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Vesta departure and journey to Ceres, A new phase of Dawn adventure
Sarunia
post Sep 6 2012, 07:31 PM
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According to the last SPICE kernel (but based on predicted trajectory), Dawn was at 20600 km from Vesta at 9:43 UTC 06 september.
For checking Dawn in real time, Mystic seems to be better than another simulator with extra information that SPICE haven't (like engine status).
I think that the administrators of "Eyes of Solar System" and others just didn't updated the new data.

Dilo, could you ask Marc if the remaining reaction wheels were turned off during the last observation of Vesta ?
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djellison
post Sep 6 2012, 08:22 PM
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QUOTE (Sarunia @ Sep 6 2012, 12:31 PM) *
I think that the administrators of "Eyes of Solar System" and others just didn't updated the new data.


Correct - we just don't have the time.
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Marz
post Sep 6 2012, 08:37 PM
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Marc just updated his excellent blog:

http://blogs.jpl.nasa.gov/tag/marc-rayman/

So the reaction wheels are nominal and it was simply a programming error that was trying to deactivate them earlier? That is excellent news! C'mon Ceres... are we there yet? [:-p]

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Floyd
post Sep 6 2012, 09:05 PM
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My reading of Marc's blog didn't indicate things were "nominal" with the reaction wheels--rather that one malfunctioned and all were turned off a week earlier than they were going to turn them off. I believe they started with 3 and 1 backup and have now lost 2. That is not good and will have to be worked around at Ceres when the space craft will need to do lots of turning for imaging and other science. Where did you hear it was an software/programming error rather than a hardware failure?


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djellison
post Sep 6 2012, 09:55 PM
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In a talk by Marc last week for the EPO folk on lab - he echoed what you describe Floyd. Marz - I think you have things wrong.
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Marz
post Sep 6 2012, 10:32 PM
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QUOTE (djellison @ Sep 6 2012, 03:55 PM) *
In a talk by Marc last week for the EPO folk on lab - he echoed what you describe Floyd. Marz - I think you have things wrong.


darn! Here's the quote from the blog, with italics around the statement I misinterpreted and bold in the phrase I used to jump to conclusions.

"on August 8 ... one of the reaction wheels experienced an increase in internal friction. ... Protective software quickly detected the event and correctly responded by deactivating that wheel and the other two that were operating, switching to the small jets that are available for the same function, and reconfiguring other systems, including powering off the ion thrust and turning to point the main antenna to Earth.

A routine communications session the next day revealed to mission controllers what had occurred. They had planned long ago to turn the wheels off for the flight from Vesta to Ceres, so having them off a few weeks early was not a significant change. The team soon restored the spacecraft to normal operations and reformulated the departure plan, and on August 17 Dawn resumed its ascent. Because of the hiatus in thrusting, escape shifted from August 26 to September 4. The flexibility in the mission timeline provided by ion propulsion made this delay easy to accommodate."


The next paragraph hints that the reaction wheels are potentially out, since hydrazine is now a precious resource:

"In order to conserve the hydrazine propellant that the jets use, the bonus departure observations described before were curtailed, as they were not a high priority for the mission."

Hopefully we can find out more details on the reaction wheels on the Sept 8th hangout.
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dilo
post Sep 8 2012, 10:54 AM
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Update on departure progress:
Attached Image

(Note1: I smooted all previous phases in order to show only averaged distance/speed/energy without elliptic-orbit related modulations;
Note2: interpolations in top/right plot are simply 2nd order empiric curves without geometric/gravitational considerations behind).


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dilo
post Sep 11 2012, 01:09 PM
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I just received some informations from Dr Rayman which shed more light on the reaction wheel issue.
The wheels were all turned off on August 8, when the software detected the increase in friction, and were left off since then, including for the departure observations.
They restored spacecraft to normal operations, but one of "normal operational modes" is to fly with all wheels off (they flew that way from August 2010 to May 2011 and they intend to do so for the journey from Vesta to Ceres). So, "wheels off" is a normal operational mode and future Ceres exploration plan will be based on the use of the two reaction wheel + RCS "hybrid" control capability, installed on the spacecraft in April 2011. Marc highlighted that most of these information were available in previous Dawn journal articles and that he will write more about the reaction wheels, RCS control, hybrid control, and hydrazine in future Dawn Journals (although not the next one).

Herebelow an update on departure plots, showing also a brief engine stop occurred yesterday:
Attached Image

In the bottom/right, I substituted the "Energy ratio" plot with a more interesting plot showing the angle between Vesta-Dawn distance vector and Dawn velocity vector, calculated from total velocity and distance informations in Mystic Simulator (or radial velocity + distance in Marc plots). As you can see, Dawn trajectory is going from a "quasi-orbital" situation (with perpendicular vectors) to a more straightforward escape trajectory (zero angle). The spread of points in the September 5-9 interval is due to the poor precision in calculated distance differences; situation improved when I realized that this page gives more precise figures...


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dilo
post Sep 13 2012, 06:13 AM
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Marc confirmed there was a programmed engine stop on September,10 for communication with Earth (they do this each week but probably frequence will be relaxed during the journey to Ceres). Effect is clearly visible in velocity and orientation plots (right):
Attached Image

Based on my interpolations, Dawn should reach Hill sphere border on September,24 around 8:00 UTC, with 140m/s speed.


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dilo
post Sep 14 2012, 05:29 PM
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Update:
Attached Image

Based on new data points and improved interpolation, Hill sphere crossing date forecast is anticipated to September,23 17:40 UTC, if no further stops will occur...


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dilo
post Sep 16 2012, 03:36 PM
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Update:
Attached Image

No major changes on Hill sphere crossing date from interpolation; I ran also a simulation using constant engine orientation and thrust (5.1e-5 N/kg) and forecast time is only slightly different (September,23 15:30 UTC)...


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Explorer1
post Sep 16 2012, 05:36 PM
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It's amazing how symmetrical Vesta's hemispheres are on the global scale, on this topographic map that includes the far north.
But the northern basin isn't from a giant impact, is it?
http://dawn.jpl.nasa.gov/multimedia/imageo...p?date=20120911
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dilo
post Sep 16 2012, 06:51 PM
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Explorer1, the northern basin isn't a crater and blue color is due to oblate spheroid shape of Vesta.
In fact, text underline that colors represent distance relative to Vesta's center (or height relative to a sphere with 500km diameter); most important, scale of the two maps isn't the same because lower (violet) points in Northern emisphere are only 22km below such sphere while South depressions reach -38km.
Anyway, the presence of spiral patterns in both polar regions (with opposite orientations) is very intriguing... ohmy.gif

PS: I like very much the Giordano Bruno statement in your signature!


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dvandorn
post Sep 16 2012, 07:05 PM
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Yes, the spiral patterns resemble, more than anything else, the pattern of modification one sees at Mars' poles. But... Vesta couldn't possibly have ever had enough atmosphere to support ice caps, could it?

-the other Doug


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algorimancer
post Sep 17 2012, 01:18 PM
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It would be really helpful to have a 3D rendered globe of Vesta -- much would be come clear, I think.
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