Cassini's Extended Mission, July 2008 to June 2010 |
Cassini's Extended Mission, July 2008 to June 2010 |
Nov 21 2008, 12:15 PM
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#211
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Senior Member Group: Moderator Posts: 2785 Joined: 10-November 06 From: Pasadena, CA Member No.: 1345 |
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) -------------------- Some higher resolution images available at my photostream: http://www.flickr.com/photos/31678681@N07/
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Nov 21 2008, 12:35 PM
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#212
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Senior Member Group: Members Posts: 3516 Joined: 4-November 05 From: North Wales Member No.: 542 |
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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Nov 21 2008, 01:07 PM
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#213
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Junior Member Group: Members Posts: 63 Joined: 18-November 08 Member No.: 4490 |
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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Nov 21 2008, 05:07 PM
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#214
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Senior Member Group: Members Posts: 2173 Joined: 28-December 04 From: Florida, USA Member No.: 132 |
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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Nov 21 2008, 11:51 PM
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#215
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Senior Member Group: Members Posts: 1414 Joined: 26-July 08 Member No.: 4270 |
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? -------------------- -- Hungry4info (Sirius_Alpha)
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Nov 22 2008, 04:13 AM
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#216
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Member Group: Members Posts: 599 Joined: 26-August 05 Member No.: 476 |
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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Nov 22 2008, 05:28 AM
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#217
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Administrator Group: Admin Posts: 5172 Joined: 4-August 05 From: Pasadena, CA, USA, Earth Member No.: 454 |
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. --Emily -------------------- My website - My Patreon - @elakdawalla on Twitter - Please support unmannedspaceflight.com by donating here.
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Nov 22 2008, 10:57 AM
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#218
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SewingMachine Group: Members Posts: 316 Joined: 27-September 05 From: Seattle Member No.: 510 |
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.
-------------------- ...if you don't like my melody, i'll sing it in a major key, i'll sing it very happily. heavens! everybody's all aboard? let's take it back to that minor chord...
Exploitcorporations on Flickr (in progress) : https://www.flickr.com/photos/135024395@N07/ |
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Nov 26 2008, 05:41 PM
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#219
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Senior Member Group: Members Posts: 1578 Joined: 14-October 05 From: Vermont Member No.: 530 |
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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Nov 30 2008, 01:20 AM
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#220
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Senior Member Group: Members Posts: 1887 Joined: 20-November 04 From: Iowa Member No.: 110 |
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Dec 17 2008, 07:49 AM
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#221
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Member Group: Members Posts: 568 Joined: 20-April 05 From: Silesia Member No.: 299 |
-------------------- Free software for planetary science (including Cassini Image Viewer).
http://members.tripod.com/petermasek/marinerall.html |
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Dec 17 2008, 08:13 AM
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#222
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Member Group: Members Posts: 754 Joined: 9-February 07 Member No.: 1700 |
Looks like the ESPN site. Can I get the basketball scores on the Cassini page now?
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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Dec 17 2008, 08:31 AM
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#223
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Founder Group: Chairman Posts: 14431 Joined: 8-February 04 Member No.: 1 |
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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Dec 17 2008, 10:16 AM
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#224
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Member Group: Members Posts: 470 Joined: 24-March 04 From: Finland Member No.: 63 |
Looks like the ESPN site. Can I get the basketball scores on the Cassini page now? I liked the current number of moons counter they used to have on the Cassini site . That was a good way of keeping up with the scores. -------------------- Antti Kuosmanen
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Dec 17 2008, 10:30 AM
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#225
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Senior Member Group: Members Posts: 3516 Joined: 4-November 05 From: North Wales Member No.: 542 |
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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