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SpaceRock

The primary data structure in spacerocks is a class called SpaceRock. You can instantiate a SpaceRock object using any valid set of 6 Keplerian elements, or a state vector.

Instantiation

from spacerocks import SpaceRock
from spacerocks.units import Units

units = Units()
units.timescale = 'utc'

rock = SpaceRock(a=44, 
                 e=0.1, 
                 inc=10, 
                 node=140, 
                 arg=109, 
                 M=98, 
                 H=7, 
                 epoch='1 December 2021', 
                 origin='ssb', 
                 frame='eclipJ2000', 
                 units=units)

The Units object we instantiated is extremely useful for avoiding bugs, as it allows for an explicit set of units to be specified only once. You can read more about it here.

Note that you can also pass a JD or an MJD as an epoch. If you provide a string (in any format) the program will use dateutil to try to parse the date. This is slow when compared to passing explicit values and time formats.

You can then access the attributes of your SpaceRock object. The attributes are lazily computed in the interest of efficiency. The attributes all carry astropy units, with angles stored as Angle objects, distances stored as Distance objects, and times stored as Time objects. This ensures a complete lack of ambiguity, and facilitates unit conversions.

The attributes of a SpaceRock object are listed in the tables below.

Orbital Parameter Attribute
semi-major axis a
eccentricity e
inclination inc
longitude of ascending node node
longitude of pericenter varpi
argument of pericenter arg
mean anomaly M
true anomaly f
eccentric anomaly E
semi-minor axis b
pericenter distance q
apocenter distance Q
Metadata Attribute
name name
epoch epoch
Physical Property Attribute
absolute magnitude H
phase-slope constant G
albedo albedo
mass mass
radius radius
diameter diameter

Vectorization

SpaceRock objects are vectorized, allowing for the processing of multiple objects at once.

rocks = SpaceRock(a=[44, 45], 
                  e=[0.1, 1.2], 
                  inc=[10, 170], 
                  node=[140, 303], 
                  arg=[109, 23], 
                  f=[98, 124], 
                  H=[7, 8.2],
                  epoch=['1 December 2021', '3 December 2021'], 
                  origin='ssb', 
                  units=units)

Notice that a SpaceRock object can handle both elliptical and hyperbolic orbits simultaneously (though parabolic orbits are not yet supported), and it can handle nonuniform epochs.

Object Slicing

SpaceRock objects have a number of other utilities. First, you can slice SpaceRock objects in a Pythonic way

r = rocks[rocks.e < 1]

Coordinate Transformations

You can easily change the origin of the coordinate system. This is particularly useful for converting the Minor Planet Center's heliocentric elements to barycentric elements.

# convert to heliocentric coordinates
rocks.to_helio()

# convert back to barycentric coordinates
rocks.to_bary()

# convert to New Horizons-centric coordinates
rocks.change_origin(spiceid=-98)

You can also change the coordinate frame between J2000 and ecliptic J2000.

# Convert to J2000 coordinates
rocks.change_frame('J2000')

# Convert back to ecliptic J2000 coordinates
rocks.change_frame('eclipJ2000')

The propagate Method

You can use the propagate method to propagate the rocks to any epochs. This method uses rebound's ias15 integrator under the hood, and automatically synchronizes the epochs of the rocks so you don't have to. Notice that we are specifying the epochs to be in Barycentric Dynamical Time.

units = Units()
units.timescale = 'tdb'

prop, planets, sim = rock.propagate(epochs=['2 December 2021', '4 December 2021'], model='PLANETS', units=units)

Here prop is a SpaceRock object containing the test particles at all of the epochs, planets is a SpaceRock object containing the perturbers at all of the epochs, and sim is the rebound simulation object at the final epoch. The model argument sets the perturbers as follows.

model Perturbers
SUN Sun
GIANTS Sun, Jupiter, Saturn, Uranus, Neptune
PLANETS Sun, Mercury, Venus, Earth, Moon, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto
HORIZONS Full set of JPL Horizons perturbers

The perturbers' masses are from JPL Horizons, and their state vectors are computed using spiceypy.

The observe Method

You can use the observe method compute objects' ephemerides from an arbitrary location in the solar system. Here we compute the ephemerides of our rocks from DECam.

obs = rocks.observe(obscode='W84')

The only argument taken by observe is an obscode (see this link for a full list) or a spiceid (Earth, Jupiter Barycenter, -98, etc.).

The observe method returns an Ephemerides object which contains the rocks' state vectors with respect to the observer in the equatorial frame, corrected for light travel time. These values allow us to compute the rocks' observable properties, which are accessible as attriutes to the Ephemerides object.

The to_file Method

Finally, you can write and read SpaceRock objects to and from asdf files.

rocks.to_file('rocks.rocks')

rocks_from_disk = SpaceRock.from_file('rocks.rocks')

This is useful for saving your work, and getting your exact objects back at a later date.