Efficient Lie-Poisson Integrator for Secular Spin Dynamics of Rigid Bodies

A fast and efficient numerical integration algorithm is presented for the problem of the secular evolution of the spin axis. Under the assumption that a celestial body rotates around its maximum moment of inertia, the equations of motion are reduced to the Hamiltonian form with a Lie-Poisson bracket. The integration method is based on the splitting of the Hamiltonian function and so it conserves the Lie-Poisson structure. Two alternative partitions of the Hamiltonian are investigated and second order leapfrog integrators are provided for both cases. Non-Hamiltonian torques can be incorporated into the integrators with a combination of Euler and Lie-Euler approximations. Numerical tests of the methods confirm their good properties resulting in short computation time and reliability on long integration intervals. Subject headings: methods: numerical — solar system: general — celestial mechanics