2018 AH

2018 AH is a sub-kilometer asteroid, classified as near-Earth object of the Apollo group, approximately in diameter. It was first observed on 4 January 2018, by the Asteroid Terrestrial-impact Last Alert System on Mauna Loa and quickly followed-up by many other surveys, with precovery observations found from Pan-STARRS and PTF from the day previous.
It is the largest known asteroid to pass so close to Earth since in 1971, and until in 2028, although it was only discovered two days after its closest approach on 2 January 2018, at 04:25 UTC. The Tunguska asteroid was likely of a similar size, if not slightly smaller.
Before being recovered on 4 January 2022 11:49 UTC at an Earth distance of 9.8 million km, the asteroid only had an observation arc of 46 days and had not been observed since February 2018. Being a short arc object that had not been observed for years generated an uncertainty that is relatively large. Between 24–31 December 2021 it was only known to make an Earth approach of between 1-8 million km. As it came to perihelion on 3 December 2021, it was approaching from the direction of the Sun.

Date	JPL SBDB nominal geocentric distance	uncertainty region
2021-12-27.7 ± 3.6 days	4.5 million km	± 3.6 million km
2021-12-27.548	4.68 million km	± 83 km

Description

has a fairly eccentric orbit, and its distance to the Sun therefore varies from as close as 90% of the Sun-Earth distance to over 4 times that distance. Due to this, among other factors, the asteroid remained undiscovered until its 2018 approach. It is almost always dimmer than magnitude 23, dimmer than most asteroid surveys can detect. During August–October 2013 it approached within ~0.3 AU of Earth and became as bright as magnitude 22.4, still rather dimmer than most survey-discovered asteroids, and it was not noticed.

2018 Approach

On its approach to Earth in 2018, had recently passed perihelion and was moving outwards on its orbit. It therefore approached from roughly the direction of the Sun, where it was undetectable to ground-based optical observations. It reached its closest point to Earth at only 45 degrees from the Sun. It was discovered at a more observable elongation of 129 degrees and at a magnitude of 15.7, and was quickly followed up over the next several days due to its brightness. remained brighter than magnitude 23 until late February 2018, and once more became mostly unobservable until its next Earth approach in December 2021.
passed unusually close for such a bright asteroid, at an absolute magnitude of 22.5. The largest asteroid to pass so close to Earth in 2017 was only an absolute magnitude of 24.3. Since 1900, the only asteroids larger than known to pass closer than it to Earth are listed below:
Asteroid diameters marked in italics have had their size directly measured.

Designation	Date	Distance	H	Diameter
Tunguska asteroid	1908-06-30			60–190
	1914-12-31	232.9	19.4	279–900
	1971-04-11	237.0	21.2	122–393
2018 AH	2018-01-02	297.0	22.5	84–190
	2028-06-26	248.7	18.3	921–943
99942 Apophis	2029-04-13		19.7	310–340
	2075-11-08	228.1	21.9	320–400
	2101-01-02	238.8	21.0	133–431
	2140-12-01	243.6	19.3	427–593
	2148-01-22	296.2	21.1	127–411

Asteroid	Date	Nominal approach distance	Min. distance	Max. distance	Absolute magnitude	Size
	1914-12-31	0.606	0.604	0.608	19.4	279–900
	1918-09-17	0.911	0.909	0.913	17.9	556–1795
	1925-08-30	0.903	0.901	0.905	18.5	443–477
	1971-04-11	0.616	0.587	0.651	21.2	122–393
	1976-10-17	0.854	0.853	0.855	22.3	73–237
	1981-11-04	0.818	0.813	0.823	21.4	111–358
	2001-11-08	0.454	0.318	3.436	20.7	153–494
	2011-11-08	0.845	0.845	0.845	21.9	320–400
2018 AH	2018-01-02	0.773	0.772	0.773	22.5	67–216
	2028-06-26	0.647	0.647	0.647	18.2	921–943
99942 Apophis	2029-04-13	0.0981	0.0963	0.1000	19.7	310–340
	2047-06-06	0.789	0.251	38.135	22.4	70–226
	2065-05-28	0.865	0.856	0.874	20.7	153–494
	2075-11-08	0.592	0.499	0.752	21.9	320–400
	2101-01-02	0.621	0.615	0.628	21.0	133–431
	2129-10-19	0.239	0.155	0.381	22.7	61–197
101955 Bennu	2135-09-25	0.780	0.308	1.406	20.19	472–512
	2140-12-01	0.634	0.631	0.637	19.3	427–593
	2146-03-23	0.896	0.744	1.288	22.8	58–188
	2148-01-22	0.771	0.770	0.771	21.1	127–411
	2156-02-13	0.685	0.652	6.856	22.9	56–179
	2156-12-16	0.998	0.955	1.215	21.6	101–327