— If the asteroid Apophis hits Earth in 2036, it could slam into the Pacific Ocean, generating a tsunami that could devastate the west coast of North America (Illustration: Don Davis/NASA)
Kamchatkans and Venezuelans beware. A 20-million-tonne asteroid could be heading your way. Californians have even more reason to worry - the asteroid is more likely to hit the Pacific Ocean, triggering a tsunami that could devastate the west coast of North America.
These are among the scenarios projected for asteroid Apophis, which researchers now say has a 1 in 45,000 chance of hitting Earth on 13 April 2036. Calculations show it would strike somewhere along a narrow track that stretches eastward from Siberia to the west coast of Africa.
Compared to earlier estimates, the new figure represents a further reduction in the threat posed by Apophis (see Risk of asteroid smashing into Earth reduced). But the threat is real enough, experts argue, to merit a United Nations protocol for dealing with the problem.
"Someone will have to make a decision," says Russell Schweickart, a former Apollo astronaut and founder of the Association of Space Explorers. Because any plan for deflecting the asteroid away from Earth will need to be implemented well before an impact site is precisely known, he says, "this is inherently going to be an international decision".
Beginning in the next few months, Schweickart's group will host a series of meetings to provide the UN with a 'decision process' for assessing and acting on the hazard posed by Apophis and other near-Earth asteroids (NEAs). A draft document ready for consideration by the United Nations Committee on the Peaceful Uses of Outer Space is expected by 2009.
During the past 10 years, a concerted search effort by astronomers has led to the detection of an estimated 90% of the asteroids that could threaten Earth with devastation on a global scale. In the coming decade, a next-generation search is likely to uncover most of the remaining global hazards, as well as many more smaller asteroids, like the 250-metre-wide Apophis, that could threaten millions of lives and cause significant damage on a regional scale.
Currently, NASA's Near Earth Object programme lists 127 objects as potential impact risks. By 2020, Schweickart predicts, the list could number in the thousands. Because of the uncertainties involved in calculating asteroid trajectories, many will initially appear to have a small but real chance of hitting Earth in the next few decades.
In most cases, those threats will vanish with additional observations that will narrow the range of possible trajectories. However, in some cases the threat of an impact could persist long enough to require action.
"If you wait to be certain, it could be too late," says Schweickart.
Schweickart and others discussed options for dealing with Apophis and other asteroid risks at the annual meeting of the American Association for the Advancement of Science in San Francisco, California, US.
"Apophis forces us to think about what we might do if [an impact threat] reaches our threshold of pain," say Ed Lu of NASA's Johnson Space Center in Houston, Texas, US.
Lu, who led the discussion on asteroid deflection, warned that "simple methods are not so simple" when it comes to moving the mountain-sized chunks of rock that hurtle through our local region of the solar system. Among the least desirable options is the use of a nuclear warhead to blow up an approaching asteroid. "You could make life a lot worse," says Lu, by turning one potential impactor into many.
'Reshaping the solar system'
Lu's favoured option is called a gravitational tractor. It involves placing a relatively massive spacecraft near enough to an approaching asteroid to shift its trajectory using only the minuscule force of gravity between the two objects. Although the method requires significant lead time and will not work in all cases, it has the advantage of controlling a hazardous object "in one piece", say Lu.
According to Lu, Apophis is particularly amenable to this form of manoeuvring. Prior to its threatening approach in 2036, the asteroid will sweep past Earth in the spring of 2029. Any change in the asteroid's position before this will be greatly magnified by the 2029 encounter, which could, in turn, eliminate the chance of an impact in 2036.
Such a mission could succeed with a 1-metric-tonne spacecraft arriving at Apophis as late as 2027, says Schweickart, who envisions a protocol that would allow the UN to 'contract' the world's space agencies to remove the threat.
"We can't prevent a hurricane," says Schweickart. "But we can prevent an asteroid impact by slightly reshaping the solar system to ensure the survival of life on Earth."