by Alok Jha
NASA keeps its eye on more than 900 near-Earth objects (NEOs) to make sure nothing gets too close to the planet. None of these are causing concern at the moment. In any case, nowadays we might stand a fighting chance against the rocks from space. Scientists are designing space missions that could theoretically divert or destroy any incoming asteroids.
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Objects larger than six kilometers wide, which could cause mass extinction, will collide with Earth every 100 million years. Experts agree that we are overdue for a big one.
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The effects of a collision
The Earth is bombarded constantly with cosmic debris, at speeds of more than 16 km/s (10 mps miles per second). Around 100 tons of rubble hits the Earth every day, much of this material burning up in the atmosphere with little result other than the fireworks of a shooting star. Some land as small rocks, meteorites, and end up in the collections of institutions such as the Natural History Museum in London, or the Vatican.
Imagine that a dense clump of rock finds itself attracted by the combined gravitational influences of the Sun and surrounding planets and on a crash course for Earth. As it enters the atmosphere at 16 km/s, its surface would heat up and its outermost layers would begin to evaporate away, turning the incoming object into a fireball. The air around it would expand rapidly, sending shock waves and sonic booms across the world that would flatten buildings and trees hundreds of miles away.
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A 200-m object landing in the Atlantic Ocean, for example, would be enough to deluge all the coastal cities of the Americas, Europe and Africa.
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According to a report produced by NASA in 2003 on potential asteroid impacts, any object up to 150 m (500 feet) in diameter would disintegrate in the atmosphere rather than reaching the ground to form a crater, a bit like the situation in Tunguska in 1908. In that case, the immediate area would be showered with debris.
If the asteroid was bigger, a sizeable chunk would hit the Earth’s surface. If it landed in the ocean, giant tsunamis would radiate from the impact point and drown coastal cities. A 200-m (700-foot) object landing in the Atlantic Ocean, for example, would be enough to deluge all the coastal cities of the Americas, Europe and Africa.
“Those smaller events occur rather more frequently—they are talking about a once every several thousand years event,” says Duncan Steel of the University of Salford, a leading authority on asteroids and comets.
As the asteroid hit the ground, it would also throw colossal amounts of dust up into the atmosphere. With a sufficiently large impact, dust would reach the stratosphere. The particles would circulate around the Earth’s natural weather systems, and there they would stay for some time, all the while blocking the sunlight from the Earth’s surface, causing plants to die and, eventually, the animals that live off them.
Can we stop it?
There are many ideas about how to stop the worst effects of an asteroid on course for the Earth, ranging from giant mirrors floating in space that could vaporize parts of it, to methods that rely more on brute force, such as smashing a rocket into it to deflect it. The traditional Hollywood solution of sending a nuclear bomb to the surface of the asteroid, however, is nowhere in sight.
Deflection methods fall mainly into two categories: kinetic and low-thrust. Kinetic methods are those that provide an instantaneous change of properties within the asteroid—sending a nuclear warhead or some sort of exploding device against it to create a shock wave, for example. Low-thrust methods include painting the surface of the asteroid with reflective or absorbing paint so that its properties are changed by attracting more or less light, thus heating or cooling it.
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RISK OF DEATH BY
Earthquakes
1 in 130,000
Asteroid impact (mass extinction)
1 in 4,300,000
Shark attack
1 in 8,000,000
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Whatever method is used, it would only change the path of the asteroid by minute amounts. But even very small adjustments could, over the years, build to big changes in its orbit.
The exact method used would also have to vary depending on the type of asteroid. Some asteroids, known as rubble piles, are loose collections of rocks and ice, so slamming a rocket into one of these would be useless, because the energy of the impact would simply be absorbed, much like the crumple zones in a car.
In this case, a more successful method might involve melting part of the surface of the asteroid by concentrating sunlight. A large solar sail or mirror could reflect the Sun’s rays on to the surface of the asteroid and burn part of it away. The jets of gas produced would create a small but constant thrust that could deviate the asteroid into a new orbit.
More traditional solid asteroids provide a range of options. You could place an engine on the surface, for example, which would create a very low thrust and move the asteroid ever so gently over an extended period of time. Or you could hurl a spaceship directly into its path. The idea here is not to physically push the asteroid away but to use the collision to gouge out a hole in the rock. The ejection of material would then push the object in a different direction.
The European Space Agency already has plans to conduct an experiment in deflecting asteroids away from the Earth. Its Don Quixote mission will consist of two spacecraft: Hidalgo and Sancho. Hidalgo will smash into an asteroid at high speed, while Sancho will watch the collision and record any shift in the asteroid’s trajectory.
Piet Hut of the Institute for Advanced Study in Princeton has championed the idea of a robotic tugboat that could attach itself to an asteroid and push it out of the Earth’s path. Based on early warning, provided by ground tracking and orbit prediction, it would be deployed ten years or more before potential impact.
