Shortest-period asteroid

As of 24 Jan 2022, the shortest-period asteroid known is 2021 PH27, which orbits the Sun in 0.314 years (114.7 days). Its orbit is entirely within Earth's, and its closest approach to the Sun is closer than Mercury's orbital distance. It was discovered by Scott Sheppard on 13 August 2021 using the Dark Energy Survey's DECam imager at NOIRLab's Cerro Tololo Inter-American Observatory in Chile. With an absolute magnitude of 17.73, it is estimated to be approximately 2 kilometers in diameter.

The previous record included a diameter for the object. Diameters are estimated from absolute magnitudes by assuming an albedo. A sample calculator is at http://www.physics.sfasu.edu/astro/asteroids/sizemagnitude.html. Objects very close to the Sun are likely to have very low albedo because of space weathering. Assuming an albedo of 5% gives an estimated diameter of 1.69 kilometers. The albedo is probably lower than that. Albedo of 2% gives a diameter of 2.67 kilometers. So "about 2 km" seems a good estimate.