Death of a Spacecraft: The Unknown Fate of Cassini
http://space.com/businesstechnology/061108_cassini_fate.html

I like the Cassini Mercury crash option, even if unlikely.

Rakhir

Yikes Rakhir, you should choose topic titles more carefully, this gave me the creeps!

About the article... Sheesh, Cassini's only halfway through the primary mission and people are already thinking of how to dispose of it. Seems a bit morbid. Smashing it into Jupiter? Mercury? Seriously?
I fail to see the need of impacting Jupiter if you exit the Saturnian system. Once you do that, it'll likely be decades before you even get a chance to get close to Saturn again. I'd rather do another close flyby of Jupiter to do some useful science. Not that it's very likely to happen, though.
How plausible would it be for Titan to be able to launch Cassini on a Hohmann transfer to Jupiter anyway? Seems like a waste of Titan flybys for sub-optimal flyby trajectories just to eject Cassini out of the Saturnian system. If you're gonna eject it anyway, the Mercury impact seems like a much better idea, given the enormous mass of Cassini (some 2 tons dry weight) plus a very substantial impact speed a trajectory coming in from beyond Jupiter would be. That would excavate one heck of a crater. Plus you get to to some Jupiter science beforehand. Here's hoping that by that time (say late next decade) we'll have something in Jovian orbit again and Cassini could wave goodbye, just like it did to Galileo.

Indeed, Ugordan, my heart missed a beat when I read the title of this article.

In addition of the Jupiter fly-by, maybe other fly-bys of inner planets or asteroids would be possible during the trip .
Avoiding of course the Earth because of RTGs.

Actually, Rakhir, you had the story right - just the wrong spacecraft:


http://www.jpl.nasa.gov/news/features.cfm?feature=1226



- John Sheff
Cambridge, MA

You're not suggesting MGS is dead, are you?

I don't understand why a Saturn impact trajectory would neccessarilly have to go through the rings. Giving it just the slightest inclination at the last Titan encounter would bring the spacecraft - er, "safely" may not be the most appropriate word here given the end result - above or below the rings and into Saturn's atmosphere in that same N or S hemisphere.

Actually, I like the idea of putting into a more distant orbit around Saturn. That would maximize the science until the very end, rather than putting Cassini on a long interplanetary cruise during which it's remote sensing capabilities would be wasted except for a rare flyby (by which time the spacecraft might be inoperable, anyway).

John Sheff
Cambridge, MA

It wouldn't be too surprising if it was dead, would it? It's been one of the most successful Mars missions ever, and has been incredibly productive for 10 years. It probably decided to pass the baton to MRO.


John Sheff
Cambridge, MA

I wouldn't write MGS off just yet. This is "just another" safing event. It had them before, it'll have them in the future.

It's been a long time since I got that A+ in Orbital Dynamics (seriously, it was the only class I completely aced in my college career), but I'll take a guess why Cassini would be forced into a ring crossing if they choose to impact Saturn.

It is true they could pump up the inclination of the orbit and then avoid the rings on the crash dive, but I suspect that the mission extension will most likely involve putting Cassini into a less inclined orbit than the end of tour currently leaves it. From what I have read, reducing the inclination involves a lot of Titan flybys and a lot of fuel while they are at it, and there was debate initially if they could ever get down to an equatorial orbit again. (I don't know what the picture is now with 2 years experience under their belts).

Since they want to keep Cassini going until it uses up most of it's manuevering fuel, then having to pump up the orbit at the end of mission would reduce the number of targeted flybys in the extended mission, and is probably something that they just don't want to save fuel for

Does anyone seriously consider the possibility of "contaminating" an icy moon with earthy micro-organisms? While life is amazingly resiliant in terms of the environments where it can take root, we have yet to identify any organism that can survive a hyper-velocity impact/vaporisation with the subsequent super-heating effects, followed by a interminably long exile in a cryogenic vacuum. Impacting Titan would vapourise the craft due to the massive atmosphere, and any surviving fragments would be very effectively sterilised before hitting the ground and supercooling to -180deg C.

Sounds like eco-political-correctness to me.

IMHO we should just run Cassini to maximise the scientific returns, and to hell with a planned disposal. When the fuel runs out, we continue to run the probe and perform science as the opportunity presents itself. Cassini would continue to wander around the system, its orbit altered by subsequent close encounters, and if it runs smack into Titan or a chunk of cratered ice, then so be it.

We could plan to leave Cassini in a useful orbit to maximise encounters with Titan and icy moons, while at sufficient orbital incline to reduce the chance of impact.

