Day: March 25, 2019

A meteor is the fiery phenomenon resulting from an asteroid or other celestial body entering the Earth’s atmosphere. Often called a shooting star, if it does not fully vaporise and a part of it hits the Earth’s surface, it is called a meteorite.

On December 18th, 2018, a meteor roughly the size of a school bus blew apart under the pressures of entry into the Earth’s atmosphere 26 km (16 miles) above the Bering Sea. The explosion released some 173 kilotons of energy – about ten times more that released by the bomb dropped on Hiroshima in 1945. It is the second largest meteor explosion recorded since NASA started officially tracking them 30 years ago, after the 2013 Chelyabinsk explosion in Russia.

And no-one actually saw the meteor until after it had blown itself apart. In fact, no-one was aware of what had happened until three months later.

It was on March 8th, 2019 that the meteor’s arrival was noted by human eyes. Peter Brown, a meteor scientist at the Physics and Astronomy department of the University of Western Ontario, was reviewing data from the system used by the Comprehensive Test Ban Treaty Organization to detect atmospheric explosions caused by nuclear tests. This system is comprised of seismic and acoustic sensors capable of picking up infrasound, inaudible to the human ear, at a distance of tens of thousands of miles.

Brown noticed that many of the system’s sensors detected the sound waves from an explosion originating over the Bering Sea, and he calculated that had anyone been below it, the sound would have been deafening. He reported his findings to the United States Air Force, and a review of logs from their spy satellites revealed the passage of the meteor had been noted. A further check with NASA revealed their database of atmospheric impacts has logged the event, which was then officially announced.

This prompted a race to verify, and Simon Proud, a meteorologist and specialist in satellite data at Oxford University in the UK, decided to check the archive of images collected by a Japanese weather satellite that sends data to his department. He found that the satellite, Himawari, had indeed visually recorded the event. And it was not alone.

NASA’s Earth-observing Terra satellite also spotted the meteor with two different instruments — the Multi-angle Imaging SpectroRadiometer (MISR) and the Moderate Resolution Imaging SpectroRadiometer (MODIS). MISR team members combined some of their imagery into an animated GIF, which NASA released Friday, March 22nd.

It is estimated that the meteor was some 10 metres (33 ft) across, and has a mass of around 1,360 metric tons. It probably entered the denser atmosphere at a speed of 115,200 km/h (71,600 mph). By comparison, the 2013 Chelyabinsk asteroid was about 20 m (65 ft) across, massed about 10,000 tonnes, and generated 440 kilotons of energy when it exploded. Even so, that event is dwarfed by the 1908 Tunguska event, which generated a force of 10-15 megatons (roughly 85 times greater than the December 18th, 2018 explosion), flattening 2,000 square km (800 sq mi) of forest through its air blast.

So why wasn’t the December meteor seen? Well, firstly, because it entered the Earth’s atmosphere above a very remote place in the world; simply put, there weren’t that many people under its path to see it. But more to the point, there is an awful lot of rocky debris in space; as I noted in my previous Space Sunday report, Earth shares its orbit around the Sun with a great cloud of dust and rock, and more is constantly falling in towards the Sun from further out in the solar system. As such, meteors are actually a common event – not that it makes them any the less dangerous.

Many of these lumps of rock and ice – as with the December 18th, 2018 rock – are simply too small to be easily located and tracked. Others, like the Chelyabinsk meteor, are occupying orbits that effectively mean they are hidden by the glare of the Sun, and remain unseen until the enter the atmosphere. Nevertheless, over the last 30 years, NASA’s Centre for Near Earth Object Studies CENOS has located and tracks some 20,000 near-Earth objects (NEOs) some of which may at some point come close enough to the Earth to enter the atmosphere, around 50% of them are between 140m and 1 km in size – large enough to pose a serious threat.

While none are as big as the one that struck Chicxulub, Mexico, 65 million years ago and brought about the extinction of the dinosaurs, those at the upper end of the scale could still result in serious loss of life were one to explode over a populated area. So tracking NEOs helps to reduce that risk by producing us with the advance warning needed to evacuate areas – or even to develop a plan to deflect the incoming object – something missions to asteroids like Ryugu and Bennu may also help us to achieve by teaching us more about the nature of asteroids.

Keeping with meteor impacts, roughly 12,800 years ago Earth went through a brief cold snap unrelated to any ice age. Geologists have, for decades, argued for and against the idea it was caused by a meteor airburst or impact, referred to as the Younger Dryas Impact Theory, which also caused the final demise of the Clovis culture in North America.

Now an international team of scientists believe they have found geological evidence in South America that could settle the debate. Led by Chilean palaeontologist Mario Pino, the team has discovered a large, young impact crater in the Osorno province in southern Chile, close to the tip of the continent. Analysis of the impact site suggest it was created around 13,000-12,800 years ago – a time coincident to the Younger Dryas Boundary (YDB), which marks the time of the Younger Dryas Impact Theory, when there were numerous impact events across the northern hemisphere.

However, it is the size of the crater that suggests it may have played a significant role in the climate change that occurred in this period, causing widespread destruction, characterised by enormous biomass burning – around 10% of the Earth’s land surface, megafaunal extinctions and global cooling. Minerals found in the region are consistent with rapid temperature changes, further indicating the impact and the fires that followed it did indeed have a catastrophic impact on the global climate at the time.

Continue reading “Space Sunday: meteors and motors” →