Could Cassini orbit Titan during extended mission?

[vogon13]

I noted the transfer orbit technique planned for the Messenger mission to Mercury to minimize delta-v for orbit insertion burn and wondered if it could also be applied to the Cassini mission in 2008-2009 time frame to allow probe to end its mission in a polar orbit around Titan? Advantages would include radar mapping a much higher percentage of surface, closer view for rest of instruments, frequent occultations of craft radio signals for more complete study of atmosphere variation over time, etc. Am I just dreaming or would this be feasible? <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[spacester]

Without doing a detailed analysis, I would have to say "no".<br /><br />You need the influence of a massive body (other than the one you're orbiting) to change the orbital period. Cassini is orbiting Saturn, so no help there.<br /><br />Cassini is going too fast relative to Titan to be directly captured by it, so no help there. <br /><br />There are no other really massive moons around Saturn, so no help there.<br /><br />They don't have a lot of extra deltaV from on-board propellant, so no help there.<br /><br />Perhaps the wizards could find some trick using weak stability boundries combined with very close Titan fly-bys and the last drops of propellant, but I doubt it. <div class="Discussion_UserSignature"> </div>

 

[thalion]

Actually, they've talked about the possibility of Cassini orbiting Titan for some time. I personally favor it, but only after exploring the other moons to satisfaction. That said, I think they'll just opt for more flybys of all the moons including Titan if they get an extended mission. With luck, this bad boy could last for decades...<br /><br />Here is a link on the topic:<br />http://digilander.libero.it/lucktam/EXT.html

 

[bobvanx]

Well, I was just looking through a stack of webpages, to see if I could find how much of Titan will be mapped after the planned 45 flybys. They mapped, 1%? During the first flyby, so with overlap and targeting, between 25% and 33% is a SWAG I could stand by.<br /><br />An orbital extended mission would allow mapping of at least 60% of the surface. One awkward thing, is Cassini has to follow its target and then turn to send data. When it's on big looping orbits, it has time to do this. If it was in an orbit with a period of just a few hours, it would be more challenging.

 

[najab]

><i>Cassini is going too fast relative to Titan to be directly captured by it, so no help there.</i><p>Don't know if you looked at the linked article, but aerocapture is possible at Titan.</p>

 

[bobvanx]

>>aerocapture<br /><br />I <i>love</i> that idea. The radio dish is already toughened, they built it to be a shield for the ring crossings at insertion. Obviously there is better stability with it oriented at the trailing side. But since they'd probably be very conservative with any braking force, Cassini has enough mass and reaction control that they could use it forward-facing with a low risk of tumbling. <br /><br />Does it have inertial guidance, or just star camera guidance?

 

[bobvanx]

>>not as much as we'd like can be seen of Titan's surface by a craft in orbit.<br /><br />The Cassini team will be changing the craft's orbital inclination all through the mission, so a polar orbit, a true mapping orbit, would be possible if the other constraints weren't insurmountable. So we really could build up a good map from Titan orbit.<br /><br />I agree that Cassini is best left in orbit around Saturn. They could still perform many more flybys of Titan, even to the high latitudes, and map more of the surface. As interesting as Titan is, to think we'd be done exploring the whole system after 4 years seems absurd. Or even after 12 years.

 

[mikejz]

I would rather keep cassini around for a lot of years. Maybe it could be used as a data relay for follow on missions. (Ie more Titan probes or smashing a small probe into the rings....)

 

[najab]

><i>Cassini is best left orbitting Saturn. Which, after all, IS its primary mission, not Titan.</i><p>You do realise the question was <i>could</i> it be done, not <i>should</i> it be done?<p>><i>Cassini is simply moving too fast to easily be put into an orbit around Titan. That's why it just dropped the Huygens off and...</i><p>You do also realise that we're talking about a hypothetical extended mission - probably the second or even third such extended mission.</p></p></p>

 

[mikejz]

I wounder if the RTGs would still have enought power to operate the Radar that long out....

