Kepler.js

A package for working with two-body Keplerian Orbits.

Installation

Release pending, but you can source it in your package.json from Github for now.

package.json

{
  "name": "my-package",
  "dependencies": {
    "kepler.js": "jordanstephens/kepler.js"
  }
}

Usage

Defining an Orbit

The easiest way to define an orbit is with common orbital elements.

var qzss = Kepler.Orbit.fromParams({
  semimajorAxis: 42164, // km
  eccentricity: 0.075,
  inclination: 43, // deg
  rightAscension: 195, // deg
  argumentOfPeriapsis: 270 // deg
});

QZSS orbital parameters from Wikipedia.

You can also use more colloquial elements like perigee and apogee instead of semimajorAxis and eccentricity.

var iss = Kepler.Orbit.fromParams({
  apogee: 426.9, // km
  perigee: 416.2, // km
  inclination: 51.65, // deg
  rightAscension: 304.1, // deg
  argumentOfPeriapsis: 117.8 // deg
});

ISS orbital parameters from Wolfram Alpha.

Or you can use position (r) and velocity (v) vectors.

var r = [-6045, -3490, 2500],
    v = [-3.457, 6.618, 2.533],
    orbit = new Kepler.Orbit(r, v);

Orbital Attributes

Defining an orbit gives you access to the object's current position (r) and velocity (v) vectors along with many other attributes, including:

• semimajorAxis
• semilatusRectum
• apoapsis
• periapsis
• eccentricity
• angularMomentum
• radialVelocity
• inclination
• rightAscension
• argumentOfPeriapsis
• trueAnomaly
• period

Updating an Orbit After dt

Once you have an initial orbit defined, you can get updated position (r) and velocity (v) vectors after a period of time (in seconds) has passed.

iss.r // => [4427.6294614883145, 662.2433879237291, 5103.355851378229]
iss.v // => [-3.000020372892516, 6.843005359324254, 1.7091866273043546]

// Update vectors after 45 minutes have passed
iss.update(60 * 45)

iss.r // => [-4662.9620885090435, -88.56494596532605, -4943.141439770972]
iss.v // => [2.5073027532818064, -6.876096362825007, -2.248327189819066]

Caveats

• Only point objects are considered in a two-body environment.
• Perturbations due to atmospheric drag, solar radiation, etc are not considered.
• Nodal precession is not considered.

Tests

Tests can be run with npm

$ npm test

Most of the specs contain values used in examples found in Orbital Mechanics for Engineering Students.