A reminder of the small but real impact threat that Earth faces from unknown asteroids came as June opened when a chunk of space rock collided with our planet.
NASA experts estimate the cosmic missile was the size of a large boulder, about two to three metres (six to nine ft) across. The asteroid was discovered only eight hours before it slammed into our atmosphere on Saturday, June 2.
The impacting asteroid, labelled 2018 LA, produced a spectacular fireball that lit up evening skies over southern Africa. It is expected largely to have disintegrated as it blazed over Botswana.
Only two other impacting asteroids have been detected by astronomers in advance. 2018 LA was first spotted early on June 2 by the Catalina Sky Survey, which is operated by the University of Arizona and funded by NASA as part of its Earth defence programme.
The asteroid was extremely faint, and about as far away as the Moon at the time of discovery. Time-exposure photos were taken with the Catalina survey’s 1.5-metre (60-in) telescope on Mt Lemmon in the Santa Catalina Mountains, Arizona. It recorded the asteroid’s rapid motion as streaks on its images.
The data was immediately forwarded to the Minor Planet Center in Cambridge, Massachusetts. Their initial calculations of the asteroid’s orbit around the Sun quickly showed that it was on course to collide.
An automatic alert was sent out to observing stations around the world, though it was clear that the impacting asteroid’s size meant it was harmless. The estimates diameter compares to the 20 meters (65 ft) of the bolide that exploded over Chelyabinsk in February 2013.
Two further observations were made by the NASA-backed ATLAS asteroid survey operated by the University of Hawaii. That indicated that 2018 LA’s impact location would be southern Africa.
Within a few hours, asteroid 2018 LA had reached Earth, entering the atmosphere at a speed of 17 km per second (38,000 mph). It was widely seen over Botswana, where it was early evening.
The impacting asteroid’s blast was also recorded by a network of stations that constantly monitor the skies for nuclear missiles.
Paul Chodas, manager of NASA’s Center for Near-Earth Object Studies (CNEOS) at the Jet Propulsion Laboratory, said: “The discovery of asteroid 2018 LA is only the third time that an asteroid has been discovered to be on an impact trajectory.
“It is also only the second time that the impact location was predicted well ahead of the event itself.”
The first, 2008 TC3, scattered meteorites over Sudan in October, 2008. The second, 2014 AA, occurred over the Atlantic in January, 2014. All three were discovered by the Catalina Sky Survey, which has recorded several near misses too.
NASA estimates that 90 per cent of potentially hazardous asteroids more than one kilometre in diameter have already been discovered. Its Near-Earth Object (NEO) programme is now focused on finding 90 per cent of those greater than 140 metres across.
Though 2018 LA was too small to be of concern, its discovery allowed scientists to test their preparedness for potentially more dangerous impacting asteroids.
NASA Planetary Defense Officer Lindley Johnson said: “This was a much smaller object than we are tasked to detect and warn about. However, this real-world event allows us to exercise our capabilities and gives some confidence our impact prediction models are adequate to respond to the potential impact of a larger object.”
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