1991 BA

1991 BA is approximately 5 to 10 meters (16 to 33 ft) in diameter and is listed on the Sentry Risk Table.^[4] It follows a highly eccentric (0.68), low-inclination (2.0°) orbit of 3.3 years duration, ranging between 0.71 and 3.7 AU from the Sun. 1991 BA was, at the time of its discovery, the smallest and closest confirmed asteroid outside of Earth's atmosphere.^[5] 1991 BA is too faint to be observed except during close approaches to Earth and is considered lost.

The asteroid has a very short 5-hour observation arc that makes future predictions of its position unreliable. Virtual clones of the asteroid that fit the uncertainty region in the known trajectory use to show a 1 in 290,000 chance that the asteroid could impact Earth on 2023 January 18.^[4] It is estimated that an impact would produce an upper atmosphere air burst equivalent to 16 kt TNT,^[4] roughly equal to Nagasaki's Fat Man. The asteroid would appear as a bright fireball and fragment in the air burst into smaller pieces that would hit the ground at terminal velocity producing a meteorite strewn field. Impacts of objects this size are estimated to occur approximately once a year.^[7] Asteroid 2008 TC3 was an object of similar size that was discovered less than a day before its impact on Earth on October 7, 2008, and produced a fireball and meteorite strewn field in the Sudan. The 18 January 2023 virtual impactor did not occur.

There is a 1 in a million chance of impacting Earth on 19 January 2114.^[4]