The U.S.S. Excelsior is an Epic Grade 5+ support ship built around the Challenge Loop, fighting V'ger Fighter hostiles and powering a support ability that boosts every deployed ship you have -- not just the Excelsior itself. You can start building it at Shipyard Level 51+, with blueprints and upgrade materials from the Challenge Refinery.
The Excelsior's active ability summons and reveals a V'ger Fighter hostile; defeating it earns ASA (Active Support Ability) fuel. Once you've banked enough fuel, activating the support ability temporarily boosts every ship you have deployed across the galaxy for 5 minutes -- useful for stacking a burst window around other combat plans, not just farming the loop itself. Its passives add Increased Iso Damage and Increased Apex Barrier specifically against V'ger hostiles, and it carries a Prototype Transwarp Drive with a torpedo-focused loadout.
You can clear the Easy and Hard Daily Challenges without owning an Excelsior, but the Excelsior Challenge specifically requires it, and having the ship speeds up progress through the wider Reward Track. Beyond the Challenge itself, using the Excelsior to clear more loop hostiles unlocks exclusive refinery items and ship-specific missions tied to its own progression.