Vesta departure and journey to Ceres, A new phase of Dawn adventure |
Vesta departure and journey to Ceres, A new phase of Dawn adventure |
Aug 26 2012, 05:45 AM
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Senior Member Group: Members Posts: 2492 Joined: 15-January 05 From: center Italy Member No.: 150 |
As suggested by "TheAnt", is time to start a new thread!
Due to lack of updates from "Mystic simulator", I asked directly to Marc Rayman for an update and he kindly sent me the following "unofficial" plots, showing evolution of distance, radial velocity and sub-spacecraft latitude (radius trend is splitted in two plots with different scales due to huge variation of the distance): I did not publish them immediately because I wanted to integrate them with my previous plots but, due to the lack of time in last week, I decided to post them directly with only minimal changes. As you can see, third engine stop occurred on Aug 8-18 at almost 2300km above Vesta while next (final) one is just started and, as reported by Marc in his latest journal, will be used to map North polar regions from 6000km distance. Thereafter thrust will resume and, based on plots, on Sept 5 total speed will overcome escape velocity (almost 45 m/s), so total energy balance will be zero; definitive exit from Vesta Hill sphere should occur after 20 days but consider that the times could slighty change due to orbit refinement and thrust plan changes. -------------------- I always think before posting! - Marco -
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Aug 27 2012, 06:15 PM
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Junior Member Group: Members Posts: 22 Joined: 23-August 11 From: France Member No.: 6134 |
I must apologize for my previous post.
According to the spice files released today, there was an attempt to observe Vesta as planned. The ck file shows that the probe has problem to maintain her orientation but i don't know if this effect is due to the ck files (always under construction) or the problem of lack of one reaction wheels. From now, I will avoid any speculations useless alarmist and I will wait the official update of wonderfull Dawn journal and Dilo states plots (specially the states of engine propulsion ) (from Mystic or Marc). And of course hope of beautiful images of Vesta's north pole Again very sorry. |
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Aug 28 2012, 04:59 PM
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#3
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Senior Member Group: Members Posts: 2492 Joined: 15-January 05 From: center Italy Member No.: 150 |
I had a private comunication from Mark, he confirmed that they successfully imaged Vesta over the weekend.
Moreover, he said there wasn't any problem with the SPK file. As explained in several Dawn Journals (including November 29, 2011), they generate a reference trajectory that goes all the way to the next destination but then, as they fly along, they generate updated flight profiles to account for all the usual error sources (orbit determination accuracy, unmodeled perturbations from RCS, deviations in IPS thrust from the modeled value, etc.). Each time an updated thrust plan is sent to the spacecraft, they only send the next thrust segment, not the full reference trajectory; in this way, in case of failure, they don't have onboard ephemeris based on thrusting that didn't occur. So when they upload the new thrust segment, they upload a new ephemeris as well. Probably, SPK issue Sarunia found is for an isolated thrust segment which then is followed by a ballistic propagation... -------------------- I always think before posting! - Marco -
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