[X] My Assistant

[pioneer]

I've heard about ideas for a future mission to Titan. I think the most feasible one is to use a balloon. Because Titan's atmosphere is more dense than Earth's, a balloon could easily float. It could also cover more ground than a rover or ground-based vehicle.

I don't think we should send a rover yet because, unlike Mars, we don't have detailed information of the surface on a global scale. For MER, we had MOC and laser altimeter data to determine a safe spot. Although we are gathering altimeter data from the RADAR and some images from SSI and VIMS, I don't think they are detailed enough to determine whether a landing area is safe enough for landing or accessible by a rover.

Also, I think the mission should go directly from cruise stage to descent and avoid orbital entry to Saturn or Titan. Over half of Cassini's weight at launch was due to fuel for SOI. If we eliminate the need for this, we reduce the mass and cost.

Of course, we would need RTGs to power the probe.

I think a mission like this is quite feasible and can be done with less than half the amount of Cassini.

[tedstryk]

It would be hard to imagine something coming at Titan that fast not burning up in the atmosphere, or, in the case that it doesn't burn up, impacting the surface. It would still need a lot of fuel to work.

[djellison]

Time it right and I see no reason why it couldnt be done at a similar approach speed as say a Mars entry. The atmosphere is very very 'tall' - so a high, low angle entry might not be so bad. The entry speed of Huygens wasnt THAT bad actually - and I'm sure a system could be built to handle say, double that. you just have to dedicate more mass to EDL. Might be scope to try an entry balute in this situation perhaps - I believe the Russians tried it in LEO once, not sure how succesfull it was.

Doug

[pioneer]

It would be hard to imagine something coming at Titan that fast not burning up in the atmosphere

The Galileo probe impacted Jupiter's atmosphere at 104,000 mph and survived.

in the case that it doesn't burn up, impacting the surface. It would still need a lot of fuel to work

You're probably right about that. I suppose even if you use a parachute as big as the biggest one Huygens had and rockets you will still impact the surface. Perhaps airbags would work.

[Guest_BruceMoomaw_*]

There's been a really surprising amount of design study already on a follow-up Titan mission -- even given the enormous amount we didn't know about the place (like, say, the most basic physical nature of its surface...) and "Pioneer" is on the right track but hasn't gone quite far enough.

Most proposals do indeed involve a balloon -- either free-drifting, or a blimp equipped with motors -- and capable of either periodically landing on the surface for samples and them taking off again, or at least hovering a short distance above the surface and lowering a sampling device on a cable. (One particularly weird idea from JPL is an "Aerover" -- a rover with huge inflatable wheels, as has also been proposed for near-future Mars rovers, but with the tires this time inflated with helium. Periodically the rover will just inflate another gasbag with helium and take off to fly to a new location for local roving. In short, a cross between a monster truck and Chitty Chitty Bang Bang.)

It's increasingly clear, though, that we do want a mission at least capable of periodically sampling the surface -- for Huygens' confirmation that Titan has very large amounts of ammonia-water cryovolcanism has also vastly increased the importance of analyzing its surface organics in detail. Since they must be frequently exposed to considerable amounts of temporary liquid water, their prebiotic chemical evolution must go MUCH further than if they were never exposed to liquid water -- and, as Jonathan Lunine confirmed in his recent NAI lecture, this now looks like by far the most important reason to explore Titan.

As for "going directly from cruise stage to descent": this would be perfectly practical if it wasn't for the need for a Titan orbiter to relay back data at a high speed from the probe. (We'll want to return both very detailed chemical analyses, and LOTS of pictures -- plus nailing down the suface probe's precise location by radio tracking from orbit.) But the concepts so far also call for "aerocapture" of the orbiter directly from solar orbit into orbit around Titan by skimming through Titan's outer atmosphere behind a heat shield, which requires precise targeting but eliminates the need to carry more than a very small stock of maneuvering fuel. (By the way, the thermal problem even for a Saturn entry probe -- let alone a Titan one -- isn't much more serious than for a Venus entry probe. Jupiter is in a category entirely by itself where the difficulty of atmospheric entry is concerned.)

Costwise, though, we're still talking about something that will fall into the "Flagship" category of Solar System mission -- that is, a billion plus -- like the first Europa probe. Right now I'd say that a Titan mission has risen in importance and is neck and neck with a Neptune orbiter as the next Flagship mission to be chosen after the Europa one -- but then, the Solar System Decadal Survey recommended only one Flagship mission per decade, so we'll still have to wait quite a while.

