Rocks from the sky

Why do people talk about a ‘meteoric rise’, when what meteors do is fall? Very odd.

Recently I had the great pleasure of visiting Meteor Crater, Arizona, an enormous hole in the ground that’s not far from another famous hole in the ground, the Grand Canyon. If you ever visit the latter, I highly recommend taking a slight detour to see the former.

Meteor Crater – a very matter-of-fact name – was formed about 50,000 years ago by a meteorite 50 metres across and weighing around 300,000 tons, releasing 10-20 megatons of TNT. That’s about the same force as a hydrogen bomb.

Yours truly standing next to Meteor Crater, Arizona, a hole in the ground 1.2 km across and 170 m deep (click to embiggen)
Yours truly at Meteor Crater, Arizona. This hole in the ground is 1.2 km across, 170 m deep and was formed by an iron-nickel meteorite weighing around 300,000 tonnes that hit the ground 50,000 years ago, with an explosion equivalent to releasing 10-20 megatons of TNT, or one (1) hydrogen bomb.

The crater was known to Native Americans, but the earliest historical record of it is from 1871. Early studies concluded that it was a volcano, but in 1902, meteorite-proponent Daniel Moreau Barringer acquired it, with an intent to mine it for iron.

The idea that rocks or anything else could fall from the sky was initially very controversial. People laughed at German physicist Ernst Florens Chladni, who in 1794 was the first person to suggest the idea. But he was vindicated in 1795, when a 25 kilogram stone fell in broad daylight in Wold Cottage England. Some were still sceptical though, notably the French Academy of Sciences; until 1803, when about 3,000 meteorites fell on France. That pretty much shut them up.

Now it’s important to get some terminology right here: a meteorite is the rock or mineral that actually hits the ground. Meteor is what it’s called when it’s flying through the sky, aka a shooting star. And when it’s merely a rock floating in space, it’s a meteoroid, unless it’s really big, in which case it’s an asteroid.

Barringer was far from the first to try and exploit meteorites for their iron. Before people learned how to smelter ores they were the main source of iron for tools and the like.

Overall though, fewer than 10% of meteorites are iron-nickel like the one that hit Arizona, or that formed the Wolfe Creek Crater in Western Australia (0.87 kilometres across and up to 300,000 years old, one of 27 Australian meteorite craters listed in the Earth Impact Database, mostly in Western Australia and the Northern Territory). They’re believed to come from cores of asteroids that have broken up, which explains their rarity.

Most other meteorites are a stony material called chondrite, made of small round particles called chondrules. These are thought to be rock formed at the birth of the Solar System.

Some (about 4.6%) contain carbon and are known as carbonaceous chondrites. These sometimes include organic compound and are eagerly sought, like the one that hit California on 22 April 2012.

With so many rocks bombarding the planet, and giant impact craters like that in Arizona, they’ve got to be pretty dangerous, right?

Well, yes and no. So far, there are no recorded human meteorite fatalities. A dog was allegedly killed in Egypt in 1911, but that hasn’t been proven. There is at least one verified account of a person being struck without fatality: in 1954 Ann Hodges of Sylacauga, Alabama was hit on the hip by a meteorite that crashed into her living room and bounced off her radio. There’s also a report from 1992 of a Ugandan boy who was hit on the back of the head by a meteorite, but it only weighed 3 grams and had been slowed down by some banana leaves.

But this is not to say they’re harmless. Chondrites can break up and explode in the Earth’s atmosphere with incredible force. The most famous example would be the Tunguska event in 1908, an explosion of about 10-15 megatons of TNT in a remote part of Siberia. Although, there is still some debate over whether it was a stony meteor or a fragment of a comet, made of ice and dust.

Trees flattened by the explosion at Tunguska, Siberia in 1908
The 1908 Tunguska impact in Siberia flattened 80 million trees over an area of 2,200 square kilometres, and was probably caused by an asteroid only about 40 metres wide (Photo taken during the Kulik expedition, 1927)

Then of course there are the really big impacts. The buried Chicxulub crater in the Yucatan Peninsula, Mexico is about 70 km across and is believed to have been caused by a carbonaceous chondrite 10 kilometres across creating an explosion equivalent to 96 teratons of TNT, or 96 million megatons. That all happened 65 million years ago, which was the end of the Cretaceous Era and the Age of Dinosaurs.

Around the turn of the millenium there were a couple of movies about a similar scenario happening today. However, a report tabled in the United Nations earlier this year by the scientific group Secure World Foundation says dinosaur extinction-level asteroids are highly unlikely, with the threat more likely to come from ‘smaller’, Tunguska-scale objects. These could only destroy a single city, or cause a devastating tsunami.

Because of their relatively small size of these objects, any warning may be only a matter of hours, and the group are concerned there is no early warning system in place. They claim that governments do not understand the threat of asteroids, and that politicians and emergency services need to be better informed on how to deal with impact events. This involves the need to communicate in non-scientific terms about the realities of near earth objects.

The report also highlighted the media’s alarmist response to recent uncontrolled re-entry of space vehicles, or crashing satellites, and how unhelpful that can be in these situations.

Of course, if we have warning of an object approaching, can’t we just send up Bruce Willis in a space shuttle to nuke the sucker, and get Aerosmith to sing a song about it?

Unfortunately, Bruce would die hard and so would we. According to a group of UK physicists who’ve published two papers in the University of Leicester’s Journal of Special Physics Topics, the end of the US shuttle program is only the beginning of the problems with the scenario of smashing a giant asteroid with a nuclear device.

Drilling into an asteroid the size of Texas in order to crack it open would require a nuclear detonation a thousand million times more powerful than any device ever detonated on earth. In other words, no such weapon exists.

The asteroid would also have to be detected far earlier than the 18 days given in the film. In order to travel to the asteroid and have it split in half, in enough time to miss the Earth, it would have to be cracked 13 billion kilometres from earth, which would take about 52 years at the (now defunct) shuttle’s top speed of about 28,157.5 kilometres per hour.

But they do suggest such an asteroid could be diverted through use of external propulsion attached to the rock, although such a mission would need to be planned well in advance to achieve success, and detection would still need to be in the realm of months and years, rather than days.

You can read more about these earth-shattering papers, Could Bruce Willis save the world? and Could Bruce Willis predict the end of the world?, at the University of Leicester.


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