[X] My Assistant

[edstrick]

The Cassini de-scoping was a catastrophic move forced as an official DIKTAT from NASA HQ. My impression is that it was largely a YOU WILL DO THIS EXACTLY AS WE ORDER, OR ELSE affair. A spacecraft with a grossly descoped 1 axis of motion scan platform (toward and away from the antenna boresight) would have had far more capability for combined radar/radio-science and remote sensing observations because spacecraft roll maneuvers would have done the work of an azimuth slew on the scan platform without removing the antenna from Earth-pointing or target pointing orientation. The only instruments being severely compromised would have been the particles instruments in the fields-and-particles suite.

I very much doubt that loss of all scan platform capability compared with a 1 axis platform would have saved much $ and cost more in the long run in terms of complicated misison operations.

Ultimately, the fault boils down to apalling fiscal mis-management of the Mariner Mark 2 program by JPL and NASA HQ. There was a major article in Science in the later 80's on the Mariner Mark 2 concept on how standardized spacecraft designs and components could dramatically reduce the per-mission cost of Voyager/Galileo class missions. Uh... duh. I've never seen a clear explanation of how they blew it so badly.

[Holder of the Tw...]

"Penny wise and pound foolish" is a good way of stating it. "Catastrophic" may be a little too harsh. Even with the limitations imposed by fixed optical instruments, quite a bit has been accomplished with them.

In regards to the fields and particles suite, Cassini couldn't have done any better with them, given the fact that (for good reason) nobody was going to do another Galileo design. If you want an optimal fields and particles mission to Saturn, you're going to need a Pioneer type spin stabilized orbiter.

[Guest_AlexBlackwell_*]

The Cassini de-scoping was a catastrophic move forced as an official DIKTAT from NASA HQ.

The projected cost savings (20%) of the redesigned Cassini mission, which included deleting the scan platform and substituting pointing of the entire spacecraft, were discussed in a 1992 letter from the Space Studies Board to NASA.

It would be nice to know how much money was actually saved.

BTW, I seem to recall that one of the final "let's-kill-Cassini" attempts resulted in NASA planning a launch of Cassini and its upper stage(s) into low Earth orbit in segments (using the Shuttle) and then having astronauts assemble the spacecraft.

This post has been edited by AlexBlackwell: May 17 2007, 06:03 PM

[edstrick]

"...If you want an optimal fields and particles mission to Saturn, you're going to need a Pioneer type spin stabilized orbiter...."

The original Cassini design had one high-precision scan platform with the remote sensing instruments and, (I think) TWO low precision platforms on opposite sides of the spacecraft to oscillate fields and particle instruments back and forth to provide full coverage of opposing hemispheres. (At least one instrument on voyagers, the LECP - low energy charged particle spectrometer - stepped it's field of view around the accessible parts of the sky) Not exactly cheap, but a lot cheaper and less likely to have problems than the Galileo dual-spin design, which was worse than a jimson-weed nightmare trip. It was a miracle beyond understanding and an engineering effort of trancendental magnitude to have pulled off the dual spin design without more than occasisonal spacecraft "upsets" due to noise from the dual-spin signal commutator.

[Exploitcorporati...]

Expanded version of PIA08365:

EDIT: Moved

[Exploitcorporati...]

Scale comparison:

[mcaplinger]

I've never seen a clear explanation of how they blew it so badly.

http://www.gao.gov/docdblite/info.php?rptno=NSIAD-94-24 is a good place to start.

[David]

As someone who's lived for much of his life in lake country, I have to say that those large-scale images of Titan's lakes/seas are remarkably beautiful. They may be filled with ethane and methane, the landscape may be unbearably cold and barren, and the atmosphere filled with unbreathable gasses -- but it still looks more like a home away from home than any other place in the Solar System.

John Wilkins, writing in 1638 on behalf of the concept that there was "a world in the Moone", stressed his belief that there were seas, rivers, and an atmosphere on the Moon, that would make it a "world" like the Earth. We now know, of course, that Wilkins was wrong; but what was false of the Moon is true of Titan, and it is in those terms the only "world" we know of other than the Earth.

[belleraphon1]

Scale comparison:

Exploitcorporations

Beautiful... my mind is on an awesome tour right now....