 

[newtonian]

Steve -I agree. Considering tje detail Huygens is showing, such as the analysis of the crust just shown live on the Science channel [a special is now airing] - the right choice was made.<br /><br />The surface would be largely hidden, though radar images do show some detail.<br /><br />A few minutes left if you want to tune in to science channel special now airing.<br /><br />The last probe text aired was a graph showing consistency of the soil, harder thin crust, crumbly underneath.<br /><br />Perhaps snow with a crust on top? <br /><br />OK, 5 miles up photo now airing. <br /><br />Perhaps clouds over a sea? Clearly the photo needs study.

 

[mikejz]

Humm... Not Aero Capture, but maybe with some aerabreaking on Saturn's atmosphere or Titan's it would be able to reduce its eliptical orbit, then use its remaing fuel to enter orbit around Titan (aerobreaking again to lower it's orbit). <br /><br />The key question of course is how much fuel is remaining. It might be intresting if a follow on mission could launched to refuel Cassini as sort of an experiment in automatic docking and repair.....

 

[najab]

><i> Cassini doesn't have that, so it'd burn up rather than slowing down, just as Huygens would have without its heat shield.</i><p>This is an apples to elephants comparison. Hygens was a <i>lander</i>, Cassini is an orbiter. Cassini would be able to aerobrake into a caputure orbit without entering more than the fringes of Titan's atmosphere.<p>I suppose, using your logic, that Mars Odessey and Mars Global Surveyor didn't succesfully aerobrake/use aerocapture into Martian orbit since they didn't have the heat shields that the MER rovers did.<img src="/images/icons/rolleyes.gif" /></p></p>

 

[bobvanx]

Yes.<br /><br />All in all, I think we'd have to see a very compelling reason to orbit Titan in order to do so. But a "heat shield" clearly isn't necessary. The big radio dish would do the job just fine, lowering the orbit little by little in successive passes through the upper atmosphere.<br /><br />Aerocapture at Mars required near daily weather maps, so they could gauge the density of the atmosphere during each pass. This was done by the orbital assets already in place. Cassini doesn't have that support, but perhaps Titan's high altitudes are less variable.

 

[mikejz]

To be honest I think this is the sort of question that has two option<br /><br />1) We keep debating if it can be done<br /><br />--or--<br /><br />2) We find someone in the know and ask them about it<br /><br />It seems to be that this is the sort of idea that has been batted around already at JPL, and some numbers have had to be run on it. Its just a matter of finding the right person in the know.

 

[bobvanx]

>>find someone in the know<br /><br />Those overworked JPL scientists are prob'ly simply aching to have one of us fire off a short email, asking if someone has thought about this! Well, I'll Google it and see if something turns up.

 

[bobvanx]

From the January 16, 2000 issue of SpaceRef:<br /><br />"Indeed, according to JPL, there are mission concepts which would use aerobraking within Titan's atmosphere to place Cassini into orbit around Titan so as to perform detailed observations."<br /><br />The pertinent info has been removed from the Cassini website. The reason could be that it turned out to be impossible, or it could be they just didn't want to talk about it yet.

 

[bobvanx]

Steve, the radio dish on Cassini is sturdy enough to use for aerobraking.

 

[CalliArcale]

Because it wasn't designed for it does not mean it is impossible. Magellan was never designed for aerobraking, yet successfully accomplished it. (It is worth pointing out, however, that Magellan was travelling a lot slower relative to Venus than Cassini is relative to Titan, and less delta-vee was required to acheive the effect.) Magellan lowered its orbit several times deliberately in order to study the upper atmosphere of Venus by observing how it affected Magellan's radio signal. Eventually, of course, this put it onto a doomed trajectory, and eventually it deorbited. (Therefore, this was not done until the end of the extended mission.)<br /><br />MGS and Mars Odyssey 2001 lack heatshields. They mainly made use of their solar arrays for aerobraking. The vehicle does not pass through so much of the atmosphere that a heat shield is neccesary; all that is needed is a nice big surface, really. The Mars probes feathered their solar arrays to maximize drag during the aerobraking maneuvers.<br /><br />(BTW, the rovers did not aerobrake, at least not in the sense of using atmospheric drag to reduce one's orbital energy. They never entered Mars orbit at all.)<br /><br />Aerocapture, of course, is a different kettle of fish. That has never been attempted.<br /><br />I would expect an effort to enter Titan orbit to involve a lot of delicate work with orbital mechanics. Saturn has a very complex moon system, which could theoretically be exploited to gently nudge Cassini into a trajectory which would make orbit insertion possible with its limited remaining propellant, probably exploiting the Saturn-Titan lagrange points. Unfortunately, most of these moons are not very large, with the exception of Titan itself. That would complicate the maneuver. I would not be surprised if the neccesary encounters to acheive Titan orbit insertion are not completely impractical within the time available, without completely changing the primary mission schedule. And that's not going to h <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[Leovinus]

