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30 Apr 2021

Great Balls of Fire!

Asteroid - meteor - meteorite. Some space rocks get to be all three, but very rarely do we get to study each of those stages for a single object. The transition of the asteroid, 2018 LA, into the meteorite, "Motopi Pan", is just the second time that an asteroid has been detected in space prior to impact and then its pieces have been collected from the ground. The first was in 2008, when meteorites from the asteroid 2008 TC3 were collected from the sands of Sudan. In 2018, SkyMapper got to play a small, but important, role in the story of 2018 LA / Motopi Pan, a story which stretches back to the beginning of the Solar System, 4.5 billion years ago.

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, as it flung around the globe. Cameras in South Africa and Botswana recorded a fireball in the sky, so bright that it cast distinct shadows on the ground. The asteroid had a ~1.5 metre diameter, ~5,700 kg mass, and a speed of 60,000 kph before it broke up 28 km 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 a few days later picked up a single meteorite weighing 18 g. 

Scientists from the meteorite research team, led by Peter Jenniskens (NASA Ames and SETI institute), contacted the SkyMapper group to find out whether any additional images of 2018 LA may exist. Consulting the image archives for the night in question, we found a series of 4 images (shown above) in which the faint streak of 2018 LA appeared at the very bottom of the image. These proved to be the last known images of 2018 LA prior to impact with the atmosphere. With the refinement of the trajectory afforded by the SkyMapper measurements, the research team 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 a 5-km-long arc. These fragments were given the collective name "Motopi Pan", after a local watering hole, and have provided material for precise laboratory analysis.

2) It pinpointed the original orbit of the small asteroid, a dynamical region associated with the much larger asteroid Vesta, which had been visited by the NASA Spacecraft Dawn ten years ago.

The lab analysis revealed more Vesta-related geological properties, as well as three characteristic age points for Motopi Pan, which paint the following picture:

1) 2018 LA had been exposed to the Solar Wind and cosmic rays for 22 million years. Hence, 2018 LA was buried under the surface of Vesta until 22 million years ago, when another body smashed into Vesta. Most likely, the event that threw 2018 LA into space was the same impact that created the 10-km-wide Rubria crater, which was mapped in detail by the Dawn spacecraft.

2) The rock in 2018 LA had experienced a melting event 4230 million years ago, which is usually a consequence of a massively energetic impact. 2018 LA 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. 

This research was published on 23 April 2021 in Meteoritics and Planetary Science (https://onlinelibrary.wiley.com/doi/10.1111/maps.13653)