How to Destroy the Universe (5 page)

Other scientists have proposed fighting hurricanes from the opposite direction—the ocean surface. One proposal is to lay down a slick of biodegradable oil over the sea to temporarily prevent warm water vapor from escaping. US entrepreneur Bill Gates has entered the fray, backing a plan involving barges that would sit in the path of a hurricane pumping cold water up from the ocean depths to cool the sea surface.

Many hurricane experts, however, are pouring cold water on these and other ideas. They argue that rapidly
moving weather systems thousands of kilometers across are simply beyond the realms of human influence. Not only that, many worry about unforeseen consequences. Hurricanes, like other weather phenomena, are fiendishly difficult to forecast. Then again, given the potential for damage (over $100 billion in the case of Katrina), these may be technologies we cannot afford to ignore.

• Asteroid impacts

• The Tunguska impact

• Planetary defense

• The nuclear option

• Kinetic impact weapons

• The Yarkovsky effect

• Asteroid evolution

Sixty-five million years ago, the dinosaurs had a very bad day indeed. An asteroid 10 km (6 miles) across slammed into Earth and exploded with the force of 200,000 gigatons of TNT—4 million times more destructive than the largest nuclear bomb. The blast gouged a crater 180 km (110 miles) across and unleashed global firestorms, quenched only by gargantuan waves that swept round the planet. Very little survived as the dinosaurs, along with many other species, became extinct. Asteroids bigger than 5 km (3 miles) in diameter hit Earth once every 10 million years. Will the arrival of the next one mean extinction for humanity? Not likely.

Asteroids are chunks of rocky debris left over from the formation of the Solar System. As dust particles swirling around the young Sun 4.5 billion years ago randomly bumped into each other, they began to stick together like clods of soil. Eventually they became big enough for their gravity to draw in material that wasn't directly in their path, and they grew larger still. As these cosmic boulders collided, they continued to grow until a handful became so large that they formed into the planets and moons of the Solar System. Not that the collisions stopped. During the so-called “late-heavy bombardment,” about 4 billion years ago, the closest planets to the Sun were pelted with asteroids as the shrapnel left over from planet formation was pulled in by their gravity. On Earth, our atmosphere and weather have eroded all but the biggest craters—such as the Barringer crater in Arizona—but look no further than the pocked and scarred surface of the Moon for evidence of how unimaginably violent this phase of Earth's history really was.

The threat hasn't gone away. On November 3, 2008, at around 10:30 pm, a solitary asteroid whizzed past our planet at a distance of just 38,500 km (24,000 miles). It sounds like a wide birth, but in astronomical terms that's a mere hair's breadth—just a tenth of the distance from Earth to the Moon. And just a month before that, another space rock actually hit our planet with
just a day's warning that it was coming, exploding in the sky over Sudan. Luckily the Sudan impact was a small one, just a few meters across, but the November 3 near-miss was much bigger. At 250 m (820 ft) in diameter and hurtling in from outer space at 20 km/s (12 mps)—nearly 60 times the speed of sound—the rock would have struck the ground (or exploded in the atmosphere as a result of heating caused as it compressed the air in front of it) with a force equivalent to a 500 megaton nuclear weapon—over 30,000 times the power of the Hiroshima bomb.

Asteroid impacts threaten Earth with startling regularity. While asteroids of the size that wiped out the dinosaurs only strike once every 100 million years or so, smaller rocks come calling far more frequently. In 1908, an asteroid 45 m (150 ft) wide exploded in the skies over the Tunguska River in Siberia. The blast was big enough to flatten a modern city—indeed, had it landed on central London, everything within the M25 ring road would have been obliterated. Astronomers believe that Earth is pummeled by at least one Tunguska-size impact every few hundred years. After much lobbying from the international scientific community, the international political community agreed that action was necessary to combat this threat. The United Nations Working Group on Near-Earth
Objects held its first workshop in February 2009. Their goal is to coordinate the world's response to the detection of an asteroid on collision course with Earth.

