The rock fell from the sky, smashing through the roof of the house, bouncing off a couch, and destroying a bed and TV. When the residents of the house in San Carlos, Uruguay came home that day, on September 18, 2015, they were in for a shock.
They were even more surprised to find the hole in their roof had not been caused by a rock thrown by a person, but rather a meteorite that had crashed to Earth from space. This would become arguably the first confirmed meteorite in Uruguay (a previous discovery is thought to have been smuggled from Argentina), which was of huge interest to local researchers, who recently published a paper about the finding.
“At first they thought someone had thrown a stone,” says Gonzalo Tancredi, co-author on the paper, from the University of the Republic in Montevideo. “But they found a strange stone covered with black material, and they thought it could be a meteorite.”
The meteorite itself was fairly typical, belonging to the LL6 Chondrite family, a group of stony meteorites that are the most common on Earth. But of more interest was that it was an example of a rare “damaging fall”, a meteorite that damages property – or even people – on Earth.
No human has ever been killed by a meteorite, and only one person has ever reportedly been directly hit. That was Ann Hodges, a local resident living in Alabama in 1954, who was struck by a space rock on her thigh after it crashed through the roof of her house.
But weighing in at 712 grams and travelling at 87 metres per second, the San Carlos Meteorite – as it has been called – would almost certainly have proved fatal had it hit someone. And it serves as a rather astonishing reminder of the danger posed to our planet by falling space rocks.
“The special fact that the [San Carlos] meteorite bounces on the couch makes the story simply unreal,” says Jorge Zuluaga from the University of Antioquia in Colombia. “Although meteorites ‘rain’ almost continuously on Earth, most of them fall in remote and vast areas such as oceans, deserts and jungles.”
When a space rock enters our atmosphere, it is called a meteor. Those that hit the ground are called meteorites, but they’re not the only ones that pose a danger to us. Meteors that explode near the ground can cause huge amounts of damage, as witnessed several times in recent history.
Perhaps the most notable event was the Chelyabinsk meteor, a rock that exploded above the Russian city of Chelyabinsk on February 15, 2014. Measuring 20 metres in size and moving at 19 kilometres per second, the rock exploded about 30 kilometres above the city with up to 30 times the energy of the Hiroshima atomic bomb. The resultant shockwave injured 1,500 people and damaged thousands of buildings.
On December 18, 2018 a huge meteor exploded over the Bering Sea, releasing as much energy as ten Hiroshima atomic bombs. The event had gone unnoticed until March 2019 as it had been so remote. And on February 1, an exploding meteor in Cuba caused a sonic boom, witnessed by thousands of people.
At the upper end of the scale is the Tunguska event on June 30, 1908, when a sparsely populated area of Russia 2,000 square kilometres in size was flattened by an air burst from a meteor with the energy of 1,000 Hiroshima atomic bombs. And, of course, at the extreme end is the asteroid that wiped out the dinosaurs 66 million years ago.
While we’re getting better at tracking these larger events – more than 98 per cent of near-Earth asteroids over one kilometre in size are known – it’s the smaller events like the Chelyabinsk meteor or the San Carlos Meteorite that are of more concern. Only an estimated one percent of asteroids smaller than 100 metres in size have been discovered.
“Meteorites fall on the ground regularly,” says Detlef Koschny, co-manager of the Near-Earth Object Segment of the European Space Agency’s Space Situational Awareness Programme. “We are mainly worried about events like the Chelyabinsk event where a 20-metre object entered the Earth's atmosphere over a big city.”
By some estimates, about 49 metric tonnes of asteroid material rains down on Earth every day. Based on existing data, Tancredi says that seven falls are witnessed by humans every year, but a far higher number of meteorites reach the surface – an average of 5,600 over the entire Earth per year, and 1,600 over land.
The number of reported damaging falls has steadily been increasing over the years, with 14 reported between 1994 and 2005, and 22 between 2006 and 2017. This is almost certainly due to an increase in global communications though, rather than an influx of space rocks.
Nonetheless, this data – coupled with recent events – highlights the need for us to keep a close eye on incoming space rocks. New telescopes like the Large Synoptic Survey Telescope, due to begin operations in 2020, will provide us with much better data on potentially hazardous debris. But the ever-present danger is plain to see.
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