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Cassini's Extended Mission, July 2008 to June 2010
Juramike
post Nov 21 2008, 12:15 PM
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QUOTE (jasedm @ Nov 21 2008, 06:15 AM) *
2) Design a spectacular and daring final two-year segment to the mission
Pros:
Cheaper


I think that's the key argument right there. (lower risk AND cheaper)


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ngunn
post Nov 21 2008, 12:35 PM
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Well a Solstice Mission has already been talked about, so some on the team must already be inclining to the long option. It might cost more in the long run, but it might actually cost less per year which I'm sure would be welcomed in the current budget squeeze.
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sci44
post Nov 21 2008, 01:07 PM
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I would go with keeping Cassini going as long as possible, with occasional flybys using the lowest energy trajectories possible.
Taking "Guest Analysts" post earlier in this thread as a basis, in 2006 Cassini used 40 kg biprop (480kg down to 440kg) and 11 kg mono (104kg down to 93 kg) during a very active tour year. Assuming monoprop to be the limiting factor, if using 27g/day mono for "tour" mode, thats 10 kg year = 9 more years usage (from end 2006). If you are prepared to reduce towards "cruise" mode usage for long periods (6g/day) thats about 2kg/year = 45 more years usage. Of course the limit would then be the RTGs, batteries, gyros and other instrument life - but taking Voyager as an indication of what may be possible, we *could* have 20-30 years more use out of Cassini before its "death plunge", with sufficient biprop left for occasional major maneuvers & targetted flybys. Again, thats all just based assuming those original figures are right.
It would also be really useful to still have Cassini to hand if there is a future mission to Titan, a decade or more hence.

The largish amount of remaining biprop on board is intriguing - I wonder how much would be needed for orbital insertion into Titan, together with an aerobrake procedure? I know this idea was mooted/dismissed a long time back, but it would be interesting to know what the figures are. Would a high orbit around Titan be considered "bio-safe" and stable long term?
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centsworth_II
post Nov 21 2008, 05:07 PM
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QUOTE (Juramike @ Nov 21 2008, 07:15 AM) *
I think that's the key argument right there. (lower risk AND cheaper)

"Lower risk" because you are killing the craft before it has time to die itself.
Cheaper because you are killing the craft relatively quickly and ending ongoing costs.

I think the key argument is which mission will gain the more valuable information. Given the long life of various satellites and probes recently, I think its reasonable to think Cassini can last until the fuel runs out. I do recall some concern about gyros, and maybe that is a legitimate concern for and "untimely" end.
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Hungry4info
post Nov 21 2008, 11:51 PM
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QUOTE (sci44 @ Nov 21 2008, 07:07 AM) *
The largish amount of remaining biprop on board is intriguing - I wonder how much would be needed for orbital insertion into Titan, together with an aerobrake procedure? I know this idea was mooted/dismissed a long time back, but it would be interesting to know what the figures are. Would a high orbit around Titan be considered "bio-safe" and stable long term?


I haven't done the math, but I would guess that it might be tricky. I would venture to guess that Titan's atmosphere probably fills a decent portion of the volume of Titan's gravitational sphere of influence. Orbit around Titan would have to have an altitude of several hundred km of course, but how high can you get without Saturn's gravity plucking you from orbit?


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mchan
post Nov 22 2008, 04:13 AM
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Plugging in the numbers into the approximation from the Wikipedia page (caveat emptor), Titan's Hill radius is ~55,000 km.

Ttan's atmosphere is extended. Cassini's flybys had to be raised from the early mission plans to over 1000 km to reduce aerodynamic heating at the flyby speeds. It's a little less dense over the poles, so the polar flybys go in near the original planned 950 km.

