With earth in ruins, three UNSC Cruisers are sent to scour the universe for a suitable replacement for earth. With only 1,800 marines and 4 squads of ODST's between the 3 Cruisers, with well over 1000 colonists, the commanding officers only allow 2 teams of 10 soldiers to scout the entire planet

For this planet in particular, many large creatures roam, so a squad of ODST's are deployed to the surface for extra protection