I can see putting Cassini into orbit around Titan. <br /><br /><br />Orbit 523: Hazy.<br /><br />Orbit 524: Still hazy.<br /><br />Orbit 525: You think London fog is hazy, you ain't seen nothing like Titan.<br /><br />Orbit 526: Yep, still hazy. <div class="Discussion_UserSignature"> </div>

 

[najab]

><i>If that is true, would it be too damaged by aerobraking, beyond all reasonable doubt or not, to work?</i><p>I haven't the means to do a comprehensive engineering study, but I'm reasonably confident that the dish would not be damaged by aero-braking. It is the most sturdy part of the probe, remember that during the ring crossing the dish was turned into the velocity vector to protect the rest of the probe from debris hits. Despite the numerous hits from dust and small particles the dish is, to the best of my knowledge, working perfectly.<p>><i>Whether or not it's not impossible is not the point.</i><p>Actually, if you go back to the original question, that <b>is</b> the point. We aren't Cassini mission managers (at least I'm not) so it's not our decision to make. We're just doing some idle speculation on what might be possible.<p>><i>I'm not interested in possibilities.</i><p>Oh. Never mind then.</p></p></p></p></p>

 

[chew_on_this]

I would rather see an orbit of Titan than smashing Cassini into Saturn ala Galileo. Maybe then we can monitor changes.

 

[yesrushgen]

Having Cassini enter orbit about Titan would be easy, if lengthy. Entering orbit would be all about reducing the required capture delta-V. For example, on the first 3 flyby's of Titan, we passed a relative velocities of some 5km/s. The required capture delta is less than that, but not much. Clearly, capture deltas of this magnitude are not practical.<br /><br />So, how to reduce this? Simple. We engage in a campaign to make Cassini's (Saturn centered) orbit more closely match that of Titan's. These are known as resonant encounters. Actually, the cancelled Europa orbiter would've used this technique to enter orbit around Europa - by reducing the capture delta-V.<br /><br />Now, with Cassini, we have an added challenge. At the end of the nominal mission, we are in a HIGHLY inclined, near polar, orbit around Saturn. With an inclination like that, relative encounter velocities with Titan are extremely high.<br /><br />Step 1 would be a series of Titan flybys that would bring Cassini back into an equatorial orbit around Saturn.<br /><br />Step 2 would be a series of resonant encounters with Titan to "pump down" or "pump up" Cassini's orbit to closely match Titan's. Depending on how far we wish to go, we would get the capture delta-V down to less than 100m/s - easily within reach of the main engine, or HGA dish if we wish to aerocapture.<br /><br />Step 3 is capture into Titan orbit. The initial orbit will be highly eccentric, depending on (a) how far we dip in atmosphere if aerocapturing or (b) how much fuel we use if using a TOI burn. The other consideration with this step is to capture into a polar orbit about Titan. Equatorial would be boring. That has to be planned VERY carefully. Once in orbit of Titan, we won't be able to change inclination very much.<br /><br />Step 4 If we aerocaptured, we need to immediately perform a burn on the next Apoapsis to prevent hitting the atmosphere again. After that, we need to evaluate the next phase of...<br /><br />Step 5 Aerobraking. We need t

 

[chew_on_this]

This makes so much more sense to me if all the craft is going to do is flit around saturn for four years. There's only so many pics one can look at of dull, cratered, gray moons. Conserve fuel for this to be it's end. This allows for a base of communications for future missions not to mention intense mapping for such missions.

 

[brellis]

Here's a LINK to an academic site where extended mission ideas are listed. I think they should leave Cassini in orbit with enough fuel to be able to support future missions. When it flew by Jupiter, they got some cool 'stereo' observations with Galileo, didn't they? <div class="Discussion_UserSignature"> <p><font size="2" color="#ff0000"><em><strong>I'm a recovering optimist - things could be better.</strong></em></font> </p> </div>