The performance of the tugboat, he says, would depend on the development of a high-performance electric propulsion system called an ion engine. Instead of burning chemicals for fuel, these engines propel a spacecraft forward by ejecting charged particles the other way. The thrust is minuscule—equivalent to the pressure of a piece of paper on your hand—but the engine is extremely efficient and lasts far longer than a conventional rocket engine. Professor Hut calculates that such a spacecraft could be used to deflect NEOs up to half a mile across.
Ion engines would also be crucial for another type of probe, the “gravity tractor.” Instead of landing on an asteroid, though, the gravity tractor would hover near it, using the slight gravitational attraction between the probe and the NEO to change its path.
Stopping an asteroid will require decades of planning, so that the very slight deflections that can be created will have some effect in keeping the object out of the Earth’s way.
What are the chances?
A 90-m (300-foot) object crashes into the planet every 10,000 years, triggering a 100-megaton explosion in the air, greater than the largest H-bomb ever tested. A 900-m (3,000-foot) object scores a direct hit on the planet every 100,000 years, with the force of 10 million Hiroshimas.
Monica Grady, an expert in meteorites at the Open University, says it is a question of when, not if, an NEO collides with Earth. “Many of the smaller objects break up when they reach the Earth’s atmosphere and have no impact. However, an NEO larger than 1 km [wide] will collide with Earth every few hundred thousand years and a NEO larger than 6 km, which could cause mass extinction, will collide with Earth every hundred million years. We are overdue for a big one.”
Mega Tsunami
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In the Atlantic Ocean, just off the northwest coast of mainland Africa, there is an island on the verge of falling apart. If it fell into the sea, the resulting waves would wipe out many of the major cities of the world and lead to a catastrophic loss of life. And it could go at any time.
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Formed by volcanic activity several million years ago, La Palma in the Canary Islands is a beautiful location, a popular destination for holiday makers looki
ng for idyllic sunshine or deep gorges filled with a diverse array of animals and plants.
Of all the Canary Islands, La Palma is the most volcanically active, having suffered seven major eruptions since it was occupied by the Spanish in the 15th century. The eruptions themselves are dangerous enough to locals, but there is something else about the island that keeps geologists worried, on behalf of the rest of the world. They believe that a future eruption might dislodge from the island’s southern edge a chunk of rock twice the volume of the Isle of Man that is currently under pressure from the gases trapped underneath it.
If this rock fell into the Atlantic Ocean, it would trigger waves that would rise up to over half a mile in height and move at the speed of a jumbo jet. The colossal wall of water, a mega tsunami, would destroy any islands in its path, and when it reached the shores of the US, Europe, South America and Africa, its effects would be catastrophic. Tens of millions of people live on the eastern seaboard of the United States alone—New York City, Boston, Miami and Washington DC would all be under water if La Palma fell apart.
“This would be the biggest natural catastrophe in history,” says Bill McGuire, director of the Aon Benfield Hazard Research Centre at University College London. “There’s a problem with all major natural catastrophes. Because we’ve never experienced these things we don’t think that they’re going to happen to us. We just ignore them, but these sorts of events have occurred throughout geological history. They’re not going to stop happening just because we’re around. La Palma is going to collapse into the North Atlantic. It’s not a question of if, it’s just a question of when.”
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There’s a problem with all major natural catastrophes. Because we’ve never experienced these things we don’t think that they’re going to happen to us.
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Even worse, La Palma is just one of the volcanic islands that could collapse and cause a catastrophic mega tsunami—there are dozens of others dotted around the world’s oceans.
What is a mega tsunami?
A tsunami is the name given to a wave caused by the displacement of a very large volume of water. They are usually caused by underwater earthquakes or volcanoes, but can also result from landslides and asteroid impacts.
Tsunamis travel at more than 800 km/h (500 mph), and in the open ocean, the height of the waves they produce might seem nothing out of the ordinary; perhaps just a slight swell in the normal sea surface. A typical tsunami wave has a wavelength (the distance between successive wave peaks) of more than 200 km (125 miles), compared with around 90 m (300 feet) for a typical wave caused by winds. When a tsunami reaches land, and the water it is in becomes shallow, the wave compresses and slows down, and its height grows enormously.
The first directly observed mega tsunami was recorded in 1958, when a magnitude 7.7 earthquake caused 90 million tons of rock to drop into the deep water at the head of Lituya Bay, south-east Alaska. The force of the rock in the water led to a 490-m (1,600-foot) high tsunami wave that inundated the land around the bay, stripping away topsoil, snapping trees and submerging boats. Based on the evidence of the rocks in the area, geologists think that Lituya Bay has suffered several mega tsunamis in the past, as recently as the 19th and early 20th centuries.
The Indian Ocean tsunami of 2004 was caused when part of the sea floor fell by several meters after an earthquake. This meant that an entire column of water above moved by the same amount and the resulting wave spread at hundreds of kilometers an hour in all directions. When the waves reached land, they bunched up, got considerably higher and devastated human dwellings.
