pympc

Perform checks for the presence of minor bodies at astronomical locations for a given epoch.

Installation

pip install pympc

or download/clone source and:

python setup.py install

Setup

First we need to import the package and grab the orbit element catalogue. This must be done at least once prior to any searches and can be run to overwrite the catalogues with the latest versions. The default call signature is shown.

import pympc
xephem_cat =pympc.update_catalogue()
print(xephem_cat)
# e.g. /tmp/mpcorb_xephem.csv

The catalogue downloaded will be the mpcorb catalogue from the Minor Planet Center.

The Near Earth Asteroid and Comets catalogues will be downloaded and used to update the mpcorb entries based on the values of the include_nea and include_comets arguments (both default to True).

It will create a csv file for each catalogue downloaded in the xephem database format and return the filepath to this file. By default the file will be saved in the user's temporary directory - this can be changed by setting the cat_dir argument.

Usage

Having downloaded the catalogue (see Setup), we can now search for minor bodies at a given location.

Interactive searching

Within an interpretor session, define a search location, epoch and radius and run the search.

import astropy.units as u
from astropy.time import Time
ra = 230.028 * u.deg
dec = -11.774 * u.deg
epoch = Time("2019-01-01T00:00")
search_radius = 5 * u.arcmin
observatory = 950  # observatory code for La Palma
pympc.minor_planet_check(ra, dec, epoch, search_radius, observatory=observatory)

Results are returned as an astropy table.

The above example uses explicit quantities, but if passed simple float arguments, and the program will assume the units (see comments below and pympc.minor_planet_check() docstring for unit assumptions).

ra = 230.028  # assumed degrees
dec = -11.774  # assumed degrees
epoch = 58484.  # assumed MJD
search_radius = 30  # assumed arcseconds
observatory = 950  # observatory code for La Palma
pympc.minor_planet_check(ra, dec, epoch, search_radius, observatory=observatory)

By default, the search will use a default filepath for the catalogue. if the file has been moved - or a custom cat_dir was passed to pympc.update_catalogue() - then the filepath can be specified.

pympc.minor_planet_check(ra, dec, epoch, search_radius, xephem_filepath='/path/to/mpcorb_xphem.csv', observatory=observatory)

Defining an observer

By default, if the observatory argument is not passed, the program will return geocentric coordinates. However, for relatively nearby objects like minor bodies, there can be signicant parallax introduced by the location of an observer on the Earth's surface. For this reason it is crucial to pass either an observatory code or a tuple containing the observatory information. See the documentation for pympc.minor_planet_check() for more details.

Speed and multiprocessing

The check should take of order a second or two, depending on multiprocessing capabilities.

The private function which actually performs the calculation is _minor_planet_check() (note leading underscore). This can be called directly, to avoid the overhead associated with converting input arguments in minor_planet_check(), if you provide them directly as required (see _minor_planet_check() docstring). Note that in this case a list of tuples is returned, rather than an astropy table.

By default the program will calculate positions of bodies in the catalogue multiprocessed. To switch this off set chunk_size = 0, i.e.:

pympc.minor_planet_check(ra, dec, epoch, search_radius, chunk_size=0)

Console script searching

Installation of the package will create a minor_planet_check script, which can be accessed from the command line. The options follow the same as the interactive searching, and results are displayed as a table. For help on the command line use:

minor_planet_check --help

It is not currently possible to pass a custom set of observatory coordinates to the script - an existing observatory code must be passed.

Limitations

The orbits are propagated following xephem (via the pyephem package), and this does not account for perturbations of the orbits. Thus, the accuracy of the position is dependent on the time difference between the epoch of the orbit elements and the epoch at which the search is being performed. Epoch differences between orbital elements calculation and observation of around a month or two are fine for typical positional accuracies of less than a few arcsecond for the vast majority of minor bodies. Note, however, that a small number of bodies (those undergoing strong perturbations and close to Earth) maybe quite inaccurate (arcminutes). A fuller analysis is given in notebooks/positional_accuracy.ipynb.

The xephem package can only provide geocentric astrometric positions. pympc will calculate the topocentric correction as a post-processing to the initial position. The simple geometric correction applied is more than sufficient for the overwhelming majority of minor bodies, but for some near earth objects the correction can be large and the relatively simple treatment by pympc may not be sufficient. Additionally, in order to find matches in geocentric positions prior to applying the topocentric correction, a buffer is added to the search radius - this should capture the vast majority of cases where the geocentric position is outside the seach radius but the topocentric position is within it - although no guarantees. To work around this you can artifically inflate your search radius and filter yourself afterwards.

The filtering of matches based on magnitude via max_mag argument to minor_planet_check() is limited by the accuracy of the magnitude information in the database so some buffer should be applied to the desired magnitude cutoff to allow for this.

Acknowledgements

This package makes use of data and/or services provided by the International Astronomical Union's Minor Planet Center.

Orbit elements are also sourced from Lowell Observatory, which is funded by the Lowell Observatory Endowment and NASA PDART grant NNX16AG52G.

Based from a package developed by Chris Klein and Duncan Galloway.