In the game's campaign, which can be played alone or cooperatively, players assume the role of Noble Six, a supersoldier engaged in combat with an alien collective known as the Covenant. The Covenant come in eight distinct varieties with different ranks and classes for each type; for example, Elites are the leaders of a group, while Grunts are less intelligent and only dangerous in large groups. :11 The player is equipped with a recharging energy shield that absorbs damage from weapons, fire and impacts. When the energy shield is depleted, the player loses health. When the player's health reaches zero, the character dies and the game reloads from a saved checkpoint. Health is replenished using health packs scattered throughout Reach's levels. The campaign's encounters with enemies are typically large, open spaces with weapons caches, cover from enemy fire and strategic vantage points. New to the Halo series are dogfight sequences set in space.