# THREE_BODY_SIMULATION Planar Three Body Problem Simulation

THREE_BODY_SIMULATION is a C program which simulates the solution of the planar three body problem.

Three bodies, regarded as point masses, are constrained to lie in a plane. The masses of each body are given, as are the positions and velocities at a starting time T = 0. The bodies move in accordance with the gravitational force between them.

The force exerted on the 0-th body by the 1st body can be written:

```        F = - m0 m1 ( p0 - p1 ) / |p0 - p1|^3
```
assuming that units have been normalized to that the gravitational coefficient is 1. Newton's laws of motion can be written:
```
m0 p0'' = - m0 m1 ( p0 - p1 ) / |p0 - p1|^3
- m0 m2 ( p0 - p2 ) / |p0 - p2|^3

m1 p1'' = - m1 m0 ( p1 - p0 ) / |p1 - p0|^3
- m1 m2 ( p1 - p2 ) / |p1 - p2|^3

m2 p2'' = - m2 m0 ( p2 - p0 ) / |p2 - p0|^3
- m2 m1 ( p2 - p1 ) / |p2 - p1|^3
```

Letting

```        y1 = p0(x)
y2 = p0(y)
y3 = p0'(x)
y4 = p0'(y)
```
and using similar definitions for p1 and p2, the 3 second order vector equations can be rewritten as 12 first order equations. In particular, the first four are:
```        y1' = y3
y2' = y4
y3' = - m1 ( y1 - y5  ) / |(y1,y2) - (y5,y6) |^3
- m2 ( y1 - y9  ) / |(y1,y2) - (y9,y10)|^3
y4' = - m1 ( y2 - y6  ) / |(y1,y2) - (y5,y6) |^3
- m2 ( y2 - y10 ) / |(y1,y2) - (y9,y10)|^3
```
and so on. This first order system can be integrated by a standard ODE solver.

Note that when any two bodies come close together, the solution changes very rapidly, and very small steps must be taken by the ODE solver. For this system, the first near collision occurs around T=15.8299, and the results produced by the ODE solver will not be very accurate after that point.

### Languages:

THREE_BODY_SIMULATION is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version.

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### Author:

Original MATLAB version by Dominik Gruntz, Joerg Waldvogel; C version by John Burkardt.

### Reference:

1. Dominik Gruntz, Joerg Waldvogel,
"Orbits in the Planar Three-Body Problem",
Walter Gander, Jiri Hrebicek,
Solving Problems in Scientific Computing using Maple and Matlab,
Springer, 1997,
ISBN: 3-540-61793-0,
LC: Q183.9.G36.

### List of Routines:

SIMPLE_RKF45 simulates the problem by calling the ODE integrator RKF45. This approach loses accuracy when the bodies come close to colliding, which is likely to happen often.

• MAIN is the main program for SIMPLE_RKF45.
• R8MAT_WRITE writes an R8MAT file.
• SIMPLE_RKF45_RUN runs the simple three body ODE system.
• SIMPLE_F returns the right hand side of the three body ODE system.

You can go up one level to the C source codes.