Death of a Spacecraft: The Unknown Fate of Cassini, Decision on Cassini's fate Options

[Rakhir]

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

[ugordan]

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.

[Rakhir]

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

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.

[jsheff]

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

[ugordan]

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

[jsheff]

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

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

[jsheff]

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

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

[ugordan]

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.

[Mariner9]

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

[Big_Gazza]

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

[Stephen]

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.

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.

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.

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

[ugordan]

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.

[Jim from NSF.com]

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

[tasp]

Let's go to Chiron!

[ugordan]

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?