A first officer (aka executive officer, XO, and number one) is the second-highest ranking officer aboard a starship or space station after the commanding officer. They take command if the commanding officer is unable to. If the first officer is also unable to take command, the second officer is in charge, followed by the third officer, and so on. On ships assigned to the Starfleet Corps of Engineers, such as the USS da Vinci, the first officer is concurrently the leader of the team.