Most scientists believe that a lead time of several decades will be needed to do anything about a rock on a collision course with our planet. That means that as our first line of defense we must set up a cosmic early warning system. NASA has been given a congressional mandate to log 90 percent of asteroids bigger than 140 m (460 ft) by 2020. The US space agency intends to detect and track hundreds of thousands of stony wanderers in the depths of space, monitoring them night after night so that their orbital trajectories through the Solar System can be established.

There's no way human astronomers working at their telescopes could achieve this, so most asteroid detection today is done by robot telescopes. Each night the telescope scans a predetermined area of sky and computer software compares its images to those taken on previous nights. Anything found to be moving from frame to frame, and which isn't a known asteroid or planet, is earmarked for further monitoring, and human astronomers are alerted.

Tracking a new asteroid over many nights tells the
astronomers how fast it's traveling. The mathematical laws of orbits then allow them to infer how far away it is and to work out its precise trajectory around the Sun. Comparing this to the orbit of Earth enables them to flag up any which look set to get uncomfortably close. But how close is close? Astronomers gauge the danger posed from near-Earth asteroids on what's called the Torino scale. It's rather like the Richter scale for earthquakes, and gives an indication of the danger posed by any particular asteroid. The name comes from a scientific conference held in Turin (Torino in Italian), Italy, in 1999, where the scale was first proposed. The scale ranges from zero up to 10. Zero corresponds to an asteroid that poses no risk at all, while 10 indicates a certain collision that's going to cause global devastation. The highest an asteroid has ranked on the Torino scale so far is asteroid 99942 Apophis, discovered in 2004, which scored a 4—meriting concern but not alarm. Further observations later gave astronomers a better understanding of its orbit and they downgraded the risk to a 0.

If and when the automated sky surveys do turn up a Torino-topping object, scientists have come up with a number of possible options. The long-standing favorite of Hollywood when it comes to dealing with calamitous rocks from space is nuclear weaponry. And
yet experts agree that in most cases going nuclear is the worst course of action. The main problem is that atomic blasts are so violent that rather than deflecting an asteroid onto an orbit that misses Earth, the most likely outcome is to shatter it into a blizzard of smaller fragments that will still hit the planet. Any of these bigger than 30 m (100 ft) across (and it is likely there would be very many of these) will still be capable of penetrating the atmosphere and causing catastrophe.

Turning a rifle bullet into a shotgun blast like this also carries with it a hidden danger. Dotted around Earth are a number of points in space known as “gravitational keyholes.” Although a rock passing through one of these keyholes will miss us on this pass, the planet's gravity will bat it onto a new path bound for collision one or more orbits down the line. With a spread of rocks heading toward the planet—as you'd get after a nuclear attack—it's likely that at least one of them will pass through one of these keyholes in space. Asteroid 99942 Apophis is due to make another close flyby of Earth in 2029 and, although it's unlikely to hit, there are concerns that it will pass through a keyhole, bringing it back to Earth in 2036.

If nuclear is no good, then what are the other options? One method is to use a so-called kinetic impactor.
This is a solid mass with no explosive charge that slams into the asteroid and gives it a kick simply by virtue of its momentum, knocking it onto a new orbit. In 2005, NASA's Deep Impact mission did just that, firing a solid projectile at the nucleus of comet 9P/Tempel. The impact threw up a plume of material from the nucleus so that instruments on the probe could analyze the comet's composition. The mechanics of guiding a projectile to deflect a hazardous comet or asteroid are much the same. So much so, the European Space Agency is planning a mission to road test such a deflection technology. Called
Don Quijote
, it consists of a pair of spacecraft. One spacecraft will crash into a target asteroid at a speed of 36,000 km/h (22,000 mph) while the other measures how the rock's course is affected by the smash. ESA plans to launch the mission in 2011, but as of mid-2012 it is still only a study.

