Long-term evolution of the neptune trojan 2001 QR322

We simulated more than a hundred possible orbits of the Neptune Trojan 2001 QR322 for the age of the Solar system. The orbits were generated randomly according to the probability density derived from the covariance matrix of the orbital elements. The test trajectories librate around Neptune's L-4 point, with amplitudes varying from 40degrees to 75degrees and libration periods varying from 8900 to 9300 yr. The nu(18) secular resonance plays an important role. There is a separatrix of the resonance so that the resonant angle switches irregularly between libration and circulation. The orbits are chaotic, with observed Lyapunov times from 1.7 to 20 Myr, approximately. The probability of escape to a non-Trojan orbit in our simulations was low, and only occurred for orbits starting near the low-probability edge of the orbital element distribution (largest values of initial semimajor axis and small eccentricity). This suggests that the Trojan may well be a primordial object.