Anyone have any idea how long it will be until RTG output decay prevents operation of the 3-axis stabilisation? That would seem to be the ultimate limiting factor on Cassini's useful life (that, and willingness of politicins to continue the funding).

Cheers,
G

The same thing could also have been said about Galileo & Europa. The problem is that there is a difference between being absolutely certain nothing survived and being only probably certain about that survival.

When NASA sent Galileo diving into Jupiter it decided to go for the "absolutely certain" option (that no earthly critters would contaminate Europa) rather than leaving the orbiter circling Jupiter and being only able to say that it was probably certain that if one day the craft ever did hit Europa nothing living would survive to contaminate the ocean that (probably) lies beneath the ice.
NASA could again only be probably certain. It surely could not be absolutely certain that nothing survived; and that may be enough for the critics to bash NASA round the head with. After all, if parts of the spacecraft could reach the ground intact at all there wouild always be the chance some earthly bug would prove equally hardy and do so as well, if only by surviving the plunge by being in some part of the craft where the temperatures reached less extreme heights than elsewhere.

As for the "supercooling to -180deg C", would all of Cassini be at those temperatures once it hit the ground? Wouldn't the RTGs be generating a certain amount of heat, enough perhaps to keep an earthly bug nice and warm for a good many years?

Then there are the RTGs themselves. Doubtless they would survive the plunge, still around and (presumably) in one piece but ready to "irradiate" NASA all the way back on Earth (via its earth-bound critics) for daring to crash a plutonium-laden spacecraft into what said critics would doubtless characterise as the "fragile" Titanian environment.

======
Stephen

Since Huygens wasn't sterilized, I can't see a valid excuse for crashing Cassini to avoid contaminating Titan. As Big_Gazza said, this just looks like PC. As for Enceladus, IMHO it would be a cold day in hell when Cassini just happened to impact none other than the tiger stripes. I say leave Cassini in a safe orbit, period.

It has to do with the residual heat of the RTG's, nothing else. If there was no RTG's, this wouldn't an issue

Let's go to Chiron!

Huygens had 35 approximately 1 Watt radioisotope heaters. A vastly lower heat output than 3 RTGs so I suppose that was deemed a non-issue?

There's something to that.

The idea is sound on one level: if there's no upside to risking a miniscule probability of forward contamination and there's little downside to avoiding it, why not be careful?

Of course if the worry were so damn great about forward contamination, we wouldn't launch a mission out there at all for fear that a ludicrously unlikely loss-of-spacecraft event and a random crash landing would "infect" one of those worlds. "Do I dare to eat a peach?" Maybe Pioneer 8 will one day be ejected from the inner solar system and will contaminate Enceladus with microbes that survived in space for 500 years. Let us all tuck into the fetal position and do no harm.

The reality is that no one is able to calculate reliably the probabilities (much less the payoff matrix) for contamination, and the decisions are made in a hand-waving way until the point in the mission where there is no upside to risking contamination. Then you may as well avoid it.

Of course, as long as we're worried about things so far to the right of the decimal, I'm not so sure that the probability of Galileo infecting Jupiter was precisely zero.

A great idea!

I tend to think the EOL (end-of-life) considerations for Cassini must be much different than for Galileo. Does anyone expect Cassini to be as radiation saturated and ill-suited for some sort of hibernation state as Galileo? At the least point it towards something interesting and let the RTGs trickle down like Pioneer and Voyager. Don't crash it unless you expect some scientific benefit... radio science as it plunges into atmosphere or something.

Maybe it's unprobable but, they can put Cassini in a trajectory to ... Mars, and try to orbit it. No? His radar instrument could be a little usefull to imaging the surface of the red planet.

No......it can use gravity assists to try and get out of Saturnian orbit....but where will it get the Delta V from to do an orbital insertion burn at Mars? There isn't the fuel to do it.

Doug

What I'd like to know is how plausible these Saturnian system ejection scenarios really are? Getting Cassini into heliocentric orbit that's similar to Satun's is fairly possible (sort of like Apollo S-IVBs that went into solar orbit). Launching the craft to another planet I don't buy that easily. The delta-V required would be very large, it's doubtful a single Titan flyby could do it (even if Cassini was coming in from a last, very eccentric orbit already going far out). It's said the average Titan flyby delta-V is 770 m/s. Consider that to the approx. 600 m/s Cassini used during SOI. The thing to note here is that Titan-imparted hyperbolic excess velocity would probably be much lower than a similar delta-V would bring if done close to Saturn (such as SOI) due to orbital dynamics, Titan being much higher in Saturn's gravity well. It's doubtful Cassini could get injected towards another planet that easily as is portrayed here.

