21021

21021 is a numbered minor planet located in the central region of the asteroid belt. It was discovered on 5 February 1998 by the Lincoln Near-Earth Asteroid Research (LINEAR) program at the White Sands Missile Range near Socorro, New Mexico. The discovery was originally catalogued as 1998 TC4 before the Minor Planet Center assigned it the permanent designation 21021.

The asteroid follows a typical S‑type orbit with a semimajor axis of approximately 2.5 astronomical units and an orbital period of 4.6 years. Its orbit has an eccentricity of 0.12 and an inclination of 5.8 degrees relative to the ecliptic. The object's diameter is estimated at roughly 4 kilometers based on infrared measurements from the NEOWISE mission, which also indicate a moderate albedo of about 0.18. Photometric observations have revealed a rotation period of around 5.9 hours, although the light curve amplitude suggests a relatively spherical shape.

21021 has not yet been formally named; it retains its provisional designation in the Minor Planet Center database. The asteroid appears in several surveys of main-belt populations and is occasionally referenced in dynamical studies of asteroid families. Its well-characterized orbit makes it a useful test case for numerical integration of minor-planet trajectories.