MESSENGER Science Orbit(s)

[Aphyle2007]

Now that MESSENGER has gone into orbit, and will undergo formal commissioning of the science mission phase, I thought that I would fiddle with generating a science orbit for Cham's MESSENGER model. The basic orbit draft can be found here:

http://csmres.jmu.edu/geollab/pyle/celestia/MessengerScienceOrbit.ssc
http://csmres.jmu.edu/geollab/pyle/celestia/MessSciOrbt.ssc.txt



There are two primary problems that need to be resolved to complete this add-on. First of all, I am having difficulty resolving the orientation of the spacecraft, in a manner similar to Grant Hutchison's modification, such that the -X axis (sun shield) always faces the Sun and the Y axis always faces Mercury. I'm quite sure it is a matter of syntax in the .ssc file, but I'm frequently known to be wrong. My inexperience should be readily apparent by this point.

The second issue deals with the evolution of MESSENGER's science orbit over the next few months. This orbit will change over four cycles, but this should get things started. The actual orbit will change from periapsis of 200 km on 2011 Mar 18 (MOI) to 500 km on 2011 Jun 16 for Orbital Correction Maneuver 1 (OCM-1, -2), where it will return to 200 km. The latitude of periapsis will change from approx. +65 degrees to approx. +71.5 degrees in this same period, with OCM-1, -2 returning it to +65 degrees. Additional OCMs will be required about 2011 Sep 10 and 2011 Dec 16.

It would seem fairly straight forward to introduce revised orbits on the particular dates by epoch for the OCMs, but how does one set up an orbit that evolves over time to get to that point? Thanks in advance for help and input!

[selden]

Now that MESSENGER has gone into orbit, and will undergo formal commissioning of the science mission phase, I thought that I would fiddle with generating a science orbit for Cham's MESSENGER model. The basic orbit draft can be found here:

http://csmres.jmu.edu/geollab/pyle/celestia/MessengerScienceOrbit.ssc

Unfortunately, I get a 404 (file not found) when trying to access it, even when when trying to access it from the directory at
http://csmres.jmu.edu/geollab/pyle/celestia/. Very strange.

There are two primary problems that need to be resolved to complete this add-on. First of all, I am having difficulty resolving the orientation of the spacecraft, in a manner similar to Grant Hutchison's modification, such that the -X axis (sun shield) always faces the Sun and the Y axis always faces Mercury. I'm quite sure it is a matter of syntax in the .ssc file, but I'm frequently known to be wrong. My inexperience should be readily apparent by this point.

I'd use a Modified BodyFrame specification, myself.

Code: Select all

"Messenger" "Sol/Mercury"
{     ... appropriate orbit and other stuff ...
   BodyFrame { BodyFixed { Center "Sol/Mercury"}} #gets replaced
   FixedRotation {Inclination 0}
}

Modify "Messenger" "Sol/Mercury"
{
 BodyFrame {
    TwoVector {
        Center "Sol/Mercury/Messenger"
        Primary {
            Axis "z" # change to be axis of model which has the shield
            RelativePosition { Target "Sol" } # Track this
        }
        Secondary {
            Axis "y" # change to be axis of model which has the cameras
            RelativePosition { Target "Sol/Mercury" } # Track this
       }
        }
    }
 }
}



The second issue deals with the evolution of MESSENGER's science orbit over the next few months.
...
It would seem fairly straight forward to introduce revised orbits on the particular dates by epoch for the OCMs, but how does one set up an orbit that evolves over time to get to that point? Thanks in advance for help and input!

Unfortunately, Celestia doesn't include a way to interpolate between Keplerian orbits. This type of situation is usually shown by creating an xyz (or xyzv) trajectory file. If they've provided the information to Horizons, JPL's ephemeris server could be used to generate that.

[Aphyle2007]

Selden,

It would seem that the work server that I use has an issue with long file names and those that end with ".ssc." Call it hypervigilant security, or call it a blunt instrument. I've changed the file name slightly to end with ".txt" and it now appears on screen. Try this link instead:

http://csmres.jmu.edu/geollab/pyle/celestia/MessSciOrbt.ssc.txt.

It will appear on the screen.

With respect to the body vector commands, I will give these a try - you've anticipated my next syntax question.

With respect to the orbital orientation, I hadn't thought of the xyz solution, so I will check on generating this. Are there useful starting links you can suggest?

[selden]

The Horizons ephemeris server is at
http://ssd.jpl.nasa.gov/?horizons
You will need to read their documentation in order to be able to use the ephemeris server.

Use its telnet intreface for the most flexibility. The Web interface is very limited. Also, the template message below can be used with its e'mail interface when asking for many xyz samples. (I copied it from my Celestia support page, which is at http://www.lepp.cornell.edu/~seb/celestia/ )

Code: Select all

Here's a sample template for an e'mail message to request xyz coordinates
from Horizons for use with Celestia. You will have to edit the returned
message into the format that Celestia uses.