Gotta admit, I like the Saturn system escape options best. I know targeting Chiron, Jupiter and/or Mercury are all long shots, but why not? Hopefully by the time a decision has to be made there will be another Saturn mission in the works which would help with the trade-off scenarios.

From what I have heard from, about as plausible as an alien showing up at your doorstep tomorrow and offering you a trillion dollars.

...okay, well that's that, then. I'll be looking out for that generous alien, though...

I do not like these options for the simple reason that *if* (and that's a big if) Cassini can be targeted to something interesting getting there takes a lot of time and by the time it arrives it will probably be uncontrollable and/or dead. You also need a significant amount of time, fuel and Titan flybys just to get ejected (assuming it is possible) and IMHO these resources can be better spent on something else.

I would prefer ending with a long-lasting 'uneventful' extended mission in equatorial orbit following the initial 2 year extended mission (perhaps also preceded by an extended-extended mission if possible). This minimizes TCMs and would concentrate on MAPS plus Saturn atmospheric observations. You would get a few NT satellite flybys as a bonus. If the spacecraft needs to be destroyed at the end of the mission crashing it into the rings should be an option.

Actually, I agree, Bjorn. However, wasn't there some concern about crashing it into the rings re controllability? Personally, I can't imagine why if they targeted the A-ring...should be kaput almost instantly.

In fact...hmm. Would it be possible to put it in an equatoral orbit very near the rings for extended close-up observations of the ring particles? That way, when it's on its last kilo of propellant it should be a simple matter to steer it into a collision...or dare I say a NEAR-style landing on a ring fragment....?

I sure i remember Cassini manager Bob Mitchell talking about possible mission extensions waayyyy back when Cassini arrived. The longevity would depend on what kind of mission they chose, one with lots of targeted flybys would be shorter, one with out could be as long as 20 years.

Extended? Pan's orbit takes about 14 hours. That means any similar orbit will cross the ring plane every 7 hours. Depends on the inclination, you're bound to get trashed or at least snowballed. Passage through a dense ring while on a very low inclination orbit would likely make you stuck in the ring plane ("viscous" drag w/ ring particles) with possibly a crippled spacecraft. No use for the cameras there as their focus is set at basically infinity and the HGA would have trouble penetrating the ring population. Not that you could get into such an orbit, anyway.

Oh, and if you meant orbiting outside the rings, that would necessarily have to be an elliptic orbit with periapsis just above the rings and apoapsis somewhere beyond Titan's orbit. The orbit would have a period on the order of several days, with most of the time spent outside the major moons. Equatorial orbit would permit "accidental" pertubations of those moons to possibly lower the periapse into the rings and you're toast.

Thanks, UG...I understand the situation much better now. The dynamic nature of the ring environment (esp. in terms of orbital mechanics) was difficult for me to visualize...great explanation, man, thanks again!

Chiron is interesting, but to me the most intriguing option, if feasible, would be Uranus.

It's key to remember that by definition, Cassini would be running on fumes by the time it was ejected. But it would have to be pre-death in order to make the mission-ending sequences work. I think all of the goals would be met by setting it up for a decades-from-now Uranus encounter, and if it can take any useful satellite images by then, great.

Note that Saturn and Uranus are currently almost on opposite sides of the Sun. That will change only moderately when Cassini might be ejecting. That would be a good start to a slow minimum-energy transfer.

The way I see it, Cassini is already halfway to Uranus and nothing else is making the trip. Jupiter and Mercury are already being targeted. And Uranus holds more interest than Chiron.

I am afraid that volcanopele most likely are right there as for sending Cassini on to another planet.
But the Chiron option might not be completely out of the question if that small and odd world happens to be in the right part of its orbit when they decide about the matter.

And I am as convinced as JRehling, that Cassini will be running on fumes, but that wouldnt stop a flyby of Chiron.

I've been impressed by Martin Lo's work on the "Interplanetary Superhighway," and I'd expect that any attempt to move Cassini from Saturn to Uranus would depend on that.

http://pr.caltech.edu/periodicals/EandS/ar.../LXV4/exit.html

Volcanopele: Has someone run the calculations and determined that this isn't possible?

