def plot_solar_system(outer=True, epoch=None, use_3d=False):
"""
Plots the whole solar system in one single call.
.. versionadded:: 0.9.0
Parameters
------------
outer : bool, optional
Whether to print the outer Solar System, default to True.
epoch : ~astropy.time.Time, optional
Epoch value of the plot, default to J2000.
use_3d : bool, optional
Produce 3D plot, default to False.
"""
bodies = [Mercury, Venus, Earth, Mars]
if outer:
bodies.extend([Jupiter, Saturn, Uranus, Neptune])
if use_3d:
op = OrbitPlotter3D() # type: Union[OrbitPlotter3D, OrbitPlotter2D]
else:
op = OrbitPlotter2D()
op.set_frame(*Orbit.from_body_ephem(Earth, epoch).pqw()) # type: ignore
for body in bodies:
orb = Orbit.from_body_ephem(body, epoch)
op.plot(orb, label=str(body))
return op
def plot_solar_system(outer=True, epoch=None, use_3d=False):
"""
Plots the whole solar system in one single call.
.. versionadded:: 0.9.0
Parameters
------------
outer : bool, optional
Whether to print the outer Solar System, default to True.
epoch : ~astropy.time.Time, optional
Epoch value of the plot, default to J2000.
use_3d : bool, optional
Produce 3D plot, default to False.
"""
bodies = [Mercury, Venus, Earth, Mars]
if outer:
bodies.extend([Jupiter, Saturn, Uranus, Neptune])
if use_3d:
op = OrbitPlotter3D() # type: Union[OrbitPlotter3D, OrbitPlotter2D]
else:
op = OrbitPlotter2D()
op.set_frame(*Orbit.from_body_ephem(Earth,
epoch).pqw()) # type: ignore
for body in bodies:
orb = Orbit.from_body_ephem(body, epoch)
op.plot(orb, label=str(body))
return op
def test_from_ephem_raises_warning_if_time_is_not_tdb_with_proper_time(recwarn):
body = Earth
epoch = Time("2017-09-29 07:31:26", scale="utc")
expected_epoch_string = "2017-09-29 07:32:35.182" # epoch.tdb.value
Orbit.from_body_ephem(body, epoch)
w = recwarn.pop(TimeScaleWarning)
assert expected_epoch_string in str(w.message)
def test_from_ephem_raises_warning_if_time_is_not_tdb_with_proper_time(recwarn):
body = Earth
epoch = time.Time("2017-09-29 07:31:26", scale="utc")
expected_epoch_string = "2017-09-29 07:32:35.182" # epoch.tdb.value
Orbit.from_body_ephem(body, epoch)
w = recwarn.pop(TimeScaleWarning)
assert expected_epoch_string in str(w.message)
def hill_radius(body, a=None, e=None):
"""Approximated radius of the Hill Sphere of Influence (SOI) for a body.
Parameters
----------
body : `~poliastro.bodies.Body`
Astronomical body which the SOI's radius is computed for.
a : float, optional
Semimajor axis of the body's orbit, default to None (will be computed from ephemerides).
e : float, optional
Eccentricity of the body's orbit, default to 0 (will be computed from ephemerides).
Returns
-------
astropy.units.quantity.Quantity
Approximated radius of the Sphere of Influence (SOI) [m]
"""
# Compute semimajor and eccentricity axis at epoch J2000 for the body if it was not
# introduced by the user
if a is None or e is None:
ss = Orbit.from_body_ephem(body, J2000_TDB)
a = a if a is not None else ss.a
e = e if e is not None else ss.ecc
mass_ratio = body.k / (3 * body.parent.k)
r_SOI = a * (1 - e) * (mass_ratio**(1 / 3))
return r_SOI.decompose()
def compute_soi(body, a=None):
"""Approximated radius of the Laplace Sphere of Influence (SOI) for a body.
Parameters
----------
body : `~poliastro.bodies.Body`
Astronomical body which the SOI's radius is computed for.
a : float, optional
Semimajor axis of the body's orbit, default to None (will be computed from ephemerides).
