Meteoroids are lumps of rock or iron that orbit the sun. The fastest travel as much as 258 000 kilometres an hour (a bullet goes 3 600 kilometres an hour). Most of them are formed from the collisions of asteroids. And just occasionally these “accidents” can send a meteoroid hurtling towards a planet or moon with devastating consequences. Speed, mass and angle of entry are the determining factors. As a meteoroid enters the earth’s atmosphere (by definition, becoming a meteor) its mass begins to burn up from the extreme friction with the air. With sufficient mass such a meteor can survive this hellish path and hit the ground.

Hoba West Meteorite, Nambia
The Hoba West Meteorite landed in Namibia in 1920

The 2008 TC3 meteor (in October 2008) entered the earth’s atmosphere at a speed of about 47 000 kilometres an hour at an angle of 19° to the horizon. Fortunately the meteor was no more than two metres in diameter and burned up in the atmosphere before making landfall in the Sudan. Had it been a larger body it would have left behind, not a bunch of aerial photographs of pyrotechnics, but a sizable crater. Its pre-entry weight was estimated at eighty tonnes, which finally exploded in a massive fireball at an altitude of about 37 kilometres. Less than ten kilogrammes of bits made landfall. Altogether scientists were able to collect nearly 600 pieces scattered over a thirty-kilometre radius.

Sixty-six million years ago an impact in the Yucatan Peninsula (named Chicxulub) destroyed 75% of all pre-historic plant and animal species. They reckon the meteor arrived a plump 150 kilometres in diameter.

Other notable events include the 1908 meteor. On 30th June 1908 this asteroid the size of half a football field entered the Earth’s atmosphere at a speed of about 55 000 kilometres per hour and exploded some eight kilometres above the Siberian forest at Tunguska, with an equivalent of 15 mega tonnes of TNT. While it did not make landfall, it exploded with such force that it decimated an area of forest in Tunguska, Russia, of over 2 000 square kilometres.

Ancient records suggest that there are major impacts some 27 million years apart. And the most tantalising theory to explain this relates to the sun, which orbits our galaxy—The Milky Way—every 230 million years or so. Scientists suggest that the sun oscillates above and below its mean orbital plane every 27 million years, alternately lifting and dropping (if you like) the Solar System through denser regions of galactic materials.

There were nine impact craters over the last 10 000 years with diameters ranging from 100 to 300 metres. Going further back in time, the last million years, there are just seven known about. The largest of these was the 14-kilometre diameter meteor that formed the Zhamanshin Crater in Kazakhstan. Other meteors recorded over this timescale include Tenoumer, landing in the Saharan Desert some 21 000 years ago, and one at Xiuyan, China that landed 50 000 years ago. Their diameters ranged from one to three kilometres.

The largest meteorite to have been found is Hoba West, having landed in Namibia in 1920 in a sufficiently organised way to preserve 66 tonnes of its body. Had it entered the atmosphere a little steeper and a little faster it would have completely disintegrated on impact leaving behind a massive crater. It is shaped like a slab a bit under three by three metres and some 90 centimetres deep. It consists of 84% iron. Having rusted somewhat there is about 60 tonnes left today.

There are three types of meteors: stony, iron and stony-iron. Most originate from the asteroid belt. But a very small percentage of meteoroids come from the Moon and Mars after they themselves were targeted by asteroids or other meteoroids.

Up to 200 objects a year also hit Earth from man-made space debris left in our ‘cosmic garbage dump’. There are millions of bits up there waiting to fall and/or collide with other bits. It is estimated that at least 6 000 tonnes of this garbage has survived re-entry hazards and landed back on earth. Eventually someone will be injured or even killed by one of these bits. Since it came from Heaven, one wonders if this would be “an act of God” and therefore covered by insurance?

Nasa’s Centre for Near Earth Object Studies operates Sentry, a highly automated collision monitoring system that continually scans the most current asteroid catalogue for possibilities of future impact with Earth. It is currently keeping an eye on 21 objects.

In October 2015 a massive asteroid, about three kilometres in diameter, zoomed towards Earth at about 64 000 kilometres an hour. Known as 86666 (2000 FL10) it passed at a distance of 25 750 000 kilometres—a “near miss” in astronomical terms. The next possible missile from the universe is the major object: 2015 RN35. But we can rest easy for some time. Its next scheduled flyby is 2038. If that fails the object can have another go in 2114.

The Planetary and Space Science Centre (PASSC) at the University of New Brunswick, Ottawa, manages the Earth Impact Database (EID). This comprises a list of 190 confirmed impact structures from around the world. The database was conceived in its earliest form when a systematic search for impact craters was initiated in 1955 by the Dominion Observatory, Ottawa, under the direction of Dr. Carlyle S. Beals. In 2001 the database was transferred to the University of New Brunswick. It notes: “An impact event with energy greater than the world’s nuclear arsenal occurs on a time-scale of less than a million years.”

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By Nigel Benetton, science fiction author of Red Moon and The Sands of Rotar.

Last updated: Wednesday, 1st April 2020