Target Earth

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Are we going to be hit by a Tunguska-like asteroid again? Astronomers suggest that the average frequency of impacts of this size (average width 75 metres) is 1 in 1,000 years. These asteroids explode in the lower atmosphere but release enough destructive energy to wipe out a large city.

The rate of impact decreases with the increase in size of the asteroid. The average interval between impacts of giant asteroids (average width 16 kilometres) is 100 million years. An impact of this magnitude could destroy an entire continent and trigger mass extinction of advanced forms of life. Such an impact is believed to have wiped out the dinosaurs around 65 million years ago (see Chapter Ten).

Do we really want to win an asteroid lottery? The probability of randomly picking six numbers in a lottery of 45 numbers is 1 in 4 million. The probability increases to 1 in 14 million if you have to randomly pick six numbers from 49 numbers, and to 1 in 19 million if you have to pick from 51 numbers. The probability of an asteroid impact, small or big, is 1 in 20,000, the same probability as for a passenger aircraft crash. From these odds it appears that the proverbial man in the street, if he is not run over by the proverbial bus (probability 1 in 100), will witness an asteroid impact long before he wins the big lotto. Why bother to buy a lottery ticket today?

Should we dismiss these risk probabilities as lies, damned lies and statistics, or lose sleep over the asteroid threat? What do the experts say?

'We do not know whether a large dangerous asteroid with our name on it is destined to hit [this] century', says Britain's Astronomer Royal, Martin Rees. 'The risk isn't large enough to keep anyone awake at night, but it isn't completely negligible either.'

The American planetary scientist Tom Gehrels has a similar opinion: 'The chances of a celestial body colliding with Earth are small, but the consequences would be catastrophic.'

'Nobody believed Chicken Little when he said that the sky was falling. But occasionally the sky does fall, and with horrendous effects.' That's what Eugene Shoemaker, who was chiefly responsible for alerting the world to the dangers of asteroid and comet impacts, said in 1994.

'Doubters can ask the dinosaurs for their opinion', advised a Scientific American editorial in November 2003.

Besides asking dinosaurs' expert opinion, what else could scientists do to stop a rogue asteroid from crashing into Earth? Once scientists have discovered an asteroid they can calculate whether it is headed our way. The plan to destroy it will depend upon how far away it is. Here are some of the plans to save us from the scenarios dramatised in the movies Armageddon and Deep Impact:

Nudge 'em. If a threatening asteroid is spotted one year before the expected collision, it would be possible to nudge it with conventional-chemical fuel missiles. A small change in the asteroid's speed, on the order of 36 metres per hour, will deflect the asteroid 6,000 kilometres - the radius of the target Earth - in one year.

Push 'em. Instead of a gentle nudge, some scientists prefer a stronger push by a nuclear-powered spacecraft that expels jets of plasma. The unmanned 'space tug' would rendezvous with the killer asteroid, attach to its surface and slowly push it so that it misses Earth.

Crush 'em. This ingenious plan suggests crushing the asteroid by placing a three-dimensional lattice made of millions of small tungsten balls in the path of the speeding offender. The collision would create enough heat to turn the asteroid into small, harmless rocks. The lattice could be launched into space by a rocket.

Cover 'em. Another ingenious plan is to wrap the rogue asteroid with a shiny plastic sheet like a giant potato in aluminium foil by sending a solar sail spacecraft that collapses around the asteroid and shrink-wraps it. An asteroid radiates heat into space after the Sun warms its surface, which imparts a tiny momentum to the asteroid, slightly shifting its orbit. A shrink-wrap or a spray of white chalk or black carbon powder across an asteroid's surface would change its reflectivity and hence the heat transfer from its surface. This heat transfer would change the momentum, which could be enough to change its path. After gift-wrapping it would be years before the asteroid changed its course.

Nuke 'em. If there is not enough time to prepare, some scientists suggest destroying the asteroid with a nuclear bomb. The idea is that the nuclear blast would melt material off the asteroid's surface, giving it a kick in the opposite direction. This is not really a good way of killing an asteroid: if the bomb detonates too close to the asteroid, it may explode it, creating millions of tonnes of radioactive dust and rubble. If the rubble rains on Earth, we may have to join the dinosaurs anyway. Neutron bombs - the bombs that kill people but leave buildings undamaged - offer a better alternative. Once the high-

energy neutrons hit the asteroid, they would heat its surface. The vaporised material would deflect the asteroid from the collision path.

Burn 'em. This plan involves placing a huge aluminium concave mirror fairly close to the asteroid. The mirror would focus a beam of light on a small spot on the asteroid. The heat would vaporise a small part of the asteroid which would shoot into space, pushing the asteroid in the opposite direction. An 800-metre wide mirror could deflect an asteroid 3 kilometres in diameter.

Dig 'em. This simple but technically difficult plan requires placing a robot on the surface of the asteroid. The robot would dig rocks from the asteroid and hurl them into space, causing it to accelerate slowly in the opposite direction.

Robert Gold of Johns Hopkins University, who considers an asteroid or comet impact 'the greatest natural threat to the long-term survivability of mankind', has proposed a comprehensive Earth defence system designed to discover, catalogue and calculate the orbits of near-Earth objects, and to deflect potential hazards. His three-part SHIELD system consists of Sentries, spacecraft designed to search for and locate threatening objects, Soldier spacecraft to deflect or disperse the object, and an Earth-based control system to oversee the network. Gold's Soldiers would use one or more of the deflection techniques discussed above. Gold believes that SHIELD can be implemented within the next 10 to 40 years. In 2002 NASA's Jet Propulsion Laboratory (JPL)

established its SENTRY programme, a highly automated collision monitoring system that continually scans the most current asteroid catalogue for possibilities of impact with Earth over the next 100 years. Whenever a potential impact is detected, it is analysed and the results are immediately published on JPL's NEO (near-Earth objects) Programme website: This website also keeps track of the asteroid 1950 DA. As the name suggests, it was discovered in 1950. It was observed for seventeen days but then it faded from view for half a century. It was rediscovered on New Year's Eve 2000. There is a 1 in 300 long shot that the 1.1-kilometre-wide 1950 DA would hit Earth in March 2880. Don't forget to pass the JPL website's good news on to your next 35 generations: 'There is no reason for concern over 1950 DA. The most likely result will be that St Patrick's Day parades in 2880 will be a little more festive than usual as 1950 DA recedes into the distance, having passed Earth by.'

On the matter of the doomsday rocks, Tom Gehrels has the last word:

Comets and asteroids remind me of Shiva, the Hindu deity who destroys and re-creates. These celestial bodies allowed life to be born, but they also killed our predecessors, the dinosaurs. Now for the first time, Earth's inhabitants have acquired the ability to envision their own extinction - and the power to stop this cycle of destruction and creation.

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