Lowell-Nbody

A N-body integrator for small bodies of the Solar System Author: Simon Grimm, ETH Zürich, University of Zürich, Switzerland

Instructions

Step 1, get perturbers file

• cd readChebyshev
• Download linux_p1550p2650.440
• Download header.440
• Download sb441-n16.bsp
• make
• set start time and end time of desired perturbers data in paramChebyshev.dat
• ./readChebyshev

This will create a binary file called 'PerturbersChebyshev.bin' The structure of the file is:

• The header contains all necessary constants:

  • time0, fp64, Start time of the entire file, in Julian Days, default = 2450800.5.
  • time1, fp64, End time of the entire file, in Julian Days, default = 2459800.5
  • AUtokm, fp64, Conversion factor AU to km, taken from DE440 file.
  • EM, fp64, Earth / Moon mass ratio, taken from DE440 file.
  • CLIGHT, fp64, Speed of light in km/s, taken from DE440 file.
  • RE, fp64, Earth radius in km, taken from DE440 file.
  • J2E, fp64, J2 term of Earth (dimensionless), taken from DE440 file.

• After the header follows a list of all perturbers. Containing for each perturber:

  • id, int32, perturber id
  • nChebyshev, int32, number of Chebyshev coefficients
  • offset0 , int32, offset of begiining of data from perturber
  • offset1, int32, offset of ending of data from perturber
  • GM, fp64, G * mass

• Now follows the data itself.

  • perturber 1, record 1
  • perturber 1, record 2
  • ...
  • perturber 1, record n
  • perturber 2, record 1
  • perturber 2, record 2
  • ...
  • perturber 2, record n
  • ...

Every record contains

• start time of record
• end time of record
• Chebyshev coefficients, (3 * nChebyshev)

Step 2, set parameters

• cd ../integrator

• make

• modify param.dat

  • Initial condition file format: format of the initial conditions file, "text" or "binary".
  • Initial condition file: name of the initial conditions file, string.
  • Output file name: name of the output files, string.
  • Path to perturbers file, string

allowed integrators are:

• LF, leapfrog
• RK4, Runge Kutta 4, fixed time steps
• RKF45, Runge Kutta Fehlberg 45, adaptive time steps
• DP54, Runge Kutta Fehlberg 54, adaptive time steps
• RKF78, Runge Kutta Fehlberg 78, adaptive time steps

Step 3, set initial conditions

The name of the initial conditions file must be set in the param.dat file. The File contains the following columns:

• id x y z vx vy vz A1 A2 A3

with:

• id, particle index, integer number
• x ,y ,z: Barecentric positions in AU
• vx, vy, vz: Barycentric velocoties in AU/days
• A1, A2, A3: Non-gravitational acceleration terms in AU/days^2

Step 4, run the integration

• ./integrator

The name of the initial conditions file must be set in the param.dat file. The output file format can either be a text file (Out_.dat) or a binary file (Out_.bin) The Format can be set in the param.dat file

The output file contains

• time in days
• id: particle index
• x: barycentric position in AU
• y: barycentric position in AU
• z: barycentric position in AU
• vx: barycentric velocity in AU/day
• vy: barycentric velocity in AU/day
• vz: barycentric velocity in AU/day
• minimal time step of last interval, in days