On June 2, 2018, asteroid 2018LA exploded over Botswana, showering a large area with meteorites. Before this, several telescopes had spotted 2018LA – only the second time we have seen an asteroid in space before it hit the Earth. The data these observations gave us on its path combined with 2018LA's' distinctive geology have allowed us to trace back the path the asteroid took 22 million years, to when a collision blasted it off the asteroid Vesta.
The worldwide network looking for movement in the skies is intended to warn us of objects large enough to pose a threat. Sometimes, however, they pick up much smaller objects. At 5,700 kilograms (12,566 pounds) you wouldn't want 2018LA to land on you, but the global risk was minuscule.
Being just 156 centimeters (5.1 feet) across, we only detected it 2018LA a day before it hit – too late to process the data on its orbit and provide a warning.
However, after a satellite detected the flash as 2018LA hit the Earth's atmosphere and people across Botswana and South Africa reported a light in the sky 20,000 times as bright as the full Moon, astronomers went digging for more. Dr Christopher Onken of the Australian National University examined the records of Australia's widefield Skymapper telescope. “I could hardly believe my eyes when I came upon a little object that appeared to be moving across images taken by SkyMapper,” Onken said in an emailed statement.
The Skymapper images from just before 2018LA hit the atmosphere, along with the records of Botswanan security cameras, established its path with great accuracy. Astronomers not only knew where to look for the fragments – of which 23 have so far been recovered around the Kalahari Desert, Botswana – but made possible the tracing back to discover 2018LA's origins.
In Meteoritics and Planetary Science 70 scientists from 42 institutions have come together to reconstruct 2018LA's orbit and analyze the pieces they found.
Putting these together, they conclude 2018LA spent 22 million years in space after being knocked off Vesta – which has been visited by NASA's Dawn probe in 2011-12. A much less likely story is 2018LA was knocked off a Vestoid, a group of asteroids that were themselves products of early Vesta collisions.
Simulating orbits over millions of years is often impossible. The gravitational influences of objects too small to seem important or the effects of the solar wind on a non-spherical body cause initially tiny perturbations that become important over time. However, Onken told IFLScience the orbital path observed for 2018LA traced it back to nu(6), a region in an orbital resonance with Saturn. Vesta's own orbit is so close to nu(6) that Vestoids frequently get scattered in and stay there for millions of years.
Onken told IFLScience the authors' confidence 2018LA is from Vesta isn't based on orbit alone. Its composition identifies it as from an asteroid large enough to have undergone internal differentiation. There are only a few of those in the inner Solar System, and Vesta's geology is distinctive among them.
Curiously, in 2015 another set of meteorites were found in Sariçiçek, Turkey that came from a collision with Vesta that also occurred 22 million years ago, but it was not the same collision. The Motopi Pan and Sariçiçek meteorites are so different chemically they don't appear to have come from the same place. Dawn observations allowed the estimation of the ages of Vesta's craters, based on the number of smaller craters within them. There are indeed two around 22 million years old – so we now have one set of meteorites from each.