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1991 BA
Discovery
Discovered by	Spacewatch
Discovery site	Kitt Peak Obs.
Discovery date	18 January 1991
Designations
MPC designation	1991 BA
Minor planet category	Apollo · NEO
Orbital characteristics
	Epoch 18 January 1991 (JD 2448274.5)
	Uncertainty parameter 9
Observation arc	4.6 hours
Aphelion	3.662±0.430 AU
Perihelion	0.7153±0.0122 AU
Semi-major axis	2.189±0.257 AU
Eccentricity	0.6732±0.0440
Orbital period (sidereal)	3.24±0.57 yr (1,183±208 days)
Mean anomaly	346.836°±2.609°
Mean motion	0° 18m 15.655s / day
Inclination	1.938°±0.104°
Longitude of ascending node	118.880°±0.012°
Argument of perihelion	70.688°±0.260°
Earth MOID	0.0003 AU · 0.1 LD
Physical characteristics
Dimensions	5–10 m
Absolute magnitude (H)	28.6

1991 BA is a sub-kilometer asteroid, classified as near-Earth object of the Apollo group that was first observed by Spacewatch on 18 January 1991, and passed within 160,000 km (100,000 mi) of Earth.^[2]^[3] This is a little less than half the distance to the Moon. With a 5-hour observation arc the asteroid has a poorly constrained orbit and is considered lost. It could be a member of the Beta Taurids.^[6]

Description

1991 BA is approximately 5 to 10 meters (15 to 30 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.

Possible impact

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.

Virtual impactor^[4]
Date	Impact probability (1 in)	JPL Horizons nominal geocentric distance (AU)	NEODyS nominal geocentric distance (AU)	MPC^[8] nominal geocentric distance (AU)	Find_Orb nominal geocentric distance (AU)	uncertainty region (3-sigma)
2023-01-18	290000	3.9 AU (580 million km)^[9]	3.9 AU (580 million km)^[10]	2.4 AU (360 million km)	2.8 AU (420 million km)^[11]	± 841 million km^[9]

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

References

^ ^a ^b ^c "JPL Small-Body Database Browser: (1991 BA)" (1991-01-18 last obs.). Jet Propulsion Laboratory. Retrieved 2 August 2017.
^ ^a ^b "JPL Close-Approach Data: (1991 BA)" (last observation: 1991-01-18; arc: 1 day; uncertainty: 9). Retrieved 20 March 2012.
^ ^a ^b "1991 BA". Minor Planet Center. Retrieved 2 August 2017.
^ ^a ^b ^c ^d ^e ^f ^g "Earth Impact Risk Summary: 1991 BA". NASA/JPL Near-Earth Object Program Office. Archived from the original on 17 August 2011. Retrieved 10 March 2012.
^ ^a ^b 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.
^ Peter Jenniskens Meteor Showers and their Parent Comets pg 463 fig 25.7
^ "Asteroid Fast Facts". NASA. Archived from the original on 26 May 2023.
^ "MPC Ephemeris Service". IAU Minor Planet Center. Retrieved 9 November 2022.
^ ^a ^b "Horizons Batch for 2023-01-18 Virtual Impactor". JPL Horizons. Archived from the original on 9 November 2022. Retrieved 9 November 2021. RNG_3sigma = uncertainty range in km. (JPL#12/Soln.date: 2021-Apr-15 generates RNG_3sigma = 841846388 km for 2023-Jan-18.)
^ "2009JF1 Ephemerides for 18 January 2023". NEODyS (Near Earth Objects – Dynamic Site). Archived from the original on 9 November 2022. Retrieved 9 November 2022.
^ "Find_Orb for 2023-01-18". Project Pluto. Archived from the original on 9 November 2022. Retrieved 9 November 2021.