2023 BU is a near-Earth object that passed 9,967 ± 1 km (6,193.21 ± 0.62 mi) from the centerpoint of Earth around 27 January 2023 00:29 UT.^[3] Since Earth's radius is about 6,378 km (3,963 mi), it was expected to pass approximately 3,589 ± 1 km (2,230.10 ± 0.62 mi) from the surface of Earth over the southern tip of South America.^[7] It passed at an altitude above low Earth orbit which is 2,000 km (1,200 mi) and below geostationary orbit which is 36,000 km (22,000 mi). The asteroid is about 3–8 meters in diameter^[5] and approached Earth from the night sky. It is the fourth closest non-impacting approach known to Earth (excluding Earthgrazers) after 2020 VT4, 2020 QG, and 2021 UA1.

The asteroid came to perihelion (closest approach to the Sun) on 27 January 2023,^[4] four hours after closest approach with Earth. 2023 BU was first imaged by Gennadiy Borisov at Nauchnyi, Crimea, on 21 January 2023 23:53 UT,^[2] about five days before closest approach. It was last observed on 31 January 2023.^[1]

The gravitational effect of the 2023 Earth approach will increase the orbital period from 359 days to an estimated 425 days.^[8] It will lift the perihelion and aphelion distances. The relatively low Earth encounter speed of 9.3 km/s (21,000 mph) is a result of a low eccentricity and Earth-like orbit.

There was no risk of an Earth impact during the 2023 Earth approach. Assuming the asteroid is at the larger size estimate of 8 meters in diameter, if it had entered the atmosphere it would not have reached the ground intact and would breakup around 30 km above the ground, thus representing only minimal threat to life.^[9]

Impacts by objects 8 meters (26 ft) in diameter occur, on average, every 5 years; impacts by objects 4 meters (13 ft) in diameter happen, on average, once every year.^[9] 2023 BU has a 1 in 17 million chance of impacting Earth on 20 January 2110.^[10]

• List of asteroid close approaches to Earth in 2023