--Greg

I think a big difference between Galileo and Cassini, assuming we are talking about when Cassini is nearly out of fuel and taking into account the fact that it won't have the radiation "sickness" issues, is that Cassini could do some great science as approaches. Galileo, with its horrific transmission rate, could have told us very little about one of the Galileans on the way "down." The same isn't true for crashing Cassini.

Yeah, but Cassini needs to turn the HGA to Earth for transmission so you can't have "approach" or better to call it "kamikaze" science and downlink at the same time.

Some questions:

Although it seems fascinating to send Cassini to Jupiter or Mercury, I have some doubts...

1.- How long will last the Cassini RTGs? If they are going to last 10 years more, I can not understand why NASA is planning to end this mission if they can stay at Saturn some years... Moreover, id the energy from the RTGs will be very low after 2010, is it really useful to send the spacecraft to Jupiter or another place? There´s a lot of things more to do at Saturn...

2.- Could NASA use Cassini as a Titan or Enceladus orbiter?

Thanks.

1. The RTGs should last for a while. Radioisotopes have an exponential decay rate hence and approx. exponential decay in power generation meaning that several years after launch the dropoff isn't nearly as steep as in the beginning. With some power management such as permitting only a few instruments online at a given time, Cassini could do good science for years to come. I agree sending Cassini on a long interplanetary cruise is just nonsense when there's science to be done at Saturn.

2. This has already been beaten to death. There's no way Cassini could enter orbit around Titan. Even less possible is Enceladus capture. Just like it's unlikely Cassini can escape into a radically different heliocentric trajectory.

In every machine that will eventually fail, there is going to be one critical part that fails first. With Cassini, it will be when it runs out of attitude-stabilizing fuel. Of course, the team can indefinitely extend that lifespan by limiting manuevers, but a spacecraft that is "alive" but cannot be manuevered isn't worth much. Theoretically, Cassini could be left in place for a very long time without making ANY manuevers and then finish its mission in the far future by making its final manuevers then. But there isn't any apparent great reason to do that... at Saturn, anyway.

The RTGs will not be an issue for Cassini. In just about any conceivable future, it will run out of attitude control before the RTGs grow weak.

Given that they want to control the final fate of the craft, they will have some sort of endgame. That will have to begin before the reserve of attitude control fuel is quite zero. Some of these elaborate endgames involve using the small margin that may be left over for one last set of observations.

I think the problem with the Saturn-ejection scenarios is that they would take so many manuevers to MAKE them happen that the fuel budget would be blown. Jason relays that they are effectively impossible, so I'll accept that... with chagrin (goodbye, Uranus encounter).

Titan/Enceladus orbit would also require way more fuel than Cassini has left. The use of the radio dish to aerobrake Cassini through Titan's upper atmosphere has been discussed and dismissed: it's just not engineered for that.

I think what we're looking at is an extended mission equal to about 50-75% the duration of the primary mission with lots of focus on Titan and some on Enceladus, some fortuitous flybys of the other icy moons, but none of Iapetus. A nice endgame might be to have the craft crash into Saturn with the HGA pointed at Earth the whole time, so that some close-up science at the rings and Saturn can be broadcast in real time as the final minutes tick off.

Aha! Thanks... simple concept, but the trade-offs had not been clear to me in that regard.

Just to give an example of just how hard it is to actually escape Saturn, here's a short scenario I made in Orbiter. Basically, I put a "craft" in a roughly 4 month orbit with periapsis slightly within Enceladus' orbit and going way out. You can see the orbit compared to Titan's orbit in the screenshot below, in the lower left window:
Click to view attachment

I set up a final Titan flyby at an altitude of 1000 km (minimum safe altitude for Cassini), the C/A point was the trailing hemisphere so Saturn-relative energy is gained not lost. The following screenshot shows the Titan C/A:
Click to view attachment

Closest approach was at 991 km altitude (hey, it's not easy to hit the targeted aimpoints in Orbiter ), over the equator. The lower left window isn't meaningful here because it shows the osculating orbital elements of Saturn-relative orbit, neglecting Titan's significant influence.

The final screenshot shows the projected orbit once I put some distance between Titan and the "craft":
Click to view attachment

Note the Ecc value in the lower-left window, it's close to but less than one! The thing still hasn't escaped Saturn. The value is pretty high and in fact the craft would probably enter heliocentric orbit once it got far enough for Sun's gravity to prevail (Saturn's SOI is roughly 0.33 AU, the projected apoapsis is 5 times that, so yes, it's on it's way to a heliocentric orbit). This goes to show that even a four-month final orbit can be insufficient to set up the conditions for and major Saturn escape. One would have to do a fair bit of orbital pumping to achieve that orbit in the first place.

