Metadata-Version: 2.1
Name: pympc
Version: 1.0.0
Summary: minor planet checking
Home-page: https://github.com/lyalpha/pympc
Author: Joe Lyman
License: GNU General Public License v3 (GPLv3)
Platform: UNKNOWN
Classifier: Programming Language :: Python :: 3
Requires-Python: >=3.6
Description-Content-Type: text/markdown
License-File: LICENSE.txt

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

The catalogue downloaded will be the [`mpcorb`](https://www.minorplanetcenter.net/data) 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](http://www.clearskyinstitute.com/xephem/help/xephem.html#mozTocId468501) 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](#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.
```python
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).
```python
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.

```python
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](https://www.minorplanetcenter.net/iau/lists/ObsCodes.html) 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.:

```python
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:
```bash
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](http://www.clearskyinstitute.com/xephem) (via the 
[pyephem](https://rhodesmill.org/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 should be 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).

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.

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](https://www.minorplanetcenter.net).

Orbit elements are also sourced from [Lowell Observatory](https://asteroid.lowell.edu/main/), which is funded by the 
Lowell Observatory Endowment and NASA PDART grant NNX16AG52G.

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