One of the deflection methods favored by scientists is known as a gravity tractor. Heavy objects attract other heavy objects thanks to the force of gravity. In 2005, two former US astronauts—Edward Lu and Stanley Love—realized that this could be used to make a gravitational tow bar that could drag an Earth-crossing asteroid off its collision course with our planet. The basic idea is to have a spacecraft fly alongside the target asteroid. As gravity makes the asteroid and the spacecraft move together, the spacecraft fires its engines—and the asteroid follows. It's a neat idea, though you
could argue that rather than going to all the trouble of sending a spacecraft to the asteroid and hovering near it for many years (because that's how long it would take to deflect an asteroid in this scheme), mission planners may as well just bolt a rocket motor directly to the asteroid. And that, too, is a possibility.

There is a more ingenious take on this idea, though—the mass driver. A rocket engine works by burning fuel in order to create a stream of high-speed exhaust gas that propels the spacecraft in the opposite direction. A mass driver does a similar job, but works by mechanically catapulting lumps of material in one direction to send the spacecraft—or, in this case, the asteroid—moving the other way. Rather like the kick from a gun firing bullets, each lump cast off produces a recoil, shoving the asteroid in the opposite direction. Over time, all the shoves add up to change its course. In practice, a mass driver would take the form of a solar- or nuclear-powered robot that sits on the asteroid's surface.

Perhaps one of the strangest ideas for saving the world from killer asteroids derives from the work of 19th-century Russian engineer Ivan Yarkovsky. In general, asteroids spin as they travel through space. Yarkovsky showed that this spinning skews the way
that heat is radiated from an asteroid's surface. This creates an acceleration on the rock that, over time, can alter its orbit around the Sun. As an asteroid rotates, it has a “dawn hemisphere,” the side where the surface is rotating from darkness into sunlight, and a “dusk hemisphere” where the surface is rotating from sunlight back into darkness again. The dusk hemisphere is warmer (because its just been in bright sunlight) and so radiates more heat than the dawn hemisphere. Because the heat is carried away as photons of electromagnetic radiation, which pack momentum (see
How to harness starlight
), the radiation exerts a recoil on the asteroid that influences its orbit over time.

The Yarkovsky effect can be enhanced or diminished by changing the asteroid's color, because different colors absorb and emit heat at different rates (in just the same way that black seat covers in a car get much hotter on a sunny day than white ones). This has led to the suggestion that one way to combat the threat posed by rocks from outer space is to send up a team of astronauts equipped with the world's largest paint rollers.

Although generally regarded as detrimental to life, some scientists suggest that, ironically, the regular upheavals caused by rocks from space slamming into our planet presented formidable challenges to life-forms
emerging on the early Earth that stimulated their evolution—giving them resilience and the problem-solving ability needed to survive in this harsh environment. And asteroids may drive our own species to even greater accomplishments, giving us the motivation to migrate away from our planet and become a spacefaring civilization. As the dinosaurs discovered to their cost, Earth is a fragile cradle—one that human beings must ultimately leave if they are to continue to prosper in this often brutal universe.

• Earth's anatomy

• The core

• Earth's deepest holes

• Reasons to dig

• Probe to the core

Human beings have explored some of the most far-flung reaches of the Solar System, but when it comes to investigating the innards of our own planet we've barely scratched the surface. Jules Verne's 1864 novel
Journey to the Centre of the Earth
told the tale of a group of explorers who venture down into the bowels of the planet. Verne's heroes discover a subterranean world populated by dinosaurs and prehistoric humans. But what really lies at the heart of our planet? Scientists think they might have the means to find out.

Our planet is a 4.5-billion-year-old ball of rock and metal just over 12,700 km (7,900 miles) in diameter. If you could take a knife and cut Earth down the middle
from pole to pole, you'd find within a layered structure rather like the inside of an onion. The outermost layer is known as the crust. It is made of various kinds of rock, and its thickness varies considerably. Beneath the oceans it can be as little as 5 km (3 miles) deep, whereas on land the thickness of the so-called continental crust can reach up to 40 km (25 miles). This is the primary reason why the continents are raised up out of the ocean. The crust accounts for roughly 1 percent of the volume of the planet.