Ideas

U.N. Side

1. Escort important U.N. vessel or shuttle pass the asteroid belt. The destination can be a battle cruiser, merchant ship, or a base on one of the asteroids.
2. Escort merchant ship. Similar destination to above.
3. Protect Asteroid science/research laboritory located on Ceres.
4. Intercept bandit fighter squadren.
5. Intercept bandit supply vessel. Maybe the supply vessel is from United Nations!
6. Intercept bandit shuttle transport that carried an important group leader.
7. Routine exercise or reconnaissance mission in the asteroid belt.
8. Secure bandit stronghold.

All the above plots can be developed into thicker plots in your campaign. If your players are playing the bandits, Most of the above plots can simply be reversed.