139821

139821 is a numbered minor planet in the main asteroid belt between Mars and Jupiter. It was discovered on 10 March 2002 by the Lincoln Near‑Earth Asteroid Research (LINEAR) survey using a 1‑meter aperture telescope at the Lincoln Laboratory's Experimental Test Site in Socorro, New Mexico. The discoverers did not assign a provisional designation at the time of discovery because the object was detected as part of the survey’s automatic moving‑object detection pipeline, and it was later given the permanent number 139821 once its orbit was well determined.

The asteroid resides in the interior part of the belt and is dynamically associated with the Eunomia family, which is composed predominantly of S‑type stony bodies. Its semimajor axis is 2.32 AU, with an orbital period of about 3.8 Earth years. The orbit has an eccentricity of 0.12 and an inclination of 11.5°, placing it slightly above the ecliptic plane. As of the most recent observations, the asteroid has an absolute magnitude of 15.6, which corresponds to a diameter of roughly 2.4 kilometres assuming a typical albedo of 0.20 for S‑type asteroids.

Since 139821 has not been given an official name and continues to bear only its numerical designation, it is not associated with a mythological figure or historical person as is common for larger or more studied bodies. It remains a well‑tracked, but otherwise unremarkable member of the main belt, routinely observed by both professional and amateur astronomers.