Abstract
We have found approximately 1500 asteroids evolving in the exterior 1:2 resonance with Mars. Looking at the histogram of semimajor axes of the asteroids in the main belt the population can be distinguished as a peak at a ≃ 2.419 AU. Approximately 400 asteroids are librating around the asymmetric libration centers and about 700 are describing horseshoe trajectories. A strong secular perturbation due to Jupiter and due to Mars' eccentricity time evolution produces switching between libration centers and between librations and horseshoe trajectories. In spite of this strong secular effect the population remains linked to the resonance over time scales of 10^8 - 10^9 years.
