# 2018 AH

2018 AH is a sub-kilometer asteroid, classified as near-Earth object of the Apollo group, approximately 100 m (300 ft) in diameter. It was first observed on 4 January 2018, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) 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 (0.001985 AU (297,000 km; 184,500 mi)) since 2002 JE9 in 1971, and until 2001 WN$_5$ 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.
2021 close approach :
| Date | JPL SBDB nominal geocentric distance | uncertainty region (3-sigma) |
| 2021-12-27.7 ± 3.6 days (as known before recovery) | 4.5 million km | ± 3.6 million km |
| 2021-12-27.548 (as known after recovery) | 4.68 million km | ± 83 km |

## Description

2018 AH 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, 2018 AH 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. 2018 AH remained brighter than magnitude 23 until late February 2018, and once more became mostly unobservable until its next Earth approach in December 2021.
2018 AH passed unusually close for such a bright asteroid, at an absolute magnitude of 22.5 (making it approximately 84–190 meters across). The largest asteroid to pass so close to Earth in 2017 was only an absolute magnitude of 24.3 (or about 31–91 meters). Since 1900, the only asteroids larger than 2018 AH 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 (thousand km) | H | Diameter (meters) |
| Tunguska asteroid | 1908-06-30 | Impact | ~23? | 60–190 |
| (152680) 1998 KJ9 | 1914-12-31 | 232.9 | 19.4 | 279–900 |
| 2002 JE9 | 1971-04-11 | 237.0 | 21.2 | 122–393 |
| 2018 AH | 2018-01-02 | 297.0 | 22.5 | 84–190 |
| (153814) 2001 WN5 | 2028-06-26 | 248.7 | 18.3 | 921–943 |
| 99942 Apophis | 2029-04-13 | 37.8 | 19.7 | 310–340 |
| (308635) 2005 YU55 | 2075-11-08 | 228.1 | 21.9 | 320–400 |
| (456938) 2007 YV56 | 2101-01-02 | 238.8 | 21.0 | 133–431 |
| (153201) 2000 WO107 | 2140-12-01 | 243.6 | 19.3 | 427–593 |
| (85640) 1998 OX4 | 2148-01-22 | 296.2 | 21.1 | 127–411 |
H < 23 asteroids passing less than 1 LD from Earth :
| Asteroid | Date | Nominal approach distance (LD) | Min. distance (LD) | Max. distance (LD) | Absolute magnitude (H) | Size (meters) |
| (152680) 1998 KJ9 | 1914-12-31 | 0.606 | 0.604 | 0.608 | 19.4 | 279–900 |
| (458732) 2011 MD5 | 1918-09-17 | 0.911 | 0.909 | 0.913 | 17.9 | 556–1795 |
| (163132) 2002 CU11 | 1925-08-30 | 0.903 | 0.901 | 0.905 | 18.5 | 443–477 |
| 2002 JE9 | 1971-04-11 | 0.616 | 0.587 | 0.651 | 21.2 | 122–393 |
| 2013 UG1 | 1976-10-17 | 0.854 | 0.853 | 0.855 | 22.3 | 73–237 |
| 2012 TY52 | 1981-11-04 | 0.818 | 0.813 | 0.823 | 21.4 | 111–358 |
| 2017 VW13 | 2001-11-08 | 0.454 | 0.318 | 3.436 | 20.7 | 153–494 |
| (308635) 2005 YU55 | 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 |
| (153814) 2001 WN5 | 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 |
| 2015 XJ351 | 2047-06-06 | 0.789 | 0.251 | 38.135 | 22.4 | 70–226 |
| 2005 WY55 | 2065-05-28 | 0.865 | 0.856 | 0.874 | 20.7 | 153–494 |
| (308635) 2005 YU$_{55}$ | 2075-11-08 | 0.592 | 0.499 | 0.752 | 21.9 | 320–400 |
| (456938) 2007 YV56 | 2101-01-02 | 0.621 | 0.615 | 0.628 | 21.0 | 133–431 |
| 2007 UW1 | 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 |
| (153201) 2000 WO107 | 2140-12-01 | 0.634 | 0.631 | 0.637 | 19.3 | 427–593 |
| 2009 DO111 | 2146-03-23 | 0.896 | 0.744 | 1.288 | 22.8 | 58–188 |
| (85640) 1998 OX4 | 2148-01-22 | 0.771 | 0.770 | 0.771 | 21.1 | 127–411 |
| 2007 UY1 | 2156-02-13 | 0.685 | 0.652 | 6.856 | 22.9 | 56–179 |
| 2011 LT17 | 2156-12-16 | 0.998 | 0.955 | 1.215 | 21.6 | 101–327 |