lkspacecraft 1.3.1

Requirements

This package relies heavily on spiceypy which wraps SPICE. It also relies on astropy.

Installation

You can install this package with pip using

pip install lkspacecraft --upgrade

You can also install this package by cloning the repo and then installing via poetry

git clone https://github.com/lightkurve/lkspacecraft.git
cd lkspacecraft
pip install --upgrade poetry
poetry install .

This package will download and store SPICE kernels into a directory in your home area when you use any of the objects. This is approximately 1Gb of data at time of writing. This means that if you install this package in multiple environments, the SPICE kernels will not be redownloaded and will be shared between multiple installs.

To uninstall this package from your machine entirely you should clear the cache of SPICE kernel files using

from astropy.utils.data import clear_download_cache
clear_download_cache(pkgname='lkspacecraft')

or by deleting the .lkspacecraft/cache/ directory in your home area. You can then uninstall with pip, or if you cloned the repository you can delete the directory.

This package now installs with a lightweight set of truncated kernels which can be used to test the functionality, but cover a very limited time range and set of bodies. These files are available in src/data/kernels/testkernels.

Usage

lkspacecraft provides Spacecraft object which will enable you to access the orbital parameters of either the Kepler or TESS spacecraft. lkspacecraft will obtain the relevant SPICE kernels to calculate the spacecraft position and velocity. To get the orbital elements you will need to pick a time that is within the relevant window of those SPICE kernels (i.e. when the mission was operational).

You can find the start and end times of the kernels using the following

from lkspacecraft import KeplerSpacecraft

ks = KeplerSpacecraft()
ks.start_time, ks.end_time

All times in lkspacecraft use astropy.time.Time objects. Using the get_spacecraft_position or get_spacecraft_velocity functions will provide you with the position or velocity in cartesian coordinates, for example

from lkspacecraft import KeplerSpacecraft
from astropy.time import Time

ks = KeplerSpacecraft()
t = Time("2009-04-06 06:22:56.000025")
ks.get_spacecraft_velocity(t)

will result in

array([[  6.94188023, -26.24714425, -11.16828662]])

This will give the velocity with respect to the solar system barycenter by default, but you can specify the earth or moon using

from lkspacecraft import KeplerSpacecraft
from astropy.time import Time

ks = KeplerSpacecraft()
t = Time("2009-04-06 06:22:56.000025")
ks.get_spacecraft_velocity(time=t, observer="earth")

You are able to calculate the light arrival time of observations of a source at a given RA/Dec using lkspacecraft’s get_barycentric_time_correction function. This will give you the time delay in seconds from spacecraft time to time at the barycenter.

from lkspacecraft import KeplerSpacecraft
from astropy.time import Time

ks = KeplerSpacecraft()
t = Time("2009-04-06 06:22:56.000025")
ks.get_barycentric_time_correction(time=t, ra=290.666, dec=44.5)

Finally you can calculate velocity aberration using

from lkspacecraft import KeplerSpacecraft
from astropy.time import Time

ks = KeplerSpacecraft()
t = Time("2009-04-06 06:22:56.000025")
ks.get_velocity_aberrated_positions(time=t, ra=290.666, dec=44.5)

Kernels

lkspacecraft will obtain the SPICE kernels for Kepler and TESS for you store them. Kernels can be found here:

The generic kernels can be obtained from NAIF generic kernels: https://naif.jpl.nasa.gov/pub/naif/generic_kernels/ The Kepler kernels can be obtained from MAST: https://archive.stsci.edu/missions/kepler/spice/ The K2 kernels can be obtained from MAST: https://archive.stsci.edu/missions/k2/spice/ The TESS kernels can be obtained from MAST: https://archive.stsci.edu/missions/tess/engineering/ https://archive.stsci.edu/missions/tess/models/

When you first initialize an lkspacecraft.Spacecraft object in Python all the kernels will be downloaded for you, and lkspacecraft will check if there are new kernels available. This will take approximately 5 minutes if you have no kernels, depending on your internet connection. Once this has been done, the kernels will be cached. If there are new TESS kernels available lkspacecraft will retrieve them for you and update the cache.

The total file volume for the kernels is ~1GB. These cached files are stored using astropy’s cache. If you want to clear the cache you can do either of the following;

from lkspacecraft.utils import clear_download_cache
clear_download_cache()

from astropy.utils.data import clear_download_cache
clear_download_cache(pkgname='lkspacecraft')

Because the kernels need to be checked, every time you initialize a Spacecraft, like below, object there will be a slight delay, and lkspacecraft will connect to the internet to check for new kernels.

ks = KeplerSpacecraft()

import lkspacecraft
lkspacecraft.enable_test_mode()
lkspacecraft.disable_test_mode()

Caveats

t = np.asarray(hdulist[1].data['TIME'], dtype=float)
tcorr = np.asarray(hdulist[1].data['TIMECORR'], dtype=float)
# Spacecraft time:
t -= tcorr

$ python -m pip install lkspacecraft --upgrade

from lkspacecraft import KeplerSpacecraft
ks = KeplerSpacecraft()
t = Time("2009-04-06 06:22:56.000025")
ks.get_velocity_aberrated_positions(time=t, ra=290.666, dec=44.5)