Asteroid’s 22m-year journey from source to Earth mapped in historic first
Astronomers have reconstructed the 22m-year-long voyage of an asteroid that hurtled through the solar system and exploded over Botswana, showering meteorites across the Kalahari desert.
It is the first time scientists have traced showering space rock to its source – in this case Vesta, one of largest bodies in the asteroid belt that circles the sun between Jupiter and Mars.
The six-tonne asteroid punched into Earth’s atmosphere at 37,000mph in June 2018 and broke apart above the central Kalahari game reserve, creating a fireball nearly as bright as the sun. Immediate searches of the presumed landing site found a small meteorite, which was named Motopi Pan.
Nasa researchers had tracked the hazardous object from as far out as the moon using telescopes in Arizona and Hawaii. After the impact they asked astronomers in Australia to check images from the Sky Mapper telescope in New South Wales – used for studying black holes and the like – in case it too had captured the asteroid’s flight path. Much to the researchers’ amazement, it had.
“We wouldn’t have noticed it had it not been for the tip-off by the American discoverers, and it hits close to home,” said Christian Wolf, an astronomer at Australian National University. “Granted, it’s the Kalahari and I have never been there, but when your day job involves orienteering along a chain of black holes with an average step size of 1bn light years between the waypoints, the Kalahari feels awfully close to your keyboard.”
The snapshots of the asteroid, named 2018LA, from three telescopes set far apart on the Earth’s surface allowed the astronomers to reconstruct the rock’s trajectory and pinpoint its origin. The trail led to Vesta, a 300-milewide asteroid that is occasionally visible without a telescope.
Armed with a good idea of the asteroid’s trajectory, the scientists were able to refine the location of its landing in the game reserve. This led to expeditions that recovered more than 20 other meteorites spread over a 3-mile patch of ground.
Mineralogical analyses of the fragments suggest that the lump of rock that became 2018LA was originally buried deep beneath the surface of Vesta but was flung into space during an impact that left a crater on the asteroid about 22m years ago.
The ejected space rock wandered the solar system, its surface being battered by cosmic rays, until it fell into the gravitational arms of the Earth and plummeted to the ground.
Laboratory tests showed that the oldest grains inside the discovered meteorites dated to 4.56bn years ago, a time when the solar system was still forming from a super-hot disc of interstellar gas and dust. Details are published in Meteoritics and Planetary Science.
“This is a really exciting study,” said Ashley King, a planetary scientist at the Natural History Museum who was not involved in the work. “It’s only the second time ever that a rock has been detected in space before entering the Earth’s atmosphere and eventually ending up as meteorites on the ground.”
He said the recovered rocks were a mixture of igneous meteorite types known as HEDs, an acronym for howardites, eucrites and diogenites, which have long been thought to be fragments of the asteroid Vesta. The asteroid was visited by Nasa’s Dawn mission some years back.
“Because the team tracked the rock from space all the way down to the impact site, they were able to calculate a very precise orbit that is consistent with an origin near Vesta, providing one of the best links we have between an asteroid and meteorites,” King said. “Knowing where meteorites come from gives us context for how they formed and is really important for understanding the history of our solar system.”