune to be working on the Galileo mission during its Mission Design phase. Many of the techniques used by the Cassini mission designers were developed for Galileo. Disclamer: I was not on the mission design team.

    First of all, the Voyager encounters with Jupiter and Saturn were always when the spacecraft were moving away from the sun. However, during the Galileo satellite tour the mission designers realized that the Galileo spacecraft could encounter Callisto, Ganymede, and Europa when moving away and moving toward Jupiter. Furthermore, the closest approach ("encounter") could be targeted to be either in front of the moon (with respect its orbit around Jupiter) or behind it. These choices allowed the designers a great deal of freedom to use the moons' gravity to shape the spacecraft's orbit. As I understand it, they did not just plan the current encounter to obtain the next encounter, but also the encounter after that.

    The ability to use a moon to shape a spacecraft orbit depends on the ratio of the mass of the planet to the mass of the moon (for all practical purposes the spacecraft is massless.) Only Io, Callisto, Ganymede, and Europa are able to provide gravity assists at Jupiter, and only Titan at Saturn.

    I spoke to Bob Mitchell, Cassini Project Manager, a few years ago and asked him about this specifically. He told me that while it was true that having to go back to Titan every time to change the orbit was a constraint, it also provided the freedom to send the spacecraft out of the "plane" where the moons orbited. At Jupiter it was necessary to stay in the plane to make multiple visits to all the moons, but since at Saturn you must visit the same moon to change the spacecraft's orbit every time (Titan) there is fewer reasons to stay in the plane. And, as you can see from the orbit diagrams, Cassini has traveled outside of the plane many times.

    • Thank you for posting.
    • by UnixUnix (1149659)
      I worked on the Magellan mission (called VRM, Venus Radar Mapper back then); Galileo had already been designed, and we did use quite a lot of it. (Thanks!) Work had started on CRAF-Cassini. It was the mid-80's. Launch seemed so far in the future...
  • That's no moon [nytimes.com]

  • [T]he final 'reference trajectory' ... now includes 56 passes over Titan, 155 orbits of Saturn in different inclinations, 12 flybys of Enceladus, 5 flybys of other large moons — and final destruction."

    So the CD with 616,420 signatures on it is not going to out last us all after all.

    When the signature drive began in 1996 an implied promise was made that Cassini, and the disk would remain in orbit around Saturn forever. See, for example this remark: "Indeed, some of us may even experience a small tas

    • by vlm (69642)

      as we envision our signatures outliving our bodies

      Well, the worldwide death rate is supposedly 0.837% of the population per year, round to 1% per year. Estimate from 96 to seven years from now, to be about 20 years.

      So, for about 20% of the folks whom signed, their signatures DID outlast their bodies... Not bad, not bad.

  • by professorguy (1108737) on Tuesday April 20, 2010 @04:30PM (#31915146)
    Am I the only one who remembers the protesters around Cape Canaveral when Cassini launched? That's because its initial trajectory was unbelievably convoluted: the ship actually traveled to Venus first, got a gravity boost then traveled back out and used the Earth for its next boost.

    The protesters feared that a miscalculation could cause Cassini to re-enter Earth's atmosphere on this near-miss flyby and disgorge its thermopile of plutonium into the stratosphere.

    So it was a crazy flight from the very first day.
    • That's because its initial trajectory was unbelievably convoluted: the ship actually traveled to Venus first, got a gravity boost then traveled back out and used the Earth for its next boost.

      Venus twice, then Earth. What's unbelievable about it? Gravity assists have been used on so many probes now that it's basically assumed for any target other than Venus or Mars. (They even used it to get to Mercury with MESSENGER.)

      The plutonium scare was mostly an overreaction by people who like to overreact to whatever they can find. Many of them are now complaining about the LHC, I'm sure. (NASA designs the RTGs to be nearly indestructible and they biased the Cassini trajectory away from Earth on the

  • Like solving problems currently applicable to stuff on Earth. Dunno' maybe outmaneuvering volcano ash?
    • Like solving problems currently applicable to stuff on Earth. Dunno' maybe outmaneuvering volcano ash?

      I was thinking the same thing... though more on the lines of figuring out ways for cars to take advantage of gravity assists of a semi-truck passing on the highway or creating water slides where you get a few seconds of weightlessness.

      Wait, maybe they only know how to do that in space. I don't know where my mind wanders sometimes.

  • One of the keys to Cassini's success has been flexibility. Using fancy flying, NASA/ESA were able to save the payload Huygens probe mission, which had an almost fatal communications flaw.
    The story has a hero, Boris Smeds [wikipedia.org], a Swedish radio engineer for the ESA, who pushed and pushed because he found out something was very wrong with the Huygens payload to be launched from Cassini probe.


    Slashdot covered this well and humorously. See substitute links provided for 6 years of bit rot on URL's :-)
    Saving Hu [slashdot.org]

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