[MizarKey]

...Costwise, though, we're still talking about something that will fall into the "Flagship" category of Solar System mission -- that is, a billion plus -- like the first Europa probe. Right now I'd say that a Titan mission has risen in importance and is neck and neck with a Neptune orbiter as the next Flagship mission to be chosen after the Europa one -- but then, the Solar System Decadal Survey recommended only one Flagship mission per decade, so we'll still have to wait quite a while.

Well, if American can afford to 'lose' $9 billion in Iraq...surely we can afford a second $1 billion for a Titan orbiter/blimb...

Eric P / MizarKey

[pioneer]

Very interesting post Bruce.

As for "going directly from cruise stage to descent": this would be perfectly practical if it wasn't for the need for a Titan orbiter to relay back data at a high speed from the probe. (We'll want to return both very detailed chemical analyses, and LOTS of pictures -- plus nailing down the suface probe's precise location by radio tracking from orbit.)

Why not have a HGA on the blimp or hovercraft? Does a mission like this require a HGA as large as Cassini's?

But the concepts so far also call for "aerocapture" of the orbiter directly from solar orbit into orbit around Titan by skimming through Titan's outer atmosphere behind a heat shield

I think NASA would consider this to be too risky. We haven't even done this on Mars.

What instruments, besides cameras, do you all think should be on the next Titan mission?

[djellison]

HGA + Titan Atmosphere = big sail = hard to get good earth pointing

Doug

[pioneer]

HGA + Titan Atmosphere = big sail = hard to get good earth pointing

I think you mean

HGA + Titan Atmosphere + big sail = hard to get good earth pointing

?

[djellison]

No, I mean any HGA worth having would act like a sail on a titan blimp and blow it around quite a lot so you'd have difficulty pointing. It'd be hard anyway - but even worse with a big HGA. The only way to go is with an Omnidirectional + relay

Doug

[pioneer]

Fascinating discussion guys.

Do you all think we should send an orbiter before a blimp or surface vehicle?

[djellison]

imho - aerobrake Cassini into Titanian orbit and do radar obs for the entire moon - and hope that it's still functioning for an 8kbps relay from a follow on blimp mission - but design the blimp mission with the capacity for say 500bps DTE via a MGA or similar - use MSL heritage power supply on the blimp - badda bing badda bang

Doug

[Gsnorgathon]

No no no. You guys are all thinking too small here.

Send a dirigible to Titan, and that way you can mount the HGA inside it.

Errm... maybe not.

[volcanopele]

For this I am assuming that Cassini will NOT enter Titan orbit in an extended mission due to lack of propellant.

I envision a two spacecraft mission. The first would be an orbiter that would go into Saturn orbit then use Titan flybys to put it into an orbit whose apoapsis is just beyond Titan orbit. From there it would go into titan orbit. The orbiter whould have the following instruments:

1) NIR-camera: we have seen from VIMS that the optical depth of Titan's atmosphere decreases with increasing wavelength. So at VIMS' max resolution of 2 km/pixel, VIMS can seen more features than ISS can at 2 km/pixel. So for the next mission, it is obvious that an imager with similar pixel scale as ISS but with the spectral range of the VIMS NIR channel would be appropriate.
2) Laser Altimeter: one of the keys in understanding the relationship between the bright and dark terrain is to map the elevation of the interaction. Do the shorelines represent a kind of "sea level" marker like on earth?
3) SAR radar (no question)

For the other probe, I would use somekind of blimp or dirigible (not necessarily because of the HGA issue). One requirement of a probe that would be below the haze would be mobility (a simple lander or even a rover would not be effective in this regard) in both latitude and longitude. So the motion of the probe must be controllable, either by the probe itself through some kind of autonavigation algorithm or from ground controllers). It must be capable of landing or send mini probes to investigate interesting surface features up close. The instruments for the blimp (or even helicopter):

1) a clone of the orbiters NIR-camera perhaps with additional filters in the visible
2) a mini-VIMS: with less atmosphere to deal with and a lamp, an instrument like this could do a wonderful job in identifying surface constintuents
3) various atmospheric instruments
4) some kind of surface science package that would be a mix between the Huygens SSP and the MER IDD with a raman spectrometer added in