.. DO NOT EDIT.
.. THIS FILE WAS AUTOMATICALLY GENERATED BY SPHINX-GALLERY.
.. TO MAKE CHANGES, EDIT THE SOURCE PYTHON FILE:
.. "auto_examples/plot_mini_moon.py"
.. LINE NUMBERS ARE GIVEN BELOW.

.. only:: html

    .. note::
        :class: sphx-glr-download-link-note

        :ref:`Go to the end <sphx_glr_download_auto_examples_plot_mini_moon.py>`
        to download the full example code.

.. rst-class:: sphx-glr-example-title

.. _sphx_glr_auto_examples_plot_mini_moon.py:


Temporary Earth Capture
=======================

Some near-Earth asteroids pass close enough to Earth that they become
temporarily captured, orbiting the planet for weeks or months before
escaping back onto heliocentric orbits.  These so-called "mini-moons"
spend time inside Earth's Hill sphere and can have geocentric specific
energy that dips below zero during capture.

This example uses 2020 CD3 -- the second known natural object to be
temporarily captured by Earth -- to demonstrate kete's orbital analysis
tools: geocentric specific energy and Earth's Hill sphere / sphere of
influence.

.. GENERATED FROM PYTHON SOURCE LINES 16-22

.. code-block:: Python


    import matplotlib.pyplot as plt
    import numpy as np

    import kete








.. GENERATED FROM PYTHON SOURCE LINES 23-28

Fetch 2020 CD3 and set up the time window
------------------------------------------

We fetch the orbit from JPL Horizons and define a window covering
the temporary capture event (~Aug 2020 to ~Mar 2021).

.. GENERATED FROM PYTHON SOURCE LINES 28-37

.. code-block:: Python


    obj = kete.HorizonsProperties.fetch("2020 CD3")

    jd_start = kete.Time.from_ymd(2010, 1, 1).jd
    jd_end = kete.Time.from_ymd(2040, 1, 1).jd

    # Propagate to the start of the window
    state = kete.propagate_n_body(obj.state, jd_start)








.. GENERATED FROM PYTHON SOURCE LINES 38-42

Physical constants
-------------------

Earth's Hill sphere and sphere of influence.

.. GENERATED FROM PYTHON SOURCE LINES 42-51

.. code-block:: Python


    planet = "Earth"

    earth_hill = kete.hill_radius(planet)
    earth_soi = kete.sphere_of_influence(planet)

    print(f"Earth Hill radius:     {earth_hill * kete.constants.AU_KM:.0f} km")
    print(f"Earth sphere of influence: {earth_soi * kete.constants.AU_KM:.0f} km")





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    Earth Hill radius:     1471600 km
    Earth sphere of influence: 925065 km




.. GENERATED FROM PYTHON SOURCE LINES 52-57

Propagate through the capture event
-------------------------------------

Step through the time window recording geocentric distance and specific
energy at each epoch.

.. GENERATED FROM PYTHON SOURCE LINES 57-82

.. code-block:: Python


    step = 7.0  # days
    times = np.arange(jd_start, jd_end, step)

    geo_dist_km = []
    spec_energy = []

    for jd in times:
        state = kete.propagate_n_body(state, jd)

        # Geocentric state for distance and specific energy
        geo_state = state.change_center(planet)
        r_km = geo_state.pos.r * kete.constants.AU_KM
        geo_dist_km.append(r_km)

        energy = kete.specific_energy(geo_state)
        spec_energy.append(energy)


    geo_dist_km = np.array(geo_dist_km)
    spec_energy = np.array(spec_energy)

    # Convert JD to fractional year for the x-axis
    t_years = [kete.Time(jd).year_float for jd in times]








.. GENERATED FROM PYTHON SOURCE LINES 83-92

Plot the results
-----------------

Two panels show the capture event:

- **Geocentric distance** relative to Earth's Hill sphere and sphere of
  influence.
- **Geocentric specific energy** -- negative values indicate a bound
  orbit around Earth.

.. GENERATED FROM PYTHON SOURCE LINES 92-127

.. code-block:: Python


    fig, axes = plt.subplots(2, 1, figsize=(9, 6), sharex=True, dpi=150)

    ax = axes[0]
    ax.plot(t_years, geo_dist_km, "k-", lw=0.5)
    ax.axhline(
        earth_hill * kete.constants.AU_KM,
        color="blue",
        ls="--",
        lw=0.8,
        label=f"Hill sphere ({earth_hill * kete.constants.AU_KM:.0f} km)",
    )
    ax.axhline(
        earth_soi * kete.constants.AU_KM,
        color="green",
        ls=":",
        lw=0.8,
        label=f"SOI ({earth_soi * kete.constants.AU_KM:.0f} km)",
    )
    ax.set_ylabel("Geocentric distance (km)")
    ax.set_yscale("log")
    ax.legend(fontsize=8)
    ax.set_title(f"{obj.desig} -- Temporary Earth Capture")

    ax = axes[1]
    ax.plot(t_years, spec_energy, "k-", lw=0.5)
    ax.axhline(0, color="red", ls="--", lw=0.8, label="Bound / unbound boundary")
    ax.set_ylabel("Specific energy (AU$^2$/day$^2$)")
    ax.set_xlabel("Year")
    thresh = 10 ** (np.ceil(np.log10(np.abs(np.min(spec_energy)))))
    ax.set_yscale("symlog", linthresh=thresh)
    ax.legend(fontsize=8)

    plt.tight_layout()
    plt.show()



.. image-sg:: /auto_examples/images/sphx_glr_plot_mini_moon_001.png
   :alt: 2020 CD3 -- Temporary Earth Capture
   :srcset: /auto_examples/images/sphx_glr_plot_mini_moon_001.png
   :class: sphx-glr-single-img






.. rst-class:: sphx-glr-timing

   **Total running time of the script:** (0 minutes 9.311 seconds)


.. _sphx_glr_download_auto_examples_plot_mini_moon.py:

.. only:: html

  .. container:: sphx-glr-footer sphx-glr-footer-example

    .. container:: sphx-glr-download sphx-glr-download-jupyter

      :download:`Download Jupyter notebook: plot_mini_moon.ipynb <plot_mini_moon.ipynb>`

    .. container:: sphx-glr-download sphx-glr-download-python

      :download:`Download Python source code: plot_mini_moon.py <plot_mini_moon.py>`

    .. container:: sphx-glr-download sphx-glr-download-zip

      :download:`Download zipped: plot_mini_moon.zip <plot_mini_moon.zip>`


.. only:: html

 .. rst-class:: sphx-glr-signature

    `Gallery generated by Sphinx-Gallery <https://sphinx-gallery.github.io>`_