Returns
-------
astropy.units.quantity.Quantity
Approximated radius of the Sphere of Influence (SOI) [m]
"""
# Compute semimajor axis at epoch J2000 for the body if it was not
# introduced by the user
if a is None:
try:
a = Orbit.from_body_ephem(body, J2000).a
except KeyError:
raise RuntimeError(
"""To compute the semimajor axis for Moon and Pluto use the JPL ephemeris:
>>> from astropy.coordinates import solar_system_ephemeris
>>> solar_system_ephemeris.set("jpl")""")
r_SOI = a * (body.k / body.parent.k)**(2 / 5)
return r_SOI.decompose()
def do_GET(self):
self.send_response(200)
self.send_header("Content-type", "application/json")
self.end_headers()
epoch_now = time.Time(datetime.datetime.now())
res = {
"earth": str(Orbit.from_body_ephem(Earth, epoch_now)),
"mars": str(Orbit.from_body_ephem(Mars, epoch_now))
}
self.wfile.write(json.dumps(res).encode("utf-8"))
return
def compute_soi(body, a=None):
"""Approximated radius of the Laplace Sphere of Influence (SOI) for a body.
Parameters
----------
body : `~poliastro.bodies.Body`
Astronomical body which the SOI's radius is computed for.
a : float, optional
Semimajor axis of the body's orbit, default to None (will be computed from ephemerides).
Returns
-------
astropy.units.quantity.Quantity
Approximated radius of the Sphere of Influence (SOI) [m]
"""
# Compute semimajor axis at epoch J2000 for the body if it was not
# introduced by the user
if a is None:
try:
a = Orbit.from_body_ephem(body, J2000).a
except KeyError:
raise RuntimeError(
"""To compute the semimajor axis for Moon and Pluto use the JPL ephemeris:
>>> from astropy.coordinates import solar_system_ephemeris
>>> solar_system_ephemeris.set("jpl")""")
r_SOI = a * (body.k / body.parent.k) ** (2 / 5)
return r_SOI.decompose()
def hill_radius(body, a=None, e=None):
"""Approximated radius of the Hill Sphere of Influence (SOI) for a body.
Parameters
----------
body : `~poliastro.bodies.Body`
Astronomical body which the SOI's radius is computed for.
a : float, optional
Semimajor axis of the body's orbit, default to None (will be computed from ephemerides).
e : float, optional
Eccentricity of the body's orbit, default to 0 (will be computed from ephemerides).
Returns
-------
astropy.units.quantity.Quantity
Approximated radius of the Sphere of Influence (SOI) [m]
"""
# Compute semimajor and eccentricity axis at epoch J2000 for the body if it was not
# introduced by the user
if a is None or e is None:
ss = Orbit.from_body_ephem(body, J2000_TDB)
a = a if a is not None else ss.a
e = e if e is not None else ss.ecc
mass_ratio = body.k / (3 * body.parent.k)
r_SOI = a * (1 - e) * (mass_ratio ** (1 / 3))
return r_SOI.decompose()
def _plot_bodies(orbit_plotter, outer=True, epoch=None):
bodies = [Mercury, Venus, Earth, Mars]
if outer:
bodies.extend([Jupiter, Saturn, Uranus, Neptune])
for body in bodies:
orb = Orbit.from_body_ephem(body, epoch)
orbit_plotter.plot(orb, label=str(body))
def optimal_transit(date, transit_min, transit_max, planet1, planet2, vs0,
step):
date_arrival = date + transit_min # minimalna data wykonania tranzytu
date_max = date + transit_max # maksymalna data wykonania, zakonczenie petli
date_arrival_final = date_arrival
vs_temp = 0 * u.km / u.s
dv_final = 0 * u.km / u.s
step_first = True
# petla idaca po datach z okreslonym krokiem
while date_arrival < date_max:
tof = date_arrival - date # tof - time of flight
date_iso = time.Time(str(date.iso), scale='utc') # data startu
date_arrival_iso = time.Time(str(date_arrival.iso),
scale='utc') # data przylotu
r1 = Orbit.from_body_ephem(planet1, date_iso)
r2 = Orbit.from_body_ephem(planet2, date_arrival_iso)
r_1, v_1 = r1.rv() # pozycja i predkosc planety poczatkowej