The message must be mailed to the address "horizons@ssd.jpl.nasa.gov"
The subject must be "JOB" ( do not include the quotes in either )

The body of the message should consist of the following text which
is between the lines of =====. Don't include those lines.
The lines starting with "!$$" are required, however.
Replace the fields ending in "here" with appropriate values.
Don't touch any of the other lines until you've read and understood
Horizons' documentation.
==========================================================
!$$SOF (ssd)       JPL/Horizons Execution Control VARLIST       
 EMAIL_ADDR = 'your address here'        ! 'you@there.com'
 COMMAND    = 'object name or id# here'  ! e.g. 'TRITON'
 OBJ_DATA   = 'NO'
 MAKE_EPHEM = 'YES'
 TABLE_TYPE = 'VECTORS'
 CENTER     = '@10'
 REF_PLANE  = 'ECLIPTIC'
 SITE_COORD = '0,0,0'
 START_TIME = 'starting date here'    ! e.g. '1989-Aug-23 09:00'
 STOP_TIME  = 'ending date here'      ! e.g. '1989-Aug-27 09:00'
 STEP_SIZE  = 'sample period here'    ! e.g. '10m' or '2d'
 REF_SYSTEM = 'J2000'
 OUT_UNITS  = 'KM-D'
 VECT_TABLE = '1'
 VECT_CORR  = 'NONE'
 TIME_DIGITS = 'MIN'
 CSV_FORMAT = 'NO'
 VEC_LABELS = 'NO'
 ELM_LABELS = 'NO'
 R_T_S_ONLY = 'NO'
 CA_TABLE_TYPE= 'EXTENDED'
 CALIM_SB= '0.1'
 CALIM_PL= '.1, .1, .1, .1, 1.0, 1.0, 1.0, 1.0, .1, .003'
!$$EOF++++++++++++++++++++++++++++++++++++++++++++++++++++++
==========================================================


[Aphyle2007]

Thanks!

I just found it, and found out what you meant by the Web interface. I did manage to generate a file, but have to check to see if I did so correctly. It would seem, though, that the information has been supplied to Horizons. We'll see what happens! Thanks again!

[selden]

You're very welcome.

[chris]

Here are two xyzv trajectory files that I made for Messenger:

http://www.celestiaproject.net/~chrislaurel/ce ... ruise.xyzv
http://www.celestiaproject.net/~chrislaurel/ce ... orbit.xyzv

Messenger-cruise covers the period from launch on 3 Aug 2004 through orbital insertion on 3 March 2011. The second trajectory covers the orbital mission through 1 April 2012. They were generated from from the SPICE kernel file msgr_040803_091031_120401.bsp using my spice2xyzv tool, and should have positions for Messenger accurate to within 1km. The kernel file's name indicates that it was produced in October 2009; there may be another available, but I haven't been able to find it.

Note that the state vectors are heliocentric in the cruise part and Mercury centric (hermocentric?) in the orbital part.

--Chris

[Aphyle2007]

Thank you, Chris! I'll plug the right file in and see what happens.

[Aphyle2007]

The second trajectory covers the orbital mission through 1 April 2012. They were generated from from the SPICE kernel file msgr_040803_091031_120401.bsp using my spice2xyzv tool, and should have positions for Messenger accurate to within 1km. The kernel file's name indicates that it was produced in October 2009; there may be another available, but I haven't been able to find it.

Chris,

I put it in with just the xyz data (not the velocity data), starting at the actual end of the MOI burn of 2011 Mar 18 00:59:43 UTC, but I am not convinced that these are the most up-to-date data. There was no change in periapsis when advancing the clock, unless the orbit specifications are overriding the .xyz data. I'll compare the data from the orbit file that you supplied with my first attempt at using the Horizons web interface. I generated some file, but not one I would trust just yet!

Oh, and the adjective "hermocentric" is what shows up in some of the literature.

--Eric

[Aphyle2007]

Okay - some measure of success on orientation, although I want to tweak this a bit:

Code: Select all

#  MESSENGER in initial science orbit, from the end of the MOI at
#  00:59:43 UTC, 2011 March 18.  This orbit will change over five cycles,
#  but this should get things started.  The actual orbit will change from
#  periapsis of 200 km on 2011 Mar 18 (MOI) to 500 km on 2011 Jun 16 for
#  Orbital Correction Maneuver 1 (OCM-1, -2), where it will return to 200
#  km.  The latitude of periapsis will change from approx. +65 degrees to
#  approx. +71.5 degrees in this same period, with OCM-1, -2  returning it
#  to +65 degrees.  Additional OCMs will be required about 2011 Sep 10
#  and 2011 Dec 16.