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The ocean would suddenly just pull away. You’d see a tide, a low tide like you’ve never seen before in your life. It would be actually spellbinding but in the background you’d be seeing this wall and it’d keep coming at you.
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These huge, destructive waves have been happening on Earth for millions of years. A mega tsunami is likely to have occurred after the asteroid impact that wiped out the dinosaurs and created the Chicxulub crater in Yucatan, approximately 65 million years ago. A series of huge waves also occurred after the asteroid impact that created the Chesapeake Bay crater, about 35.5 million years ago.
And they happen inland too: in 1980, more than 400 m (1,300 feet) of the top of Mount St. Helens became detached and fell into a nearby lake, causing waves that reached a maximum of 260 m (850 feet) above the normal height.
What will happen at La Palma?
When looking for the location of the next mega tsunami, geologists have been examining volcanic islands, which were formed by eruptions and can be torn apart by them as well. The island of La Palma came on to the radar after an eruption of its Cumbre Vieja volcano in 1949, which caused a crack in its western flank. At the same time, a vast chunk of rock—19 km (12 miles) long with a volume of 500 Km3 (120 cubic miles)—dropped 4 m (13 feet) into the surrounding ocean. Scientists think this chunk of rock is still moving, and another eruption could be all that is needed for it to break off entirely.
Computer models of the potential collapse show that the disintegration of the rock would release an amount of energy equal to half a year’s consumption of electricity in the US.
Within two minutes of the rock entering the water, the resulting waves would be half a mile high. Around ten minutes later, the tsunami would have traveled 240 km (150 miles). After almost an hour, the wave amplitude would have fallen to 90 m (300 feet) but the surrounding islands would have been submerged, and within a few hours, the waves would reach Africa and the Atlantic coast of Europe—Britain, Spain, Portugal and France would all be hit.
The northern coast of Brazil would also be smashed by 40 m (130 foot) high waves, and around eight hours after the collapse of the volcano, the fast-moving tsunami would begin to reach US cities.
“If you were standing on a beach in what is presently Miami, the very first effects you’d probably see is what we call drawback,” said Gary McMurty of the University of Hawaii, speaking in a BBC documentary on the potential for a collapse at La Palma. “The ocean would suddenly just pull away. You’d see a tide, a low tide like you’ve never seen before in your life. It would be actually spellbinding but in the background you’d be seeing this wall and it’d keep coming at you.”
Harbors would channel the powerful waves miles inland, and the tsunami would lead to the loss of millions of lives and cause billions of dollars’ worth of damage to property and land. The long-term effect on the economy would be incalculable.
What can we do about it?
If La Palma fell apart tomorrow, there is not much we could do to stop the spread of the mega tsunami or prevent it from causing damage. Having said that, knowing it was about to happen could allow people to prepare and reduce the impact.
Any collapse of the volcano would occur during a future eruption. This would be preceded by days or even weeks of earthquakes and deformation of the land around the volcano, as the gases and hot lava from the Earth’s interior swelled up underneath. Monitoring the volcano and watching out for these signs could give several days’ notice of a collapse, crucial time for emergency services to mount a response.
“Eruptions of Cumbre Vieja occur at intervals of decades to a century or so and there may be a number of eruptions before its collapse,” says Simon Day of the Aon Benfield Hazard Research Centre. “Although the year to year probability of a collapse is therefore low, the resulting tsunami would be a major disaster with indirect effects around the world. Cumbre Vieja needs to be monitored closely for any signs of impending volcanic activity and for the deformation that would precede collapse.”
A tsunami early-warning system already operates in the Pacific Ocean, because of the number of events there, and a similar system for the Atlantic would be required to ensure that problems were detected and information passed on quickly.
If an earthquake was about to occur and scientists thought this could be the one to finally destabilize the Cumbre
Vieja’s western flank, it would be up to prime ministers and presidents around the world to make the decision whether or not to evacuate people from danger areas. This would be no easy task, but at least they would have been warned. And it would beat having to pick up the pieces of a surprise mega tsunami hitting a bustling New York City.
Supervolcano
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The first thing people thousands of miles away would hear is a series of ground-shaking bangs. Outside, in the direction of the noise, they would make out a dark cloud of ash rising into the upper reaches of the atmosphere. It would not be long before the eruption reached, and devastated, their town too.
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They might not realize it, but everything within a few hundred miles of that distant cloud, rising from the distant volcano, would already have been obliterated, burned or blanketed by dust. In a few short hours, a searing hot wind dense with sharp lumps of rock would smash through their own buildings, setting fire to anything it touched, killing any living thing in its path. That wind would reach the coast beyond their city and superheat the water, initiating a tsunami that would travel across the ocean, carrying the volcano’s devastation to the other side of the world. In just the first day, the death and destruction would reach unimaginable levels.