r_2, v_2 = r2.rv() # pozycja i predkosc planety koncowej
(vs1, vs2), = iod.lambert(Sun.k, r_1, r_2,
tof) # rozwiazanie zagadnienia lamberta
dv_vector = vs1 - (
vs0 + (v_1 / (24 * 3600) * u.day / u.s)
) # zmiana predkosci niezbedna do udanego wykonania manewru
dv = np.linalg.norm(
dv_vector / 10) * u.km / u.s # modul wektora zmiany predkosci
if step_first: # zapis wynikow z pierwszego kroku
dv_final = dv
vs_temp = vs2
step_first = False
else:
if dv < dv_final: # sprawdzenie czy kolejny krok jest bardziej korzystna
dv_final = dv
date_arrival_final = date_arrival
vs_temp = vs2
date_arrival += step * u.day
return dv_final, date_arrival_final, vs_temp # funkcja zwraca niezbedny przyrost predkosci, date przybycia
def total_delta_v(launch, arrival):
"""Calculate the total delta v for specific combination of launch date and arrival date
:param launch: launch time in isoformat string
:param arrival: desired arrival time in isoformat string
"""
date_launch = time.Time(launch, scale="utc")
date_arrival = time.Time(arrival, scale="utc")
ss_earth = Orbit.from_body_ephem(Earth, date_launch)
ss_mars = Orbit.from_body_ephem(Mars, date_arrival)
man_lambert = Maneuver.lambert(ss_earth, ss_mars)
return {
'delta_v': man_lambert.get_total_cost().value,
'total_seconds': man_lambert.get_total_time().value
}
def _plot_solar_system_2d(outer=True, epoch=None, interactive=False):
pqw = Orbit.from_body_ephem(Earth, epoch).pqw()
if interactive:
orbit_plotter = (OrbitPlotter2D()
) # type: Union[OrbitPlotter2D, StaticOrbitPlotter]
orbit_plotter.set_frame(*pqw)
else:
orbit_plotter = StaticOrbitPlotter()
orbit_plotter.set_frame(*pqw)
_plot_bodies(orbit_plotter, outer, epoch)
return orbit_plotter
def set_body_frame(self, body, epoch=None):
"""Sets perifocal frame based on the orbit of a body at a particular epoch if given.
Parameters
----------
body : poliastro.bodies.SolarSystemPlanet
Body.
epoch : astropy.time.Time, optional
Epoch of current position.
"""
from poliastro.twobody import Orbit
with warnings.catch_warnings():
warnings.simplefilter("ignore", DeprecationWarning)
orbit = Orbit.from_body_ephem(body, epoch).change_plane(self.plane) # type: ignore
self.set_orbit_frame(orbit)
def orbit_check(date, v, m, Isp):
# sprawdzenie czy orbita została osiągnieta
date_iso = time.Time(str(date.iso), format='iso', scale='utc')
r_out = Orbit.from_body_ephem(Jupiter,
date_iso) # polozenie Jowisza po asyscie
r_out1, vp_out1 = r_out.rv()
v_exit = v + (vp_out1 /
(24 * 3600) * u.day / u.s) # predkosc satelity po manewrach
epoch_out = date.jyear
ss_out = Orbit.from_vectors(Sun, r_out1, v_exit,
epoch=epoch_out) # wyjsciowe parametry orbity
print('Sprawdzanie osiągnięcia orbity ...')
print()
if ss_out.ecc >= 1: # ekscentrycznosc orbity
print('Predkosc jest okej')
else:
print('Predkosc jest za mała')
print()
print('Dostosuj się do minimalnej orbity wyjściowej ')
# minimalna orbita wyjsciowa paraboliczna:
ss_out_new = Orbit.parabolic(Sun,
ss_out.p,
ss_out.inc,
ss_out.raan,
ss_out.argp,
ss_out.nu,
epoch=epoch_out)
v_out_new = ss_out_new.rv()[1] - v_exit # obliczenia nowej predkosci
dv_out_new = np.linalg.norm(v_out_new) * u.km / u.s
m_p_new = m * (math.exp(dv_out_new / Isp) - 1
) # obliczenia brakujace masy paliwa
# Odpowiedz:
print('Potrzebna delta V: %.3f km/s' % float(dv_out_new / u.km * u.s))
print('Potrzebna dod. masa paliwa: %i kg' % int(m_p_new / u.kg))
def plot_body_orbit(self, body, epoch=None, label=None):
"""Plots complete revolution of body and current position if given.
Parameters
----------
body : poliastro.bodies.SolarSystemBody
Body.
epoch : astropy.time.Time, optional
Epoch of current position.
label : str, optional
Label of the orbit, default to the name of the body.
