1991 BA is the designation given to an asteroid which was discovered by Spacewatch on January 18, 1991 and passed within 160,000 km (100,000 mi) of Earth. This is a little less than half the distance to the Moon. 1991 BA is approximately 5 to 10 meters (15 to 30 ft) in diameter and is listed on the Sentry Risk Table. It follows a highly eccentric (0.682), low-inclination (2.0°) orbit of 3.36 years duration, ranging between 0.713 and 3.773 AU from the Sun (semi-major axis of 2.243 AU). 1991 BA was, at the time of its discovery, the smallest and closest confirmed asteroid outside of Earth's atmosphere. 1991 BA is too faint to be observed except during close approaches to Earth and is considered lost.
Virtual clones of the asteroid that fit the uncertainty region in the known trajectory show a 1 in 1,961,000 chance that the asteroid will impact the Earth on 2023 January 18. It is estimated that an impact would produce an upper atmosphere air burst equivalent to 19 kt TNT, roughly equal to Nagasaki's Fat Man. The asteroid would be vaporized as these small impacts occur approximately once per year.
- "JPL Close-Approach Data: (1991 BA)". 1991-01-18 last obs (arc=1 day; Uncertainty=9). Retrieved 2012-03-20.
- "Earth Impact Risk Summary: 1991 BA". NASA/JPL Near-Earth Object Program Office. Retrieved 2012-03-10.
- Scotti, J. V.; Rabinowitz, D. L.; Marsden, B. G. (1991). "Near miss of the Earth by a small asteroid". Nature 354: 287–289. Bibcode:1991Natur.354..287S. doi:10.1038/354287a0.