# User guide¶

## Defining the orbit: State objects¶

The core of poliastro are the State objects inside the poliastro.twobody module. They store all the required information to define an orbit:

• The body acting as the central body of the orbit, for example the Earth.
• The position and velocity vectors or the orbital elements.
• The time at which the orbit is defined.

First of all, we have to import the relevant modules and classes:

import numpy as np
from astropy import units as u

from poliastro.bodies import Earth, Sun
from poliastro.twobody import State


### From position and velocity¶

There are several methods available to create State objects. For example, if we have the position and velocity vectors we can use from_vectors():

# Data from Curtis, example 4.3
r = [-6045, -3490, 2500] * u.km
v = [-3.457, 6.618, 2.533] * u.km / u.s

ss = State.from_vectors(Earth, r, v)


And that’s it! Notice a couple of things:

• Defining vectorial physical quantities using Astropy units is terribly easy. The list is automatically converted to a Quantity, which is actually a subclass of NumPy arrays.

• If no time is specified, then a default value is assigned:

>>> ss.epoch
<Time object: scale='utc' format='jyear_str' value=J2000.000>
>>> ss.epoch.iso
'2000-01-01 12:00:00.000'


If we’re working on interactive mode (for example, using the wonderful IPython notebook) we can immediately plot the current state:

>>> from poliastro.plotting import plot
>>> plot(ss)


This plot is made in the so called perifocal frame, which means:

• we’re visualizing the plane of the orbit itself,
• the $$x$$ axis points to the pericenter, and
• the $$y$$ axis is turned $$90 \mathrm{^\circ}$$ in the direction of the orbit.

The dotted line represents the osculating orbit: the instantaneous Keplerian orbit at that point. This is relevant in the context of perturbations, when the object shall deviate from its Keplerian orbit.

### From classical orbital elements¶

We can also define a State using a set of six parameters called orbital elements. Although there are several of these element sets, each one with its advantages and drawbacks, right now poliastro supports the classical orbital elements:

• Semimajor axis $$a$$.
• Eccentricity $$e$$.
• Inclination $$i$$.
• Right ascension of the ascending node $$\Omega$$.
• Argument of pericenter $$\omega$$.
• True anomaly $$\nu$$.

In this case, we’d use the method from_elements():

# Data for Mars at J2000 from JPL HORIZONS
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 = State.from_elements(Sun, a, ecc, inc, raan, argp, nu)


Notice that whether we create a State from $$r$$ and $$v$$ or from elements we can access many mathematical properties individually:

>>> ss.period.to(u.day)
<Quantity 686.9713888628166 d>
>>> ss.v
<Quantity [  1.16420211, 26.29603612,  0.52229379] km / s>


To see a complete list of properties, check out the poliastro.twobody.State class on the API reference.

## Changing the orbit: Maneuver objects¶

poliastro helps us define several in-plane and general out-of-plane maneuvers with the Maneuver class inside the poliastro.maneuver module.

Each Maneuver consists on a list of impulses $$\Delta v_i$$ (changes in velocity) each one applied at a certain instant $$t_i$$. The simplest maneuver is a single change of velocity without delay: you can recreate it either using the impulse() method or instantiating it directly.

dv = [5, 0, 0] * u.m / u.s

man = Maneuver.impulse(dv)
man = Maneuver((0 * u.s, dv))  # Equivalent


There are other useful methods you can use to compute common in-plane maneuvers, notably hohmann() and bielliptic() for Hohmann and bielliptic transfers respectively. Both return the corresponding Maneuver object, which in turn you can use to calculate the total cost in terms of velocity change ($$\sum |\Delta v_i|$$) and the transfer time:

>>> ss_i = State.circular(Earth, alt=700 * u.km)
>>> hoh = Maneuver.hohmann(ss_i, 36000 * u.km)
>>> hoh.get_total_cost()
<Quantity 3.6173981270031357 km / s>
>>> hoh.get_total_time()
<Quantity 15729.741535747102 s>


You can also retrieve the individual vectorial impulses:

>>> hoh.impulses[0]
(<Quantity 0 s>, <Quantity [ 0.        , 2.19739818, 0.        ] km / s>)
>>> hoh[0]  # Equivalent
(<Quantity 0 s>, <Quantity [ 0.        , 2.19739818, 0.        ] km / s>)
>>> tuple(_.decompose([u.km, u.s]) for _ in hoh[1])
(<Quantity 15729.741535747102 s>, <Quantity [ 0.        , 1.41999995, 0.        ] km / s>)


To actually retrieve the resulting State after performing a maneuver, use the method apply_maneuver():

>>> ss_f = ss_i.apply_maneuver(hoh)
>>> ss_f.rv()
(<Quantity [ -3.60000000e+04, -7.05890200e-11, -0.00000000e+00] km>, <Quantity [ -8.97717523e-16, -3.32749489e+00, -0.00000000e+00] km / s>)


## More advanced plotting: OrbitPlotter objects¶

We previously saw the poliastro.plotting.plot() function to easily plot orbits. Now we’d like to plot several orbits in one graph (for example, the maneuver me computed in the previous section). For this purpose, we have OrbitPlotter objects in the plotting module.

These objects hold the perifocal plane of the first State we plot in them, projecting any further trajectories on this plane. This allows to easily visualize in two dimensions:

from poliastro.plotting import OrbitPlotter

op = OrbitPlotter()
ss_a, ss_f = ss_i.apply_maneuver(hoh, intermediate=True)
op.plot(ss_i)
op.plot(ss_a)
op.plot(ss_f)


Which produces this beautiful plot:

Plot of a Hohmann transfer.

And that’s it for the basics of poliastro! Feel free to send my your suggestions of improvement, both for the code and the docs.