"""
from poliastro.twobody import Orbit
with warnings.catch_warnings():
warnings.simplefilter("ignore", DeprecationWarning)
orbit = Orbit.from_body_ephem(body, epoch)
self.plot(orbit, label=label or str(body))
def _plot_body_orbit(
self,
body,
epoch,
*,
label=None,
color=None,
trail=False,
):
if color is None:
color = BODY_COLORS.get(body.name)
from poliastro.twobody import Orbit
with warnings.catch_warnings():
warnings.simplefilter("ignore", DeprecationWarning)
orbit = Orbit.from_body_ephem(body, epoch)
return self._plot(orbit,
label=label or str(body),
color=color,
trail=trail)
def hill_radius(body, a=None, e=None):
"""Approximated radius of the Hill Sphere of Influence (SOI) for a body.
Parameters
----------
body : `~poliastro.bodies.Body`
Astronomical body which the SOI's radius is computed for.
a : float, optional
Semimajor axis of the body's orbit, default to None (will be computed from ephemerides).
e : float, optional
Eccentricity of the body's orbit, default to 0 (will be computed from ephemerides).
Returns
-------
astropy.units.quantity.Quantity
Approximated radius of the Sphere of Influence (SOI) [m]
"""
# Compute semimajor and eccentricity axis at epoch J2000 for the body if it was not
# introduced by the user
if a is None or e is None:
try:
ss = Orbit.from_body_ephem(body, J2000_TDB)
a = a if a is not None else ss.a
e = e if e is not None else ss.ecc
except KeyError:
raise RuntimeError(
"""To compute the semimajor axis or eccentricity for Moon and Pluto use the JPL ephemeris:
>>> from astropy.coordinates import solar_system_ephemeris
>>> solar_system_ephemeris.set("jpl")""")
mass_ratio = body.k / (3 * body.parent.k)
r_SOI = a * (1 - e) * (mass_ratio**(1 / 3))
return r_SOI.decompose()
def laplace_radius(body, a=None):
"""Approximated radius of the Laplace Sphere of Influence (SOI) for a body.
Parameters
----------
body : `~poliastro.bodies.Body`
Astronomical body which the SOI's radius is computed for.
a : float, optional
Semimajor axis of the body's orbit, default to None (will be computed from ephemerides).
Returns
-------
astropy.units.quantity.Quantity
Approximated radius of the Sphere of Influence (SOI) [m]
"""
# Compute semimajor axis at epoch J2000 for the body if it was not
# introduced by the user
if a is None:
a = Orbit.from_body_ephem(body, J2000_TDB).a
r_SOI = a * (body.k / body.parent.k) ** (2 / 5)
return r_SOI.decompose()
def laplace_radius(body, a=None):
"""Approximated radius of the Laplace Sphere of Influence (SOI) for a body.
Parameters
----------
body : `~poliastro.bodies.Body`
Astronomical body which the SOI's radius is computed for.
a : float, optional
Semimajor axis of the body's orbit, default to None (will be computed from ephemerides).
Returns
-------
astropy.units.quantity.Quantity
Approximated radius of the Sphere of Influence (SOI) [m]
"""
# Compute semimajor axis at epoch J2000 for the body if it was not
# introduced by the user
if a is None:
a = Orbit.from_body_ephem(body, J2000_TDB).a
r_SOI = a * (body.k / body.parent.k)**(2 / 5)
return r_SOI.decompose()
ss = Orbit.from_vectors(Earth, r, v)
print(ss)
from poliastro.plotting import plot
#plot(ss)
a = 1.523679 * u.AU
ecc = 0.093315 * u.one
inc = 1.85 * u.deg
raan = 49.562 * u.deg
argp = 286.537 * u.deg
nu = 23.33 * u.deg
ss = Orbit.from_classical(Sun, a, ecc, inc, raan, argp, nu)
#plot(ss)
from poliastro.examples import iss
print(iss)
