Research Byte
Published in the RSAA Lunations
Vol1 Issue16 1–31 May 2021
The impact and recovery of asteroid 2018LA
On June 2, 2018, an asteroid was discovered by the Catalina Sky Survey station in Arizona, and passed through images taken by the NASA/ATLAS telescope in Hawaii 3.8 hours later, and by ANU’s SkyMapper at Siding Spring Observatory 5.5 hours later, as it flung around the globe. Cameras in South Africa and Botswana recorded a fireball in the sky and shadows on the ground. Now called 2018LA, the asteroid had a ~1.5 metre diameter, ~5,700 kg mass, and a speed of 60,000 kph before it broke up 28 kms above ground and impacted in the Central Kalahari Game Reserve in Botswana. The flight path to impact was initially reconstructed from images and videos, and searches for fragments in the presumed landing area picked up a meteorite with 18 g weight.
This is only the second time that an asteroid was observed in space prior to impacting on Earth. Why is that important? In fact, we know about 1 million asteroids today, including their sizes and orbits, but for the vast majority of them we have no idea of what they are made of. For meteorites, we have the opposite problem: we know quite well what most of them are made of, but we have little idea about where they came from, what orbits they were on before they crashed onto Earth. A future task in studying the formation of the Solar System is then to match meteorites types to asteroid types.
SkyMapper has never attempted to look for asteroids, but following the request from the Solar System researchers, we were serendipitously able to help refining the flight path as well as measuring the size, rotation period, colour and thus mineralogical type of the asteroid. As the flight path of 2018LA is defined by three telescope sightings from Arizona to Australia, the team led by Peter Jenniskens (NASA Ames and SETI institute) was able to do two things:
1) It pinpointed the landing site in the Kalahari desert to within 5 km, and new search expeditions found 22 more rocks spread over an area of 5 km. These fragments are now collectively called Motopi Pan (pieces #1 to #23) and are plenty of material for lab analysis.
2) It pinpointed the origin of the small asteroid, leading back to the much larger asteroid Vesta, which had been visited by the NASA Spacecraft Dawn ten years ago.
The lab analysis revealed three characteristic age points for Motopi Pan:
1) 2018LA had been exposed to the Solar Wind and cosmic rays for 22 million years. Hence, 2018LA was initially a rock buried under the surface of Vesta until 22 million years ago another body smashed into Vesta, creating a crater 10 km in size.
2) The rock in 2018LA had experienced a melting event 4230 million years ago, which is usually a consequence of a massively energetic impact. 2018LA is believed to originate from a small crater inside the large Veneneia basin on Vesta, and this is likely the age of the Veneneia basin, that would have been created right when the “Early Bombardment” started in the Solar System, a period where many bodies changed face rapidly because everything was hitting everything.
3) Finally, 4563 +/- 6 million years is the measured age of the oldest Zircon grains in the fragments from Motopi Pan, and this is precisely the same as the oldest ages that have ever measured for anything in any lab. This point in time is usually considered the formation time of the Solar System, the time when the first solids formed in the cooling proto-solar nebula.
Christopher Onken, Christian Wolf (ANU), and Hadrien Devillepoix (Curtin University)
This research was published on 23 April 2021 in Meteoritics and Planetary Science (https://onlinelibrary.wiley.com/doi/10.1111/maps.13653)
Further links:
https://edition.cnn.com/2021/04/28/world/meteorites-asteroid-vesta-earth-scn/index.html
https://www.theguardian.com/science/2021/apr/23/astronomers-map-asteroids-22m-year-journey-to-earth
ABC Morning News with Joe O’Brien, Friday 23 April, minutes 22 to 31
