[Spice_discussion] Converting MPC elements to SPICE ephemerides

[Boris Semenov]

Hi Bill --

Your understanding of the cause of the error is correct: Type 17 
(Equinoctial Elements) is intended for elliptical orbits and will not 
work for hyperbolic orbits.

The right SPK type for the elements that you have is Type 5 (two body 
propagation).

With the following modified setup using Type 5, correct NAIF ID for 
comet Elenin (1003113), and corrected reference frame for the elements 
(ECLIPJ2000 instead of J2000 because the orbital inclination in MPC is 
relative the ecliptic plane):

\begindata
INPUT_DATA_TYPE   = 'ELEMENTS'
OUTPUT_SPK_TYPE   = 5
OBJECT_ID         = 1003113
CENTER_ID         = 10
REF_FRAME_NAME    = 'ECLIPJ2000'
PRODUCER_ID       = 'W. Thompson, NASA/GSFC, USA'
DATA_ORDER        = 'EPOCH RP E INC NOD PER MEAN'
INPUT_DATA_UNITS  = ('ANGLES=DEGREES' 'DISTANCES=AU')
START_TIME        = '2011 MAY 01'
STOP_TIME         = '2012 MAY 01'
DATA_DELIMITER    = 'EOL'
LEAPSECONDS_FILE  = 'naif0009.tls'
PCK_FILE          = ('pck00008.tpc', 'gravity.tpc'  )
SEGMENT_ID        = 'SPK_ELEMENTS_5'
\begintext

and with the input without the D*/DT terms:

2011-09-10T17:22:33.600
0.482462
1.000024
1.8391
323.2242
343.8085
0.0

I was able to make an SPK file that showed agreement with a Elenin SPK 
made using Horizons Web interface

http://ssd.jpl.nasa.gov/x/spk.html

to better than 5,000 km for a few month period in 2011 (see attached plot).

Regards,
Boris.

William Thompson wrote:
> I would like to generate a SPICE ephemeris for Comet Elenin.  The IAU 
> Minor Planet Center gives these elements for this comet:
> 
>> The following orbital elements are taken from MPC 75009:
>>
>>     C/2010 X1 (Elenin)
>> Epoch 2011 Aug. 27.0 TT = JDT 2455800.5
>> T 2011 Sept. 10.7240 TT                                 MPC
>> q   0.482462             (2000.0)            P               Q
>> z  -0.000049       Peri.  343.8085      +0.6023572      +0.7979954
>>  +/-0.000004       Node   323.2242      -0.7287529      +0.5399501
>> e   1.000024       Incl.    1.8391      -0.3257069      +0.2676887
>> From 1621 observations 2010 Dec. 10-2011 May 10, mean residual 0".5.
> 
> (from 
> http://scully.cfa.harvard.edu/cgi-bin/returnprepeph.cgi?d=c&o=CK10X010)
> 
> I tried making an SPK file out of this with the following setup file
> 
> \begindata
> INPUT_DATA_TYPE   = 'ELEMENTS'
> OUTPUT_SPK_TYPE   = 17
> OBJECT_ID         = -900
> OBJECT_NAME       = 'ELENIN'
> CENTER_ID         = 10
> CENTER_NAME       = 'SUN'
> REF_FRAME_NAME    = 'J2000'
> PRODUCER_ID       = 'W. Thompson, NASA/GSFC, USA'
> DATA_ORDER        = 'EPOCH RP E INC NOD PER MEAN DPER/DT DMPN/DT DNOD/DT'
> INPUT_DATA_UNITS  = ('ANGLES=DEGREES' 'DISTANCES=AU')
> START_TIME        = '2011 MAY 01'
> STOP_TIME         = '2012 MAY 01'
> DATA_DELIMITER    = 'EOL'
> LEAPSECONDS_FILE  = '/solarsoft/stereo/gen/data/spice/gen/naif0009.tls'
> PCK_FILE          = ('pck00008.tpc' 'gravity.tpc')
> SEGMENT_ID        = 'SPK_ELEMENTS_17'
> \begintext
> 
> and the following data file
> 
> 2011-09-10T17:22:33.600
> 0.482462
> 1.000024
> 1.8391
> 323.2242
> 343.8085
> 0.00
> 0.00
> 0.00
> 0.00
> 
> However, when I try to run mkspk, I get the following error message:
> 
>  > mkspk -setup elenin.setup -input elenin.dat -output elenin.bsp
> 
> MKSPK Program; Ver. 4.3.0, 20-DEC-2004; Toolkit Ver. N0060
> 
> Loading setup file ...
> 
> Processing setup file keyword values ...
> 
> Writing comments to the output SPK file  ...
> 
> Started processing input data ...
> 
> Writing next SPK segment ...
> 
> ================================================================================ 
> 
> 
> Toolkit version: N0060
> 
> SPICE(BADSEMIAXIS)
> 
> The semimajor axis supplied to the SPK type 17 evaluator was 
> non-positive. This
> value must be positive. The value supplied was -3.0073036604979E+12.
> 
> A traceback follows.  The name of the highest level module is first.
> MKSPK --> SPKW17
> 
> ================================================================================ 
> 
> 
> I think this error is caused by the fact that the eccentricity is 
> slightly above 1, i.e. the orbit is hyperbolic.  Is this my problem, or 
> have I misunderstood the input parameters to mkspk?
> 
> Bill Thompson
> 
> 
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