from poliastro.maneuver import Maneuver
dv = [5, 0, 0] * u.m / u.s
man = Maneuver.impulse(dv)
man = Maneuver((0 * u.s, dv))
from astropy import time
epoch = time.Time("2018-01-25 20:00")
from poliastro import ephem
new = Orbit.from_body_ephem(Earth, epoch)
print(new)
def test_orbit_from_ephem_is_in_icrs_frame(body):
ss = Orbit.from_body_ephem(body)
assert ss.frame.is_equivalent_frame(ICRS())
def test_orbit_from_ephem_with_no_epoch_is_today():
# This is not that obvious http://stackoverflow.com/q/6407362/554319
body = Earth
ss = Orbit.from_body_ephem(body)
assert (Time.now() - ss.epoch).sec < 1
date_liftoff = time.Time('1969-07-16 14:32', scale='tdb')
date_launch = date_liftoff + (2 * u.h + 44 * u.min)
#date_launch is the time at which Translunar Insertion Maneuver is performed
date_arrival = date_launch + (75 * u.h + 54 * u.min)
#date_arrival is the time at which Cislunar Insertion Maneuver is performed
tof = date_arrival - date_launch
#Apollo Parking Orbit Around Earth
apollo = Orbit.circular(Earth,
alt=185.21 * u.km,
inc=32.521 * u.deg,
epoch=date_launch)
#Moon Orbit aqcuisition and conversion to GCRS
EPOCH = date_arrival
moon = Orbit.from_body_ephem(Moon, EPOCH)
moon_icrs = ICRS(x=moon.r[0],
y=moon.r[1],
z=moon.r[2],
v_x=moon.v[0],
v_y=moon.v[1],
v_z=moon.v[2],
representation=CartesianRepresentation,
differential_cls=CartesianDifferential)
moon_gcrs = moon_icrs.transform_to(GCRS(obstime=EPOCH))
moon_gcrs.representation = CartesianRepresentation
moon_gcrs
moon = Orbit.from_vectors(
Earth, [moon_gcrs.x, moon_gcrs.y, moon_gcrs.z] * u.km,
from poliastro.examples import iss iss #plot (iss) iss.epoch iss.nu.to(u.deg) iss.n.to(u.deg / u.min) iss_30m = iss.propagate(30 * u.min) iss_30m.epoch iss_30m.nu.to(u.deg) #plot (iss_30m) earth = Orbit.from_body_ephem(Earth) #plot(earth) earth_30d = earth.propagate(30 * u.day) #plot(earth_30d) from poliastro.maneuver import Maneuver dv = [5, 0, 0] * u.m / u.s man = Maneuver.impulse(dv) man = Maneuver((0 * u.s, dv)) ss_i = Orbit.circular(Earth, alt=700 * u.km) ss_i #plot(ss_i) hoh = Maneuver.hohmann(ss_i, 36000 * u.km) hoh.get_total_cost()
def test_orbit_from_ephem_is_in_icrs_frame(body):
ss = Orbit.from_body_ephem(body)
assert ss.frame.is_equivalent_frame(ICRS())
import numpy as np from astropy import units as u from poliastro.bodies import Earth, Jupiter, Sun from poliastro.twobody import Orbit from poliastro.util import norm # Orbits ss_Earth = Orbit.from_body_ephem(Earth) ss_Jupiter = Orbit.from_body_ephem(Jupiter) # Radius vector r_Earth = ss_Earth.r.to(u.m) r_Jupiter = ss_Jupiter.r.to(u.m) # Radius vector norm R_E = norm(r_Earth) R_J = norm(r_Jupiter) # Velocity vectors v_Earth = ss_Earth.v.to(u.m / u.s) v_Jupiter = ss_Jupiter.v.to(u.m / u.s) # Velocity vectors direction v_Earth_dir = v_Earth / norm(v_Earth)
def test_orbit_from_ephem_with_no_epoch_is_today():
# This is not that obvious http://stackoverflow.com/q/6407362/554319
body = Earth
ss = Orbit.from_body_ephem(body)
assert (Time.now() - ss.epoch).sec < 1
import numpy as np
from poliastro.twobody import Orbit
from poliastro.util import norm
from astropy.time import Time
from poliastro import iod
from poliastro.threebody.flybys import compute_flyby
from brentq import brentq
import matplotlib.pyplot as plt
T_ref = 150 * u.day
k = Sun.k
a_ref = np.cbrt(k * T_ref**2 / (4 * np.pi**2)).to(u.km)
energy_ref = (-k / (2 * a_ref)).to(u.J / u.kg)