The TSSM3 pdf from the November OPAG meeting mentions Titan orbit altitudes. For aerobraking, periapsis is ~600 km (dual purpose of direct atmospheric sampling for the mass spectrometer). Circular orbit is ~1500 km to avoid having to burn propellant to maintain altitude.
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elakdawalla
post Nov 22 2008, 05:28 AM
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QUOTE (mchan @ Nov 21 2008, 08:13 PM) *
Cassini's flybys had to be raised from the early mission plans to over 1000 km to reduce aerodynamic heating...

Just a small correction, AFAIK it wasn't aerodynamic heating that was the problem, it's that the atmosphere appeared to be exerting more torque on the spacecraft than their models predicted, which creates a problem for planning the spacecraft's attitude during the flyby.

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Exploitcorporati...
post Nov 22 2008, 10:57 AM
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A few asides from the peanut gallery...for the iceball-obsessed crowd, myself included, I think we've gotten a virtually miraculous tour and XM. I'd like to see another reasonably close pass at Hyperion from a different perspective, a couple more close views of Dione's cliffs, preferably in low sun, and as many visits to the various rocks as are possible. Another Iapetus encounter is probably out of the question. Gapfill imaging of the north polar regions of each world would be nice wherever possible as well. In the XM, we have high-resolution coverage of the poorly imaged leading hemisphere of Enceladus, the fractured side of Dione, Rhea again, and Herschel on Mimas to look foreward to already, not to mention the Helene flyby and a few decent nontargeted encounters. A few more SAR swipes at Titan would give us 50% coverage in the long term, and maybe complete IR coverage of the emerging northern hemisphere. I'm inclined to support the long option, with the primary focus on atmosphere and rings, especially in light of the ugly financial picture. It would be cool to see those ring-shadows spread again, and watch the south turn blue.


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stevesliva
post Nov 26 2008, 05:41 PM
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QUOTE (jasedm @ Nov 19 2008, 08:19 AM) *
I suppose it must come down to a decision over which science instruments are considered 'most important' and which draw the most power - a very difficult call, which will change on each orbit according to target importance/flyby distance.


I never did reply to this because I really don't know, although it is important to remember that without a scan platform good chunks of the instruments aren't necessarily being used at times. Fly-bys now will change pointing several times, but if you're assuming that (for example) the imaging instruments are off for an entire fly-by, you probably get much simpler mission planning as well as lower power consumption. I don't know if you could do it without reaction wheels. Fields and particles, I guess.
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alan
post Nov 30 2008, 01:20 AM
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Stumbled across this kodak moment using the solar system simulator
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peter59
post Dec 17 2008, 07:49 AM
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Completely new design of Cassini-Huygens site.
http://saturn.jpl.nasa.gov/
I liked the old. mad.gif


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brellis
post Dec 17 2008, 08:13 AM
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Looks like the ESPN site. Can I get the basketball scores on the Cassini page now? huh.gif

Seriouserly, I fondly recall the way the Cassini page looked during the long flight to the Saturn system. The status reports gave me comfort, and made me feel very close to the mission itself. IMO there's not much need to go for the "wow" factor with a splashy front page -- there's plenty of "wow" in the images themselves to attract people to the amazing scientific achievements of the Cassini mission.
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djellison
post Dec 17 2008, 08:31 AM
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I like it. The stuff I regularly look at is still easy to find, and the stuff I didn't know about is now there as well. The old site was a navigational mess.
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akuo
post Dec 17 2008, 10:16 AM
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QUOTE (brellis @ Dec 17 2008, 09:13 AM) *
Looks like the ESPN site. Can I get the basketball scores on the Cassini page now? huh.gif

I liked the current number of moons counter they used to have on the Cassini site smile.gif. That was a good way of keeping up with the scores.


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ngunn
post Dec 17 2008, 10:30 AM
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For me the new site takes ages to start. The raw image thumbnails have a bizarre horizontal stretch. Worst of all searching the raw images is so slow as to be completely impractical. When I eventually got page 2 of 'Titan' 'Newest' to open just now I fiound myself looking at an image from February. I seem to recall we've had more that 10 images of Titan since then.

Call me when it's fixed.
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