Which asteroid is officially numbered 24?

The asteroid officially numbered 24 is 24 Themis.

Themis is a large main-belt asteroid discovered by Italian astronomer Annibale de Gasparis on 5 April 1853. Its formal designation combines its catalogue number and name: 24 Themis, often written as (24) Themis. The name honors Themis, the Greek personification of divine order and natural law.

At roughly 200 kilometers across, Themis is one of the larger bodies in the asteroid belt and the parent body of the Themis asteroid family. It travels around the Sun at an average distance of about 3.1 astronomical units and takes roughly 5.5 years to complete one orbit.

The asteroid became especially interesting after observations reported water ice and organic compounds on its surface in 2009–2010. That discovery surprised scientists because Themis orbits in the warmer inner Solar System compared with icy bodies beyond Neptune. The number 24 refers to its order of discovery and assignment, not its size, orbit, or chemical composition.