Handling EphemerisRangeError when observing a minor planet with missing data

On running the test below:

#!/usr/bin/env python3

import datetime, io, skyfield.api, skyfield.constants, skyfield.data.mpc

print( "Skyfield:", skyfield.__version__ )

# Download and save  
# https://minorplanetcenter.net/iau/Ephemerides/Distant/Soft00Distant.txt
# and adjust the date to the download date.
now = datetime.datetime.strptime( "2020-09-16", "%Y-%m-%d" )

latitude = -33
longitude = 151
timeScale = skyfield.api.load.timescale( builtin = True )
t = timeScale.utc( now.year, now.month, now.day, now.hour, now.minute, now.second )
topos = skyfield.api.Topos( latitude_degrees = latitude, longitude_degrees = longitude )
ephemeris = skyfield.api.load( "de421.bsp" )
sun = ephemeris[ "sun" ]
earth = ephemeris[ "earth" ]
with open( "Soft00Distant.txt" ) as f:
   for line in f:
       with io.BytesIO( line.encode() ) as f:
           dataframe = skyfield.data.mpc.load_mpcorb_dataframe( f ).set_index( "designation", drop = False )
           name = line[ 166 : 194 ].strip()
           if "2012 DR30" in name:
               body = sun + skyfield.data.mpc.mpcorb_orbit( dataframe.loc[ name ], timeScale, skyfield.constants.GM_SUN_Pitjeva_2005_km3_s2 )
               ra, dec, sunBodyDistance = sun.at( t ).observe( body ).radec()
               ra, dec, earthBodyDistance = ( earth + topos ).at( t ).observe( body ).radec()

I get the following:

skyfield.errors.EphemerisRangeError: ephemeris segment only covers dates 1899-07-28 23:59:18Z through 2053-10-08 23:58:51Z UT

which occurs during both the sun and earth observation.

On looking at the downloaded data, there is a piece of data missing from one field when compared to lines before/after. I presume this missing field is causing the exception. Is this is something that should/could be handled within Skyfield, or rather I just wrap a try/except around the code?

[Bernmeister]

I didn’t realise that the file could be loaded into a dataframe in one go. Yes, I’ve found that comets and minor planets run an order of magnitude or two slower than PyEphem (and was going to post about a little later on). My use case is in two parts: first I download the MPC data files (one for comets and four for minor planets). For each file, I remove a comet/minor planet if it exceeds (apparent) magnitude of 15. I then write out a binary blob file containing what’s left. Secondly I use the binary blob on each update of my software loop to read in this much smaller subset.

I presume what you have written above in the code sample (open the text file and load into dataframe) is what you refer to speeding things up, then I’ll have a go at using that. Much appreciated!

[brandon-rhodes]

@Bernmeister — Thanks for following up! I have updated the docs to describe filtering the dataframe to remove invalid orbits, and hopefully any users who miss that advice will be led here or to the docs when they search for the exception name.

Oh, and, for the record, I believe that this line — which is missing its semimajor axis — was the culprit from Soft00Distant.txt:

K12D30R  7.1   0.15 K194R   0.04515  195.57163  341.47709   77.98645  0.9909325  0.00001530                MPO434970   209   8 2000-2014 0.26 M-v 38h MPC        0000         2012 DR30