[Spice_discussion] Converting MPC elements to SPICE ephemerides

[William Thompson]

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


-- 
William Thompson
NASA Goddard Space Flight Center
Code 671
Greenbelt, MD  20771
USA

301-286-2040
William.T.Thompson at nasa.gov