A small asteroid was discovered on an impact trajectory with Earth on June 2, 2018. Eight hours later, a video camera in South Africa recorded a bright fireball over Botswana. An international team of researchers, including Western's Peter Brown and the Western Meteor Group, have now published their findings in the journal Meteoritics and Planetary Science after they tracked down the surviving meteorites deep inside the Central Kalahari Game Reserve in central Botswana.
"Combining the observations of the small asteroid in space with information gleaned from the meteorites shows it likely came from Vesta, target of NASA's DAWN mission," said lead author and meteor astronomer Peter Jenniskens of the SETI Institute and NASA Ames Research Center.
The asteroid, called 2018 LA, was first spotted by the University of Arizona's Catalina Sky Survey as a faint point of light moving among the stars. The Catalina Sky Survey searches for Earth-crossing asteroids as part of NASA's Planetary Defense program.
"Small meter-sized asteroids are no danger to us, but they hone our skills in detecting approaching asteroids," said Eric Christensen, director of the Catalina Sky Survey program.
The Western Meteor Group contributed to the international study by analyzing sound waves from the fireball as it penetrated the atmosphere. From these sound waves, Brown and his collaborators in Western's department of physics and astronomy estimated the energy deposited by the impacting body equaling one-thirtieth (1/30) the energy of the atomic bomb that was dropped on Hiroshima during World War II.
"The infrasound shockwave measured in South Africa was not as strong as expected from U.S. Government sensor detections of the bright light," said Brown, Canada Research Chair in Planetary Small Bodies.
Each element of Western's Elginfield Infrasound Array (ELFO) is housed in a vault designed to minimize temperature variations and possible water intake. The equipment housed in the vault includes the microbarograph (microphone), a UPS/GPS and digitizer. This is the Center Element, or CELE.
Combining this new acoustic information from Brown and the Western Meteor Group with other measurements made by the international team, the consortium was able to estimate the original size of the small asteroid as 1.5m with a mass of approximately six tonnes.
"This study was significant as it represents only the second time an asteroid has been detected prior to impacting Earth where meteorites have subsequently been recovered," said Brown, a Western astronomy professor. "This is important as we can then link the detailed lab work on the rocks to the spatial context of where this object originated in the solar system. Even more exciting in this case was the fact that the meteorite came from the second largest asteroid, 4 Vesta."
The research team found additional observations in archival data from the SkyMapper Survey program in Australia that showed the asteroid spinning in space, alternatingly presenting a broad and a narrow side to us while reflecting the sunlight.
"From that data, we inferred that the asteroid was about twice as wide as it was thick and was spinning once every four minutes," said Darrel Robertson of NASA's Asteroid Threat Assessment Project at NASA Ames Research Center.
Cosmic rays that created radioactive isotopes bombarded the asteroid while it was in space. By analyzing those isotopes, the researchers determined that 2018 LA was a solid rock about 1.5 m in size, which reflected about 25 per cent of sunlight.
"This is only the second time we have spotted an asteroid in space before it hit Earth over land," said Jenniskens. "The first was asteroid 2008 TC3 in Sudan ten years earlier." Jenniskens also guided the search for fragments of 2008 TC3.
This time, fewer observations led to more uncertainty in the asteroid's position in its orbit. Davide Farnocchia of NASA JPL's Center for Near-Earth Object Studies combined astronomical observations of the asteroid with U.S. Government Satellite data of the fireball to calculate the fall area. Esko Lyytinen of the Ursa Finnish Fireball Network made a parallel effort.
After confirming the fall area, Moses and Jenniskens joined geologist Alexander Proyer of the Botswana International University of Science and Technology (BIUST) in Palapye and geoscientist Mohutsiwa Gabadirwe of the Botswana Geoscience Institute (BGI) in Lobatse and their colleagues to search for the meteorites. The search area was in the Central Kalahari Game Reserve, home to diverse wildlife, including leopards and lions. Researchers were kept safe by the staff of the Botswana Department of Wildlife and National Parks. The team coordinated the search with the Department of National Museum and Monuments in Botswana.
Fragment of asteroid 2018 LA recovered in Central Kalahari Game Reserve in central Botswana. (Submitted)
