The vulnerability of the Earth to catastrophic collision with another large object was dramatically revealed a few weeks ago. It was reported in the media that a 1.6km-wide asteroid was headed towards us and might pass as close to Earth as 48,000km in 2028, with the possibility of a direct collision not entirely out of the question.
Twenty-four hours later, revised calculations showed that the asteroid, named 1997 XF11, would actually miss Earth by 1 million km. The world heaved a sigh of relief, but the asteroid threat has firmly entered public consciousness, and we now look at the sky with wariness.
Asteroids are celestial bodies that orbit the sun, mainly between Mars and Jupiter. They range in diameter from 670km (Ceres) to less than 1km. Before 1970 asteroids were considered low-class astronomical objects and there was little interest in them.
In the 1970s some astronomers noticed the abundance of asteroids that pass close to Earth, and began a programme to catalogue them. Computer models showed that the gravity of the planets caused a significant number of asteroids from the main belt to tumble down into lower orbits, approaching or crossing the orbit of the Earth.
It is now estimated that there are about 300,000 near-Earth asteroids over 100 metres in diameter, and about 100 over 1km in diameter.
The media concentrate on impacts of Earth with asteroids 1km wide, or bigger. These impacts are catastrophic and have consequences for all life on the planet. Luckily, they are extremely rare and very unlikely to happen over the next few thousand years. Collisions with smaller asteroids are much more common. One such event occurred at Tunguska, a remote forested area in Siberia, on June 30th, 1908.
The Tunguska asteroid (or comet fragment) was 30 to 60 metres wide. It came in on a grazing trajectory and exploded about 5km above Tunguska, releasing energy equivalent to a nuclear bomb. The forest was flattened out to about 30km from ground zero. Carpenters were thrown off a building 200km away by the shock wave.
Reindeer herders 80km away were hurled into the air. About 1,500 reindeer were killed. Large seismic vibrations were recorded 1,000km away. Seventy per cent of the Earth's surface is covered by water, and an impacting asteroid is far more likely to hit the ocean than land. A small asteroid ocean impact would cause much more damage than a land hit. A land hit from a small asteroid would cause mainly localised damage. An ocean hit could cause a huge wave (tsunami) that could inflict massive damage on coastal regions.
An earthquake-induced tsunami in Chili in 1960 produced waves in Hawaii, 10,600km away, up to 10 metres high, and five-metre waves in Japan, 17,000km away. These waves caused huge damage and loss of life. Tsunami damage is not just caused by the heavy wall of water, but more by the solid debris carried by the water, which batters everything in its path.
It is estimated that a tsunami exceeding 100 metres in height occurs along Atlantic coasts once every few thousand years. Written history in these regions does not recount such a tsunami, so we may be due another soon!
Such a wave would cause enormous damage along the east coast of the USA and would submerge vast areas in Europe, including Holland and Denmark. A 100-metre wave would travel inland about 22km, a 200-metre wave would travel inland about 55km.
Earth has often been hit by large objects from space and over 150 impact craters mark the Earth's surface. The most famous of these is a circular 195km diameter feature in Mexico's Yucatan Peninsula. This is believed to be the impact site of a 12km asteroid that struck about 65 million years ago.
About 50,000 years ago, a small nickel-iron asteroid, about 30 metres across, blasted a crater 1km across and 210 metres deep (Meteor Crater) in Arizona. In another location such an impact would destroy a city.
A land impact from a 1km asteroid would have serious consequences worldwide. Life would certainly be obliterated near the impact site. But also the dust cloud from the impact would blot out the sun for weeks, perhaps months.
A larger asteroid would create enough dust to blot out the sun for much longer, possibly ending civilisation. The Yucatan asteroid is popularly credited with wiping out the dinosaurs and 70 per cent of Earth's other species. It is estimated that as many as 2,000 asteroids larger than 1km either cross or come close to Earth's orbit, but astronomers have discovered fewer than 200 of them. Just as worrying are the estimated 300,000 asteroids wider than 90 metres that come near or intersect Earth's orbit. Perhaps 100 million Earth-crossing asteroids larger than 20 metres exist, and a direct hit by one of these could destroy a large city.
Asteroid hunting to date has been grossly under-funded. NASA contributes only $1.8 million annually to this task.
What if an asteroid is discovered with our name on it? If the asteroid is small and detected many years, and many orbits, before projected impact, it could be easily handled by slightly deflecting it from its path, using conventional explosives, at its closest approach to the sun.
Nuclear weapons would be necessary to deflect an asteroid wider than 100 metres and detected only shortly before impact. Isn't it ironic that nuclear weapons, regarded by many as the single biggest threat to civilisation, may yet prove to be the key to our future?
William Reville is senior lecturer in biochemistry at UCC.