flyby_1_time = Time("2018-09-28", scale="tdb")
r_mag_ref = norm(Orbit.from_body_ephem(Venus, epoch=flyby_1_time).r)
v_mag_ref = np.sqrt(2 * k / r_mag_ref - k / a_ref)
d_launch = Time("2018-08-11", scale="tdb")
ss0 = Orbit.from_body_ephem(Earth, d_launch)
ss1 = Orbit.from_body_ephem(Venus, epoch=flyby_1_time)
tof = flyby_1_time - d_launch
(v0, v1_pre), = iod.lambert(Sun.k, ss0.r, ss1.r, tof.to(u.s))
V = Orbit.from_body_ephem(Venus, epoch=flyby_1_time).v
h = 2548 * u.km
d_flyby_1 = Venus.R + h
V_2_v_, delta_ = compute_flyby(v1_pre, V, Venus.k, d_flyby_1)
theta_range = np.linspace(0, 2 * np.pi)
def func(theta):
V_2_v, _ = compute_flyby(v1_pre, V, Venus.k, d_flyby_1, theta * u.rad)
# print ("a: " + str(perDiff(a, aJPL)) + "%")
# print ("e: " + str(perDiff(e, eJPL)) + "%")
# print ("i: " + str(perDiff(i, iJPL)) + "%")
# print ("Lon ascending node: " + str(perDiff(lOmega[0], lOmegaJPL)) + "%")
# print ("Perihelion: " + str(perDiff(aPerihelion, omegaJPL)) + "%")
# print ("M: " + str(perDiff(M, MJPL)) + "%")
a = a * units.AU
ecc = e * units.one
inc = i * units.deg
raan = lOmega[0] * units.deg
argp = aPerihelion * units.deg
nu = v * units.deg
epoch = time.Time(t2, format='jd', scale='utc')
earth_orbit = Orbit.from_body_ephem(Earth)
earth_orbit = earth_orbit.propagate(time.Time(t2, format='jd', scale='tdb'),
rtol=1e-10)
magellan_orbit = neows.orbit_from_name('2018ez2')
magellan_orbit = magellan_orbit.propagate(time.Time(t2,
format='jd',
scale='tdb'),
rtol=1e-10)
estimated_orbit = Orbit.from_classical(Sun, a, ecc, inc, raan, argp, nu, epoch)
op = OrbitPlotter()
op.plot(earth_orbit, label='Earth')
op.plot(magellan_orbit, label='2018 EZ2')
op.plot(estimated_orbit, label='Estimated')
plt.show()
from poliastro.maneuver import Maneuver
from poliastro.iod import izzo
from poliastro.plotting import plot, OrbitPlotter
from poliastro.util import norm
solar_system_ephemeris.set("jpl")
## Initial data
# Links and sources: https://github.com/poliastro/poliastro/wiki/EuroPython:-Per-Python-ad-Astra
date_launch = Time("2022-08-05 16:25", scale='tdb')
C_3 = 31.1 * u.km**2 / u.s**2
date_flyby = Time("2024-10-09 19:21", scale='tdb')
date_arrival = Time("2026-07-05 03:18", scale='tdb')
# Initial state of the Earth
ss_e0 = Orbit.from_body_ephem(Earth, date_launch)
r_e0, v_e0 = ss_e0.rv()
print("Position of Earth: ",r_e0)
print("Velocity of Earth: ",v_e0)
# State of the Earth the day of the flyby
ss_efly = Orbit.from_body_ephem(Earth, date_flyby)
r_efly, v_efly = ss_efly.rv()
# Assume that the insertion velocity is tangential to that of the Earth
dv = C_3**.5 * v_e0 / norm(v_e0)
man = Maneuver.impulse(dv)
# Inner Cruise 1
ic1 = ss_e0.apply_maneuver(man)
a = (r_p + r_a) / 2
roadster = Orbit.from_classical(attractor=Sun,
a=0.9860407221838553 * u.AU,
ecc=0.2799145376150214 * u.one,
inc=1.194199764898942 * u.deg,
raan=49 * u.deg,
argp=286 * u.deg,
nu=23 * u.deg,
epoch=date)
for date in days_as:
apophis_orbit = neows.orbit_from_name('99942')
spacex = neows.orbit_from_name('-143205')
op.orbits.clear()
earth = Orbit.from_body_ephem(Earth, date)
mars = Orbit.from_body_ephem(Mars, date)
op.plot(earth, label=Earth)
op.plot(mars, label=Mars)
op.plot(roadster, label='Roadster')
op.plot(apophis_orbit, label='Apophis')
op._redraw()
plt.pause(0.01)
input('type to exit')
op.plot(Orbit.from_body_ephem(Mars, time.Time("2018-07-28 12:00",
scale='utc')),
label=Mars)