#  Sources:  McAdams, J. V., Farquhar, R. W., Taylor, A.H., & Williams,
#            B. G. (2007).  MESSENGER mission design and navigation.
#            Space Science Review  131:219-246.
#
#            NASA (2011).  MESSENGER Mercury orbit insertion press kit,
#            http://messenger.jhuapl.edu/news_room/Mercury_PK_TST_Rev7_Web.pdf


"Messenger" "Sol/Mercury"

{
   Class         "spacecraft"
   Mesh         "messenger2.3ds"
   Radius         0.006

   Beginning 2455639.70814     # 2011 Mar 18 00:59:43 UTC
   
   EllipticalOrbit
   {
   Epoch         2455639.70814
   Period         0.5
   PericenterDistance   2639.7
   SemiMajorAxis      8799
   Eccentricity      0.7
   Inclination      82.5
   AscendingNode      315.0
   ArgOfPericenter      120
   MeanAnomaly      0
   }
   
   BodyFrame { BodyFixed { Center "Sol/Mercury"}} #gets replaced
   FixedRotation {Inclination 0}

}

Modify "Messenger" "Sol/Mercury"

{
BodyFrame {
   TwoVector {
      Center "Sol/Mercury/Messenger"
      Primary {
         Axis "-x" # change to be the axis of model with the shield
         RelativePosition { Target "Sol" } # track this
         }
      Secondary {
         Axis "y" # change to be the axis of model with the cameras
         RelativePosition { Target "Sol/Mercury" } # track this
           }
        }
      }
}

The -x axis is oriented towards the Sun, but the -y axis, with the cameras, is oriented correctly at periapsis. I want to go back and look at the actual spacecraft specs to see what other instruments are oriented where.

Otherwise, great help! Thanks again.

--Eric

[chris]

I put it in with just the xyz data (not the velocity data), starting at the actual end of the MOI burn of 2011 Mar 18 00:59:43 UTC, but I am not convinced that these are the most up-to-date data. There was no change in periapsis when advancing the clock, unless the orbit specifications are overriding the .xyz data. I'll compare the data from the orbit file that you supplied with my first attempt at using the Horizons web interface. I generated some file, but not one I would trust just yet!

I'll try and post some images of the trajectory in a bit... I made this a couple weeks ago, but I recall that it looked roughly as expected based on the mission description.

I highly recommend retaining the velocities: you'll get a much more accurate trajectory.

Oh, and the adjective "hermocentric" is what shows up in some of the literature.

Cool!

[Aphyle2007]

I'll try and post some images of the trajectory in a bit... I made this a couple weeks ago, but I recall that it looked roughly as expected based on the mission description.

This document http://messenger.jhuapl.edu/news_room/Mercury_PK_TST_Rev7_Web.pdf had some pretty good graphics of the initial science orbit and MOI trajectory, too.
The other document that I referenced discusses the evolution of the orbit over a Mercurian year (88days) and the timing of the OCM's.

I highly recommend retaining the velocities: you'll get a much more accurate trajectory.

I guess that I need to read some of the documentation to fully understand how .ssc and .xyz files work together. I am using Celestia 1.6.0 on a MacBookPro, so I'm not at all sure about the operation of .xyzv files. I agree with you - I'd love to include the velocities.

--Eric

[Aphyle2007]

Okay - Now I've gone and done it!

I generated an .xyzv file for MESSENGER, using the Horizons web interface, and formatted things a bit, and it not only shows the shift in orbits, but it shows ALL of the orbits at once:



Is there anyway to get it to display just the current orbit? Also, it would seem that many of the orbits intersect the planet, although I cannot find a particular date that causes a collision. Is this simply an artifact of the orbital variations of Mercury and the fact that, with all orbits at once, some appear to intersect at the current position of Mercury? Or is it just that I requested the wrong coordinate system. The first few lines of the .xyzv file are:

Code: Select all

#   *******************************************************************************
#   Ephemeris / WWW_USER Sat Apr  2 13:54:33 2011  Pasadena, USA     / Horizons   
#   *******************************************************************************
#   Target body name: MESSENGER Spacecraft (-236)     {source: STK created (pnts=    7,}
#   Center body name: Mercury (199)                   {source: DE405}
#   Center-site name: BODY CENTER
#   *******************************************************************************
#   Start time      : A.D. 2011-Mar-18 00:00:00.0000 CT
#   Stop  time      : A.D. 2012-Mar-31 00:00:00.0000 CT
#   Step-size       : 60 minutes
#   *******************************************************************************
#   Center geodetic : 0.00000000,0.00000000,0.0000000 {E-lon(deg),Lat(deg),Alt(km)}
#   Center cylindric: 0.00000000,0.00000000,0.0000000 {E-lon(deg),Dxy(km),Dz(km)}
#   Center pole/equ : IAU_MERCURY                     {East-longitude -}
#   Center radii    : 2439.7 x 2439.7 x 2439.7 km     {Equator, meridian, pole}   
#   Output units    : KM-S                                                         
#   Output format   : 02
#   Output type     : GEOMETRIC cartesian states
#   Coordinate systm: Body Mean Equator and Node of Date                           
#
#
#   XYZV file generated for Celestia by Eric Pyle (Aphyle2007)
#
#   *******************************************************************************
#   JDCT ,   , X, Y, Z, VX, VY, VZ,
#   *******************************************************************************               
#                  
      2455639.000000      6498.36562680     -1052.11434700       548.61736331        -1.54390537         0.45203034         1.38180049

The complete .xyzv file can be found at http://csmres.jmu.edu/geollab/pyle/celestia/Messenger.xyzv.txt.

Still trying, ever forward?