In short, this leaves me wondering just what escape delta-Vees can be achieved at all -- Jupiter and Uranus indeed both seem like unreachable targets.

You need the Encounter MFD
http://www.orbitermars.co.uk/

Doug

Hm... and I thought TransX was the advanced one out there... will give this thing a try.

Trans X is all a bit complicated for me - the Encounter one is perfect for flybys etc

Doug

Yes, it's complicated and has a convoluted interface, but it's powerful even if awkward to use. I once managed to execute a Voyager-style tour of the solar system via hyperbolic transfers between Jupiter, Saturn, Uranus and Neptune. It took ages to get to the outer planets on the maximum time acceleration and many, many course corrections due to not-so-great accuracy. In the end I think I depleted all fuel before the Neptune encounter.
I still don't understand some of the functions and how to use it. Plus it has a few strange bugs.

If a crash is to be the end, I'd much prefer to see a "Silent Running" dive thru one of the "gaps" in the rings with HGA pointed at Earth.

Sad to read that Iapetus will obviously not be observed closely during the extended mission
as the orbit of CASSINI will not go much farther out than Titan's.

I don't know exactly which regions of Iapetus will be imaged in next year's close encounter,
the following image shows some frames - but they're probably not up to date...
Click to view attachment

I guess there are regions that won't be observed at all then, whether they're in shadow,
not in the right position for 1000 km passing CASSINI or not visible at all.

Wouldn't it be senseful to put CASSINI - after the extended mission - on a longer elongated orbit passing Iapetus outbound
over its anti-saturn hemisphere showing Roncevaux Terra between the 'Snowman' and the 'White Mountains' a few times?
I don't know how manageable this scenario concerning CASSINI's fuel resources is,
but it sounds far not so weird like some ideas discussed here - like going to Chiron, a KBO, Uranus, Jupiter or even Mercury (!).

Bye.

Sounds interesting to me too, TritonAntares. Speaking from the depths of my orbital ignorance, you'd think it would be possible to do a Hohmann-type transfer out to Iapetus via Titan, if the geometry ever lines up correctly. Save enough propellant for a good terminal burn at apogee, and have a nice, leisurely flyby...of course, the disposal problem would still remain.

Why dispose of Cassini as if it were some embarrassment or threat (it has been pointed out several times in this forum that Huygens was not sterilized when it landed on Titan)? We might want to retrieve it and put it in a museum some day. Yes, a long shot, but wouldn’t we appreciate it if one of Magellan’s ships had been saved for posterity?

Leave Cassini in a wide, distant orbit and let it watch Saturn until its RTGs run low in 2025 or 2030.

Wouldn't that fragment have to lie on the very outer reaches of the outermost ring (otherwise Cassini would find itself having to play dodgem with umpteen billion other fragments first in order to reach it)?

And how would it get there? In order to match orbits with such a fragment it would have to be in a nearly circular orbit itself. That would presumably mean using propellant to get there since Titan would be unavailable to do so.
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Stephen

I appreciate your sentiments, but I'm actually a lot more worried about an accidental impact with Enceladus than with Titan. Enceladus is the only place known besides Earth that definitely has liquid water, at least in some places, so we have an ethical responsibilty IMHO to avoid contaminating it. Again in my opinion, if Titan has some sort of life then it's probably a good bet that it's both sufficiently different from ours and so well-adapted to that environment that Earthly bugs couldn't compete.

All that being said, I wonder if anyone has been calculating the probabilites if such an accidental impact with Enceladus or Titan in these scenarios? The odds should certainly be an integral part of ANY decision...risk vs. benefit seems to be the main theme in this debate. Come to that, if Cassini is placed in almost any sort of equatorial orbit then its most likely final fate presumably would be to be demolished in the rings someday...

Yeah...it would be a long shot. I'm not even sure that it could traverse the F-ring without sustaining major damage, even with fairly low relative velocity.

As explained previously, the problem is not the absence of sterilization but the combination of piggybacked microbes and long-lasting heat.
Once Huygens batteries were dead, Huygens temperature reached quickly the temperature of Titan surface (around 94 K). At this temperature, the terrestrial microbes are unable to reproduce.
It's not the same thing with Cassini.
RTGs are designed to have chance to survive a launch failure so they may also reach the surface of Titan almost intact (and thus not fully sterilized by the heat of the entry). They will then produce a significant heat for many decades.