I live just 30 miles from the shore of Lake Erie. That is the bottom most lake in the Great Lakes image.
Every winter I travel to the lake shore to watch the pressure ice ridges push up. They ger sculpted by the grit and wind and snow into wonderful patterns. I can almost imagine myself hiking the karst-like terrain in the other image.. the Titan Seas...

If this is truly to the same scale, notice how much more rugged the Titan terrain appears .... of course, I assume the Earth image is from visible wavelengths and not radar, even so... how much more muted is the
srrounding landscpake on Earth compared to Titan. Is this due to the radar or an actual function of the two landscapes? Note the other small Earth lakes (water filled), that dot the landscape here... the Titan lakes further north that dot that world are much larger in comparison... but there are visual similarities....
Titan sure looks like a rough place at these latitudes... what a wonder it will be to hike there....

Thanks... I will be now be on a virtual hike all afternoon.... look forward to bouncing my grandsons on my knee and letting them trace alien tributaries with their little fingers... and inspire them to hike a world with orange skies, ruddy, craggy ice-ammonia shelves, puddles and rivers and seas of hydrocarbons, littered with a mix of pre (or post) biotic chemistries...

Craig

[belleraphon1]

All... a bit of fun...

This is a link to an image I took at Headlands on Lake Erie in February of this year. I have played a bit with the coloration to imagine if the scene was on Titan. Everything seen here is now melted and sunk into the lake waters..... I'd have to swim to be at this location today.

http://www.flickr.com/photo_zoom.gne?id=505044868&size=l Sorry... this is the kid in me playing

On a more serious note...

Looking at the Comparisin Great Lakes Titan Seas radar image I am struck by the large methane sea to the right. Note how we can see the continuation of the same topography under what seems to be a shallow layer of liquid. And that topography is not modified at all compared to the shore and the land adjacent to it. So if this is liquid , there has not been time for erosion or settling of hydrocarbon sediment to obscure the flooded terrain. What a neat place THAT would be to scuba...

Craig

[Exploitcorporati...]

That's really a very evocative image, Craig. No need to apologize! As for the scaling in the comparison image, it's taken directly from PIA09184... important to remember the curvature over these surfaces are very different, but it gives a nice idea.

[Holder of the Tw...]

Sorry everyone, hate to take things off topic again.

The original Cassini design had ... TWO low precision platforms on opposite sides of the spacecraft to oscillate fields and particle instruments back and forth to provide full coverage of opposing hemispheres.

Well, I'll have to modifiy my stance a little bit. Cassini could have done a bit better in fields and particles than it did, but it's hard for me to imagine, for instance, building a platform to swing the magnetometer around. With regards to the paper Alex quoted, I can understand that slewing the spacecraft would make interpreting the field readings more complicated, but isn't there also an offsetting gain by sampling in more than one direction?

At this point I'm still convinced that remote sensing vs. fields/particles is best resolved by using two different spacecraft optimized for each.

"...one of the final "let's-kill-Cassini" attempts resulted in NASA planning a launch..."

What is it, exactly, with NASA's suicidal tendencies in regards to it's own science missions?

[edstrick]

Magnetometer data needs to 1.) know the attitude of the spacecraft and 2) be able to clean the data of any spacecraft generated field. It can stay on a fixed boom.

OTHER instruments, essentially particle instruments (not fields) need more or less omindirectional coverage of the sky to see where particles of different energies and copositions are "streaming" (or beaming or blowing or whatever) from. Solar wind spectrometers can survey the sunward field, but inside magnetospheres, they can see plasmas coming from all directions. Electron spectrometers and other instruments pretty much have to look everywhere.

With a mission like Cassini, you can have multiple detectors looking -- and maybe scanning -- in various directions, or when not doing pointed observations, you can put the spacecraft in a slow roll to do a Pioneer or Galileo-like scan around the roll axis. For specific observation sequences, you can orient the entire spacecraft and do field-of-view pointings or scans, etc. It's just that it all complicates the mission and forces you into more and more tradeoffs... I gotta get Enceladus plume composition data this pass, I can't do remote sensing etc... sort of tradeoffs. We've had some at Titan like that too.

[peter59]

Cassini Radar Images Sea, Islands and Mountains

[Exploitcorporati...]

Seas map updated with approximate position and scale of newly released image from T30: