Vesta departure and journey to Ceres, A new phase of Dawn adventure Options

[dilo]

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.

[djellison]

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.

[Marz]

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.

[dilo]

Update on departure progress:

(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).

[dilo]

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:

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...

[dilo]

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):

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