Discovery

On 2 June 2018, at roughly 08:22 UTC (01:22 local time), 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 (identified by JPL's scout program as 30% odds, calculated by Bill Gray as 82%) 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 indeed impacted Earth, rather than simply a near approach, came when two observations by the Asteroid Terrestrial-impact Last Alert System (ATLAS) arrived later the same day and extended the observation arc from 85 minutes to 3 hours and 47 minutes, significantly improving the orbital parameters.^[1] 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.^[17] The asteroid was several meters in diameter and impacted the atmosphere going about {{convert|17|km/s|mph|abbr=on}}. 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 {{convert|0.069|AU|km mi|abbr=on|lk=off}} 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.^[18]

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 {{mpl|2008 TC|3}}, several kilograms of meteorites were expected to have reached the ground and be recoverable. 2018 LA was estimated to only be about 40% as massive as {{mp|2008 TC|3}}.

On June 23, three weeks after the asteroid entered Earth's atmosphere, the first meteorite fragments were recovered by scientists. The physical characteristics of the meteorites are not yet known.^[4]

See also

• Chelyabinsk meteor
• Impact event
• List of asteroid close approaches to Earth in 2018
• 1930 Curuçá River event
• Sikhote-Alin meteorite
• Tunguska event
• Asteroid impact prediction
• 2008 TC3, the first asteroid ever discovered prior to impact
• 2014 AA, the second asteroid discovered prior to impact
• Vela Incident
• WT1190F, space debris that re-entered in 2015

References