2018 LA, also known as ZLAF9B2, was a small Apollonear-Earth asteroid in mean diameter that impacted the atmosphere with small fragments reaching the Earth at roughly 16:44 UTC on 2 June 2018 near the border of Botswana and South Africa. It was discovered only 8 hours prior by the Mount Lemmon Survey, Arizona and based on hrs of observations, was calculated to have a roughly 85% chance of impact likely somewhere between Australia and Madagascar. Hours later, a report arrived to the American Meteor Society that an observer from Botswana had seen a bright fireball. After the impact, the Asteroid Terrestrial-impact Last Alert System released observations roughly 2 hours after the other reported observations which confirmed that the asteroid had indeed impacted Earth on a grazing path as per the observed fireball. A preliminary analysis of the pre-impact evolution of this meteoroid suggests that it may be part of a dynamical grouping.
Discovery
On 2 June 2018, at roughly 08:22 UTC, the Mount Lemmon Survey picked up an 18th magnitude asteroid moving quickly against the background stars. The object was observed over the course of 15 minutes and submitted to the Minor Planet Center, and was followed up by the same survey for the next hour or so, resulting in an Observation arc of 1 hour and 17 minutes. The asteroid was soon identified as having a chance of impacting Earth and one precovery observation was found just 7 minutes prior to its discovery observation, resulting in a final observation arc of 85 minutes before the asteroid was widely considered lost.
Entry and ATLAS observations
Several hours later, at 16:44 UTC, a report arrived from southern Botswana to the American Meteor Society that an exceptionally bright fireball had been spotted. Although this was further west than projected based on the initial observations, the timing and location were consistent, although substantiated by just one observer. Confirmation that the asteroid had impacted Earth, rather than simply a near approach, came when two observations by the Asteroid Terrestrial-impact Last Alert System arrived later the same day and extended the observation arc from 85 minutes to 3 hours and 47 minutes, significantly improving the orbital parameters. The 4 hour observation arc better constrained the line of variations and showed that the asteroid was virtually certain to impact Earth, and based only on the sky observations, projected to have impacted in Namibia instead. This proves consistent with the fireball report. The bolide was also detected by infrasound at station I47 in South Africa and registered about 0.4 kt. The asteroid was several meters in diameter and impacted the atmosphere going about. Based on its velocity and energy, the asteroid was likely 2.6-3.8 meters in diameter. Asteroids in the range of several meters in diameter are very hard to detect as they are too small to reflect much sunlight. For example, on 24 May 2018, the asteroid was still from Earth and only had an apparent magnitude of 25.5, much dimmer than any major modern surveys can detect using rapid-fire 30 second snapshots meant to cover as much of the sky as possible.
Fragment recovery
Scientists promptly looked for a meteorite strewn field, hoping to recover fragments of the asteroid before they had a chance to weather too much. Fragments can achieve dark flight after deceleration to terminal velocity. Dark flight starts when fragments decelerate to about 2–4 km/s. Larger fragments will fall further down the strewn field. Assuming a similar fraction of 2018 LA survived as of, several kilograms in total were expected to have reached the ground and be recoverable, if estimates of 40% as massive as were correct. On June 23 the first meteorite fragments were recovered by scientists, the physical characteristics of which may have been published.
